application brief AB20-3 replaces AN1149-3 Electrical Design Considerations for SuperFlux LEDs Table of Contents Overview Summary Electrical Design Process Overview Circuit Design Overview Worst-Case Circuit Analysis and Validation Comparison of Different Methods of Worst-Case Circuit Analysis Characterization of Prototype LED Signal Lamp Validation of LED Signal Lamp Key Concepts for Electrical Design of LED Signal Lamps Resistor-Limited Drive Circuits Effect of LED String Length on Forward Current Regulation Forward Current Variations between LED Emitters in an Array EMC Transient Protection Circuits Stop/Tail Drive Circuits Theory Electrical, Optical, and Thermal Characteristics of LED Emitters LED Emitter Modeling Linear Forward Voltage Model Luminous Flux Models Versus Forward Current and Temperature Thermal Resistance Models Applications Resistive Current Limiting CHMSL Design Example EMC Transient Protection Special Considerations for Dual Luminous Intensity Operation Luminous Flux Variations at Low Currents PWM Drive Circuit for Tail Functions Current and Voltage Regulator Circuits Operation of Shunt, Series-Pass, and Switching Regulators 2 3 4 5 5 7 7 8 8 9 10 12 12 13 16 17 18 19 21 24 27 29 29 30 31 32 Overview Summary The electrical design is part of the overall signal used in LED signal lamps and related circuit The electrical design of an LED signal lamp has design issues. several objectives. The first objective is to operate the individual LED emitters at sufficient In addition to AB20-3, two companion electrical drive current in order to generate sufficient design application notes are also available. luminous flux to meet the lighting requirements. The second objective should be to limit the AB20-3A, titled “Advanced Electrical Design forward current through the individual LED Models,” discusses forward voltage models that emitters so as not to exceed their maximum are more accurate and usable over a larger range internal junction temperatures and maximum dc of forward currents than the simple linear models forward currents under worst-case conditions of shown in the “LED Emitter Modeling” section. In ambient temperature and input voltage. In addition, AB20-3A derives several additional addition, the electrical design should protect the thermal-modeling equations from the basic LED array from automotive EMC transients. equations shown in the “LED Emitter Modeling” Finally, the electrical design should provide section. good intensity matching within the LED array. The result of achieving these objectives will be a Application Note titled AB20-3B “SuperFlux LED maintenance-free LED signal light that operates Forward Voltage Data” gives worst-case forward reliably for the lifetime of the passenger vehicle voltage data for SuperFlux and SnapLED70 or truck. emitters. There are several potential electrical models for LED emitters (linear, diode equation Application Brief AB20-3 has been written to models, etc.), each one optimized for an simplify the electrical design of LED signal expected range of forward currents, and various lamps and is part of the Application Brief AB20 levels of “worst-casing” (i.e. min/max, average, series. This application note has been divided average ± one, two, three standard deviations, into three major blocks—Overview, Theory, and etc.). In order to accommodate these various Applications. The Overview consists of five needs, the data presented in AB20-3B gives the sections that discuss the electrical design nominal forward voltage and expected forward process and key electrical design concepts. voltage range for each SuperFlux LED Theory consists of two sections that give an characterized over a range of forward currents in-depth overview of the electrical, optical, up to 70 mA. From this data, the desired and thermal properties of LED emitters and electrical model can easily be generated. The mathematical modeling of their operation. latest forward voltage data for SuperFlux LED Applications consist of four sections that cover emitters is available from your local Lumileds specific types of circuit designs commonly Lighting or Agilent Technologies sales engineer forward voltage can cause luminous intensity or from the following URL: matching variations within groups of http://www.lumileds.com SuperFlux emitters from the same luminous flux and forward voltage category. For this Note: For best matching within an array, reason, Lumileds Lighting does not warrant SuperFlux and SnapLED 70 emitters should LED performance at currents less than 20 mA be operated at forward currents over 20 mA (40 mA for SnapLED 150) and strongly (40 mA for SnapLED 150). At forward discourages these designs. currents below 20 mA (40 mA for SnapLED 150), variations in luminous efficiency and Electrical Design Process The electrical design is part of the overall signal exceeded, portions of the electrical or thermal lamp design process described in AB20-1. design may need to be iterated in order to reduce the forward current and/ or junction The electrical design consists of several discrete temperature. Finally, additional prototypes of the steps. The first step is to determine the circuit signal lamps should be constructed and topology and to generate an electrical subjected to the appropriate reliability validation schematic of the overall circuit. First, the circuit tests. topology must be determined. Circuit topology refers to the arrangement of electrical The most important circuit topology components on the electrical schematic. Next, considerations include: the circuit must be designed. Circuit design is • Number of LED emitters in series the process where the electrical components • Whether LED emitters are connected in are selected and component values are individual series-strings, or in cross- determined. Third, the operation of the circuit connected series strings • Method of current limiting (i.e. resistors or must be analyzed. Circuit analysis refers to the active circuit) mathematical analysis of the variations in voltage and current through the electrical • Method of EMC protection (if any) components due to variations in applied voltage • Method of dimming, such as for a combined Stop/ Tail signal and component tolerances. The fourth step is to create a breadboard of the circuit and to measure the forward current, light output, and Circuit design is the solution of several thermal properties of the entire signal. If the simultaneous linear equations that model the maximum junction temperature or the maximum forward current through each loop or node of the DC forward current of any of the LED emitters is circuit. The solution of these equations exceeded, the reliability of the LED signal lamp determines the values of electrical components may be compromised. Thus, if the circuit that drive the LED array at the desired forward validation tests indicate that these limits are current at the specified external supply voltage. 3 Different mathematical models can be used to and LED emitters. With this technique, the model the forward voltage of the LED emitters voltages and currents through each component depending on the accuracy and dynamic range can be determined based on the expected needed. minimum and maximum limits of component values in the circuit. Another technique is to use Circuit analysis uses the same types of a Monte Carlo simulation. With this method the simultaneous linear equations used in the voltages and currents through each component electrical design. However, circuit analysis are determined for random combinations of generally assumes that the external voltage component values in the circuit. Then the results applied to the circuit, the values of electrical for a large number of Monte Carlo simulations components, and the ambient temperature can are statistically tabulated. The Monte Carlo vary over some predetermined range. Circuit simulation method gives a better estimate of the analysis can be done using “worst-case” expected manufacturing variations for the circuit. electrical models for the electronic components Circuit Design Overview As stated earlier, the first step in the electrical As will be shown in the section “Key Concepts design is to pick one of the circuit topologies. for the Electrical Design of LED Signal Lamps” Next, the operation of the circuit can be the different circuit topologies provide different modeled with a series of simultaneous linear levels of forward current regulation and overall equations that describe the current through electrical power consumption. Circuits with poor each electronic component as a function of forward current regulation would require the LED component values and applied voltage. For emitters to be driven at a lower forward current circuit design, it is usually assumed that all at the nominal input voltage than would circuits LED emitters have the same electrical with better forward current regulation (so as not characteristics, which greatly simplifies the to exceed the maximum forward current at the mathematical modeling. maximum input voltage). Circuits with higher amounts of power consumption would tend to In order to ensure reliable operation, the have higher internal self-heating (unless the maximum forward current through the LED circuitry is located outside the signal lamp emitters should not exceed the maximum value housing), which would also tend to reduce the obtained from Figure 4 in the HPWx-xx00 data maximum forward current of the LED emitters. At sheet. Note that the maximum forward current this point, it may be desirable to evaluate several of the LED emitters is based on the maximum different circuit topologies on paper and see ambient temperature, TA, the maximum input which one gives the “best” overall results. voltage, and the thermal resistance, R θJA, of the If the signal lamp will be exposed to high-voltage LED signal light assembly. EMC transients, then the appropriate protection circuitry can be added to the basic circuit 4 chosen. Worst-case forward and reverse transients and the minimum breakdown voltage transient currents can be estimated using the specification, VBR, on the data sheet for negative linear forward current model for positive transients. Worst-Case Circuit Analysis and Validation The next step in the electrical design should be circuit. Thus, the actual occurrence of these an analysis of the forward current through the worst-case conditions could be extremely small. LED emitters at worst-case input voltage and operating temperature extremes using worst- Another approach to worst-case analysis is to case component tolerances. All of the active characterize a number of LED emitters and and passive electronic components used in the determine the appropriate forward voltage circuit design can be modeled with their worst- model for each one. Then using a Monte-Carlo case minimum and maximum values. This simulation, random combinations of these analysis serves several purposes. First, it emitters can be assembled into a “paper” determines whether the forward current is less circuit and the actual forward currents can than the maximum dc forward current under all be calculated for the circuit based on the operating conditions. Secondly, it determines corresponding forward voltage models. Then the change in light output of the signal lamp the results from multiple simulations can be under the same conditions. Finally, it can be tabulated. This approach provides a much better used to determine the worst-case matching understanding of the forward current variations within the LED array. that would occur in actual practice. This worst-case analysis can be done in several In general, within arrays of LED emitters, the different ways. One approach is to use worst- maximum forward current occurs at the case values for one or more LED emitters in the maximum input voltage with the minimum value array such as to cause worst-case current of the current limiting resistor and minimum matching between LED emitters or to establish forward voltage model for the LED emitters. the maximum or minimum forward current Likewise, the minimum forward current occurs through individual LED emitters. The problem at the minimum input voltage with the maximum with this approach is that probability of this value of the current limiting resistor and occurrence actually happening can be quite maximum forward voltage model for the LED low. If the probability of getting worst-case LED emitters. emitters is very small, then the probability of both minimum and maximum worst-case LED The worst-case forward current variations emitters occurring in the same circuit assembly for different LED emitters within the array is is even lower. Furthermore, for the worst-case determined by the circuit topology, the drive variations in forward currents to actually occur, current, and the variation in electrical these worst-case LED emitters must both be characteristics of the individual LED emitters randomly assembled into certain parts of the in the array. When several LED emitters are 5 connected in series, the worst-case minimum range of 35 to 70 mA (70 to 150 mA for the forward current would occur when all LED SnapLED 150). These matching effects are emitters in a given series string have the worst- covered in more detail in the section “Key case maximum forward voltage. Likewise, the Concepts for Electrical Design of LED Signal worst-case maximum forward current would Lamps.” occur when all LED emitters in another seriesstring have the worst-case minimum forward The reader will need to determine whether the voltage. For these series-string circuits, the assumptions used for the worst-case designs are likelihood of all LED emitters being at their reasonable. It is possible to design with such worst-case forward voltage extremes is quite large tolerances, that the worst-case design low. When LED emitters are connected in results in an over-designed circuit. Over- parallel, the forward current through each LED designing occurs if significant cost is added to will vary somewhat from the average forward the assembly in order to protect against the current so as to generate the same forward remote possibility of occurrences that might voltage across all LED emitters in the parallel never happen in practice. In the case of LED grouping. The worst-case forward current signal lamps, over-designing might result in many variations occur when one LED emitter has more LED emitters being added to the array than the worst-case minimum forward voltage and needed. The best example might be where the another LED emitter in the same parallel designer chooses an extremely high worst-case grouping has the worst-case maximum input voltage and maximum ambient forward voltage. temperature. Then, the use of the suggested design process in the “Resistive Current Limiting” SuperFlux and SnapLED 70 emitters are section would result in a fairly small design categorized for forward voltage at 70 mA. current at the design input voltage. This design As might be expected, the smallest forward current would require a large number of LED current variations within an array of SuperFlux emitters to achieve the desired light output from or SnapLED 70 LED emitters occur at drive the array. Or the assumptions used for the worst- current approaching 70 mA. Similarly, SnapLED case input voltage and ambient temperature 150 emitters are categorized for forward voltage might require the use of a more expensive at 150 mA, so the best matching occurs at 150 constant current drive circuit, where with more mA. At lower forward currents, the variations in reasonable assumptions a resistive circuit could forward current within the LED array become have been used. These concerns about over- larger—especially when LED emitters are designing by using excessive tolerances on input connected in parallel. For series-string circuits, voltage and ambient temperature also can be acceptable forward current variations can applied to LED emitter tolerances. For example, usually be achieved over forward currents over the probability of all LED emitters in a given array a range of 20 to 70 mA (40 to 150 mA for the being at their worst-case minimum or maximum SnapLED 150). However, when LED emitters limits is very small but still is greater than zero. If are connected in parallel, acceptable forward every LED emitter is assumed to be at the worst- current variations can be achieved only over a case minimum extreme, then the external current 6 limiting resistor may be chosen to overly restrict optical transmission losses. The prototype allows the forward current through the array. Another these assumptions to be measured. Third, the possibility is that if the circuit is designed to working prototype allows the thermal properties accommodate every LED emitter in a given of the LED signal light to be evaluated. The array being at their worst-case minimum or thermal resistance, RθPIN A, can be measured by maximum limits, then design might eliminate attaching thermocouples to the cathode pins of several potential circuit topologies because of several LED emitters in the array. excessive light output variations. In practice, the likelihood of these conditions actually Based on the electrical, optical, and thermal occurring is so small, that the design process measurements of the working prototype, might have eliminated more cost effective circuit additional iterations of the design may be topologies. For this reason, Lumileds Lighting required. These design iterations will further recommends that worst-case design be used refine the estimates for the electrical component as a development tool in conjunction with values, the number of LED emitters needed for characterization and validation of the signal the signal lamp, and the thermal resistance of lamp assembly. the signal lamp. Also, it may be necessary to evaluate improved optical designs and methods Following this paper design, a working for improving the thermal properties of the LED prototype LED signal light can be constructed. signal lamp assembly. For more information on This prototype serves several purposes. First, thermal design, the reader is encouraged to it allows verification of the electrical design. The review AB20-4 “Thermal Management forward current can be measured at different Considerations for SuperFlux LEDs.” For input voltages and compared with the paper more information on optics design, the reader electrical design. Second, it allows verification is encouraged to review AB20-5 “Secondary of the optical design. In AB20-1 section, Optic Design Considerations for SuperFlux “Estimating the Number of LED Emitters LEDs.” Needed,” assumptions were made for the 7 Key Concepts for Electrical Design of LED Signal Lamps Presently, most current LED signal lamp Where: designs use resistive current limiting for the LED VIN = input voltage applied to the circuit array. Since most LED signal lamps are driven VF = forward voltage of LED emitter at forward from 12 to 24 V dc and require several LED current IF emitters, the emitters can be connected in VD = voltage drop across optional reverse series and share the same supply current. transient EMC protection diode Some of the most common circuit y = number of series connected LED emitters configurations are shown in Figure 3.1. The x = number of paralleled strings series connected string circuit in Figure 3.1a uses a separate current limiting resistor for each Thus for a given IF , the value of R depends both string of y LED emitters. The paralleled-string on the number of LED emitters per string as well circuit in Figure 3.1b uses a single resistor for as the number of paralleled strings. the entire LED array. Note that this circuit uses x strings with y LED emitters per string. The As shown later in the section “LED Emitter cross-connected paralleled string circuit shown Modeling” the forward voltage of an LED emitter in Figure 3.1c has one or more cross can be mathematically modeled by the following connections between the strings of LED equation: emitters. A mechanical analogy to this circuit is VF ≅?VO + RS IF that the circuit looks like a “ladder” with each cross connection being a “rung” on the “ladder.” In the diagram, z, refers to the number Where: of series connected LED emitters between each VO = turn-on voltage of each LED emitter “rung” with 1 ≤ z ≤ y. Most CHMSL designs RS = series resistance of each LED emitter use either several series connected strings (Figure 3.1a) or several cross-connected series This equation is known as the linear forward strings (Figure 3.1c) with z =1. Note that in voltage model since it models the forward order to obtain the same forward current for voltage of the LED with a straight line. In using all of LED emitters, both of the circuits this model it is important to remember that it can shown in Figure 3.1b and Figure 3.1c need only be used over a specific range of forward to have the same number of LED emitters in currents. Outside of this range, the model will each string and each “rung” for Figure 3.1c. give misleading results. Using the linear forward voltage model, this equation can be rewritten as: Note that if all LED emitters have identical electrical forward characteristics, then the value of the external current limiting resistor, R, is equal to: 8 The number of LED emitters selected per through each string of LED emitters as a function string (y) affects the change in the forward of input voltage. Figure 3.2 shows string lengths current of the LED emitters as the input ranging from 2 to 6 emitters. Note that series- voltage varies over some range. In general, strings with 5 and 6 emitters have the largest as the value of y is increased, the change in change in forward current and highest “threshold forward current becomes larger due to the voltage.” Since the forward voltage of the LED same input voltage variation. In addition, when emitters varies slightly over temperature, then the the input voltage is less than (yVO) the forward forward current through each LED string will current through each LED emitter is change slightly over temperature. Circuits with approximately equal to zero. As the value of y is longer string lengths will have a slightly larger increased, this “threshold” voltage increases. forward current change over temperature. Figure 3.2 shows the change in forward current Figure 3.1 Circuit Configuration of LED Arrays Used in LED Signal Lamps. 9 Figure 3.2 Forward Current Through String of HPWA-xHOO LED Emitters versus Applied Voltage. Figure 3.3 Total Supply Current versus Supply Voltage for Sixty HPWA-xHOO Emitter LED Signal Lamp Array Driven in Series Strings with Two, Three, Four, Five, or Six Emitters per String. The number of LED emitters per string also Variations in the forward voltage characteristics affects the total supply current. For a fixed of the individual LED emitters can lead to forward number of LED emitters longer string lengths current variations within the LED array. These result in fewer total strings and thus a lower forward current variations directly affect the total supply current. Figure 3.3 shows the luminous flux output of each LED emitter and can total supply current for the series-string cause noticeable luminous intensity variations configurations in Figure 3.2. Figure 3.3 assumes within the LED array. These random variations in a total of 60 LED emitters, thus 2-LED strings forward voltage characteristics affect the three would require 30 strings, 3-LED strings would circuits shown in Figure 3.1 differently. require 20 strings, etc. Note that the total supply current is much higher for series strings The “series-string” circuit shown in Figure 3.1a with 2 and 3 emitters. is least affected by random forward voltage variations between the LED emitters because the As shown by Figures 3.2 and 3.3, the choice of forward voltages of all LED emitters in a given the number of emitters per string is a tradeoff string are averaged together. In many cases, one between the regulation of forward current due emitter with a high forward voltage can cancel to input voltage variations and the total supply out another emitter in the same series-string with current for the LED array. Small string lengths a low forward voltage. In addition, the voltage give excellent forward current regulation, but drop across the current limiting resistor, R, is require a higher supply current. Long string much higher than the combined voltage drops lengths provide poor forward current regulation across the series resistors, yRS. Thus, small but require less supply current. For these variations in RS only cause small variations in the reasons, most 12 V resistive limited designs forward current through the series string. use three or four LED emitters per series Because SuperFlux and SnapLED 70 emitters string (y = 3 or 4). are categorized for forward voltage at 70 mA (150 mA for the SnapLED 150), the smallest 10 Figure 3.4 Worst-Case and Typical Variations in Forward Current Between Two Strings of HPWT-xHOO LED Emitters Driven with Individual Current-Limiting Resistors per String (Figure 1a Circuit, with y=4). Figure 3.5 Worst-Case and Typical Variations in Forward Current Between 16 HPWT-xHOO LED Emitters Driven in a Cross-Connected Paralleled String Configuration (Figure 1c Circuit with x = y = 4, z = 1). forward current variations between adjacent SuperFlux and SnapLED 70 emitters and 40 series-strings of LED emitters occur at a drive mA for SnapLED 150 emitters (see the section current of 70 mA (150 mA for the SnapLED “Electrical, Optical, and Thermal 150). However, the forward current matching Characteristics of LED Emitters). between adjacent strings is quite good even at forward currents as low as 10 mA (20 mA for The “paralleled-string” circuit shown in Figure the SnapLED 150). Figure 3.4 shows the worst- 3.1b and the “cross-connected series string” case forward current variation between two LED circuit shown in Figure 3.1c do not regulate the strings constructed using four LED emitters per forward current as well as the “series-string” string. The worst-case calculations assume that circuit. When two or more LED emitters are all HPWT-xH00 emitters are from the same connected in parallel, the forward current forward voltage category and one string uses through each emitter will be somewhat higher or four “minimum” forward voltage emitters and lower than the average forward current through the other string uses four “maximum” forward them so as to force the forward voltage across voltage emitters. The typical calculations them to be the same. Again, because SuperFlux assume that all HPWT-xH00 emitters are from LED and SnapLED 70 emitters are categorized the same forward voltage category and one for forward voltage at 70 mA (150 mA for the string consists of two “minimum” forward SnapLED 150), the smallest forward current voltage emitters and two “typical” forward variations between adjacent LED strings (Figure voltage emitters and the other string consists of 3.1b circuit) or between LED emitters in the two “maximum “forward voltage emitters and same “rung” (Figure 3.1c circuit) occur at a drive two “typical” forward voltage emitters. Due to current of 70 mA (150 mA for SnapLED 150). potential variations in luminous flux output at The variations in forward currents become much low currents, Lumileds Lighting recommends worse at lower drive currents. These variations in a minimum forward current of 20 mA for forward currents can cause unacceptable 11 luminous intensity variations even using LED SuperFlux LED emitters from only one forward emitters from the same forward voltage and voltage category within the same LED array. luminous flux category. Figure 3.5 shows the worst-case forward current variations within the LED array when the array is constructed using LED emitters from the same forward voltage category. The worst-case calculations assume that the LED array consists of 16 HPWT-xH00 emitters constructed using four “minimum” LED emitters, four “maximum” LED emitters, and eight “typical” LED emitters. Then each paralleled grouping consists of one “max,” one “min,” and two “typical” LED emitters. The typical calculations assume that the LED array Figure 3.6 Typical EMC Transient Protection Circuits for LED Signal Lamps. consists of 16 HPWT-xH00 emitters constructed using two “minimum” LED emitters, LED emitters are susceptible to permanent two “maximum” LED emitters, and twelve damage due to high voltage automotive EMC “typical” LED emitters. Then two of the transients. The addition of a high-voltage paralleled groupings consist of one “max,” one silicon diode in series with the LED array can “min,” and two “typical” LED emitters, and the effectively protect the array from high-voltage other two paralleled groupings consist of four negative transients. The LED array can be “typical” LED emitters. Lumileds Lighting protected from positive “Load Dump” recommends a minimum forward current of transients with the addition of a transient 35 mA (70 mA for the SnapLED 150), for the suppressor connected in parallel with the LED “paralleled-string” circuit in Figure 3.1b or the array. Figure 3.6 shows the addition of EMC “cross-connected parallel-string” circuit protection circuitry to the LED array. EMC shown in Figure 3.1c. At drive currents less transient protection is covered in more detail in than 35 mA (70 mA for SnapLED 150), the the following section “EMC Transient Protection.” “worst-case” forward current variations between adjacent LED emitters can exceed Some applications require the LED array to 2:1. Because of the averaging effects of several operate at two levels of luminous intensity (i.e. series-connected LED emitters, the circuit in a rear Stop/ Tail signal). Generally, it is desirable Figure 3.1b has somewhat lower typical forward that the LED emitters should appear matched at current variations than the circuit shown in both drive conditions. SuperFlux and SnapLED Figure 3.1c. Note that the forward current 70 emitters are categorized for luminous flux at matching can be improved with the addition of 70 mA (150 mA for the SnapLED 150). As shown a small resistor (ROPT > RS) in series with each in the section “Electrical, Optical, and Thermal string for the circuit shown in Figure 3.1b or Characteristics of LED Emitters,” the light output “rung” for the circuit shown in Figure 3.1c. For matching for random combinations of LED these circuits, it is important to use emitters gets progressively worse at lower 12 forward currents. For SuperFlux and SnapLED reduced luminous intensity. The Stop signal emitters, the light output varies by a factor of might operate the LED array at a high DC 2:1 at a forward current of 20 mA (40 mA for forward current. Then for the Tail signal, the the SnapLED 150). Thus, even if all of the LED array would be operated at the same peak emitters are driven at the same forward current, forward current with a low duty cycle (ratio of there would likely be unacceptable light output “on” time to “on” plus “off” time). This matching if the Tail signal is driven at a low DC approach provides light output matching under forward current. both levels of luminous intensity. A recommended PWM circuit is shown in the For best matching, it is recommended that a section “Special Considerations for Dual pulse width modulation (PWM) circuit be Luminous Intensity Operation.” designed to operate the signal lamp at Theory Overview of Electrical, Optical, and Thermal Characteristics of an LED Emitter In order to properly design an LED signal light, the incremental forward voltage. Although this it is important to have a basic understanding graph has been used traditionally to describe the of the electrical, optical, and thermal forward characteristics of a diode, in reality LED characteristics of an LED emitter. emitters are best thought of as current controlled devices, not voltage controlled devices. For an The typical forward current (IF) versus forward LED emitter, the optical properties are best voltage (VF) characteristic for an AlInGaP LED described as a function of current, not a function emitter under positive (forward) bias is shown in of voltage. In addition, operation of the LED Figure 3.7. On a linear scale of forward current emitter at a constant current gives the best versus forward voltage, negligible current flows control of light output. In contrast, operation of until a threshold voltage, also known as the the LED emitter at a constant voltage allows a turn-on voltage (VO), is exceeded. Above this larger variation in forward current and light output voltage, the current increases proportionally to from device to device. Figure 3.7 Typical Forward Current versus Forward Voltage for HPWA-xHOO LET Emitter (Linear Scale). Figure 3.8 Typical Forward Current versus Forward Voltage for HPWA-xHOO LED Emitter (Semi-Long Scale.) 13 The low-current forward characteristics of the HPWT-xx00 emitters. Operation of the LED same AlInGaP LED emitter are shown in Figure emitter in the reverse current region is not 3.8. This graph shows the forward voltage recommended. Reverse currents in excess of 50 versus the log of forward current. Note that a µA can cause permanent damage to the LED small current flows through the emitter even at junction, as discussed later in the section titled low forward voltages below the turn-on voltage “Electrical Transients.” The reverse breakdown shown in Figure 3.7. Due to the high optical voltage is essentially constant over the –40ºC to efficiency of AlInGaP material, a perceptible 100ºC temperature range. amount of light is generated from the LED emitters at forward currents as low as 10 µA. The change in luminous flux (ΦV) as a function of Thus the inadvertent operation at low forward forward current (IF) of an AlInGaP LED emitter is currents can cause “ghosting” within an “off” shown in Figure 3.10. Note that the change in LED signal light. luminous flux is roughly proportional to the change in forward current. At forward currents The forward voltage of an AlInGaP LED emitter over 20 mA, the luminous flux increases at a changes by about –2 mV per °C over lower rate due to internal heating within the LED temperature. Thus, the forward voltage at a emitter. The change in luminous flux due to a given current is slightly lower at elevated change in forward current ( ∆ΦV / ∆IF) varies temperatures and slightly higher at colder somewhat from unit to unit. Figure 3.11 shows temperatures. the expected range in light output for HPWTxH00 emitters that were matched at 70 mA. The reverse characteristics of an AlInGaP LED Note that the light output varies by a factor of 2:1 emitter are shown in Figure 3.9. Note that a at a 20 mA forward current. Since the SnapLED negligible amount of reverse current (< 1 µA) 150 emitter is matched at 150 mA, then the light flows through the LED until the reverse output can be expected to vary by a factor of 2:1 breakdown voltage is reached. The reverse at a forward current of 40 mA. current increases quickly at voltages higher than the reverse breakdown voltage (defined as the The luminous flux of an AlInGaP LED emitter voltage across the LED at which the reverse varies inversely with temperature as shown in current reaches 100 µA). The reverse Figure 3.12. breakdown voltage for AlInGaP LED emitters is typically in the range of 20 V. However, it can be as low as 10 V for the HPWA-xx00 and 14 Figure 3.9 Typical Reverse Current versus Reverse Voltage for HPWA-xHOO LED Emitter (Linear Scale). Figure 3.10 Typical Luminous Flux versus Forward Current for HPWA-xHOO LED Emitter (Logarithmic Scale). At 85°C, the light output will be approximately and dominant wavelength over temperature are 50% of the light output at 25°C. At –40°C, the fully reversible when the ambient temperature light output will be approximately twice the light returns to 25ºC. output at 25°C. This change is fully reversible. As might be expected, the junction temperature The peak and dominant wavelength of an varies directly as a function of ambient AlInGaP LED emitter changes by about 0.1 nm temperature. In addition, the junction per ºC. Thus, the color of the LED shifts slightly temperature of the LED emitter is hotter than the toward the red at elevated temperatures. surrounding ambient temperature due to the internal power dissipation (IFVF) within the LED By now, it should be apparent that a number of emitter. Figure 3.13 shows the internal the electrical and optical characteristics of an temperature rise, TJ – TA, for an LED signal lamp LED emitter vary as a function of ambient and over a range of thermal resistance, as a function junction temperature. All of these changes in of the forward current through the LED emitter. forward voltage, luminous intensity, and peak Figure 3.11 Expected Variations in Luminous Flux versus Forward Current for HPWT-xHOO LED Emitters (Logarithmic Scale). Figure 3.12 Typical Luminous Flux versus Temperature for HPWT-xHOO LED Emitter Driven at 60 mA (Linear Scale). 15 Figure 3.13 Expected Internal Temperature Rise (TJ -TA) for HPWA-xHOO LED Emitter versus Forward Current (Linear Scale). Figure 3.14 Straight-Line Forward Voltage Model for LED Emitter (Linear Scale). Besides affecting a number of the electrical and For these reasons, it is important to understand optical parameters, the junction temperature the thermal properties of the individual LED and maximum operating current also affect the emitter as well as the thermal properties of all the reliability of the LED emitter. For AlInGaP elements of the LED signal lamp (printed circuit SuperFlux LED emitters, operation of the emitter board, case, etc). AB20-4, titled “Thermal near the maximum operating temperature limit Management Considerations for SuperFlux and maximum operating current limit can result LEDs” discusses the proper thermal design of an in a small amount of light degradation over time. LED signal lamp. In this application note, thermal In addition, due to the different rates of thermal modeling will be covered only in sufficient detail expansion of the epoxy material used for the so as to allow proper circuit modeling to maintain emitter package and the metal pins and gold the LED junction temperature within the bond-wire within the LED emitter, there are recommended operating temperature limits. This upper and lower limits to the operating and application note will also use thermal modeling to storage temperature ranges for each LED estimate the change in electrical and optical package. Exceeding these limits, especially for parameters of the LED emitters over temperature hundred’s of temperature cycles, can result in and how these effect the operation of the LED premature catastrophic LED emitter failures. signal light. For additional detail on thermal These effects are covered in detail in AB20-6, modeling please refer to AB20-4. titled “Reliability Considerations for SuperFlux LEDs.” LED Emitter Modeling The purpose of modeling is to represent the interactions with other electronic components to electrical, optical, and thermal characteristics of be expressed mathematically. The process of a component with equations that allow their modeling also requires that mathematical 16 expressions be selected that best approximate the actual measured data. Thus, the equations for VO and RS can be written as: For operation over a restricted range of current, say from 30 mA to 70 mA, the forward current can be modeled with a linear model. As shown in Figure 3.14, the linear model draws a straight line between two points (IF1, VF1) and (IF2, VF2) at two forward currents, IF1 < IF2, to linearize the For most applications this linear model can be electrical forward characteristics between these used to model the forward characteristics of an forward currents. The linear model is shown LED emitter. For best accuracy, the use of the graphically in Figure 3.15 for the forward voltage linear model should be restricted to a range of versus forward current curve shown in Figure forward currents, IF2 / IF1, less than 4:1. For 3.7. The equation for the forward current operation at a lower range of currents, different becomes: points (IF3, VF3) and (IF4, VF4) can be selected to bracket the approximate range of operating current. However, it’s always important to recognize that the linear model only works for Where: VO = turn-on voltage, the y-intercept of the straight line (IF = 0) RS = series resistance, the slope of the straight line a specified range of forward currents (I F1 ≤?I F ≤?IF2) as the accuracy of the linear model degrades quickly outside of this range. It should go without saying that the linear model cannot be used at all for values of VF < VO. Figure 3.15 Linear Forward Voltage Model for HPWA-xHOO LED Emitter Shown in Figure 7. Figure 3.16 Worst-Case Linear Forward Voltage Models for LED Emitters. 17 Using the nominal forward voltage at the two estimated with two permutations of the linear test currents in Equations #3.4 and #3.5 would model as shown in Figure 3.16: generate the typical linear forward voltage model as shown below. The nominal linear VF min = VO LL + RS LL IF ≅ VO min + RS min IF forward voltage model (VO nom and RS nom) is based on the average forward voltages at two VF max = VO HH + RS HH IF ≅ VO max + RS max IF test currents, IF1 and IF2, for a large number of SuperFlux LED emitters from the same forward In order to model the variation in electrical voltage category. forward characteristics over temperature, another term can be added to the linear model (IF1, VF1 nom), (IF2, VF2 nom) ⇒ ( VO nom, RS nom) as shown in Equation #3.6. Note that the data shown in AB20-3B represents the forward Then: voltage at 25°C with the units measured cold (i.e. TJ = 25°C). Thus, the thermally stabilized forward VF nom = VO nom + RS nom IF voltage at 25°C will be slightly lower than the values shown in AB20-3B. The values of VF(IF1) and VF(IF2) vary for different SuperFlux LED emitters from the same forward voltage category. Statistical forward voltage data for SuperFlux LED emitters is given in AB20-3B. Then, the values of VO and RS can be calculated using the desired limits (i.e. VF max, VF min , or VF average ± n σ). Worst-case circuit analysis is concerned primarily with the highest and lowest forward voltages over the range of IF1 ≤ IF ≤ IF2. In most cases, the worst-case range of Where: = junction temperature, °C TJ ∆ VF /∆ = change in VF due to temperature, ≅ –2mV°C VO, RS = measured at a junction temperature of 25°C forward current and forward voltage can be Figure 3.17 Linear Model (m = 1) for Luminous Flux versus Forward Current for HPWA-xHOO LED Emitter Shown in Figure 3.10. Figure 3.18 Exponential Model (k = -0.0110) and Exponential Curve Fit (k = -0.0096) for Luminous Flux versus Temperature for HPWA-xHOO LED Emitter Shown in Figure 3.12. 18 ΦV (TJ ) = luminous flux at forward current IF at In general, the luminous flux output of LED emitters varies as a function of the forward junction temperature, TJ ΦV (TJ = 25°C) = luminous flux at 25°C, without current. Ignoring the effect of heating, the relationship between luminous flux and forward heating current can be modeled with the following k = thermal coefficient, k ≅ –0.01 equation: (3.7) ΦV(IF,TJ = 25°C) ≅ Φ V(IF TEST,TJ = 25°C)[IF/IF TEST]m Over the automotive operating temperature range of –40°C to 85°C, this model matches the actual data within ± 10%. Figure 3.18 shows Where: how the modeled data for ΦV as a function of ΦV(IF,TJ = 25°C) = Luminous flux at forward temperature compares to the actual data shown current, IF, ignoring heating in Figure 3.12. Note that the value selected for k Φ V(IF TEST,TJ = 25°C) = Luminous flux at test was chosen to improve the curve fit at elevated current, IF TEST, ignoring heating temperatures than at temperatures below 25°C. = forward current IF Typical values of k for AlInGaP and TS AlGaAs IF TEST = forward current at data sheet test SuperFlux LED emitters are shown in Table 3.1. conditions = linearity factor, 1 ≤ m ≤ 2 m Thermal resistance is a measurement of the temperature rise within the LED signal lamp At forward currents less than 10 mA, m ≈ 1.3 for caused by internal power dissipation as well as AlInGaP LED emitters. At forward currents over other sources of heat in close proximity to the 30 mA, the linearity factor, m ≈ 1.0 for AlInGaP LED (i.e. bulbs, resistors, drive transistors, etc). LED emitters. Figure 3.17 shows how the For a detailed discussion of thermal resistance, modeled data for ΦV versus IF compares to the please refer to AB20-4. The units of thermal actual data shown in Figure 3.10. resistance are ºC/W. For the same power dissipation, the LED signal lamp with a higher For operation at forward currents over 30 mA, thermal resistance would have a larger internal Equation #3.7 can be simplified into a simple temperature rise. The basic thermal modeling linear equation: equation is shown below: ΦV(IF, TJ = 25°C) ≅ ΦV (IF TEST , TJ = 25°C)[ IF / IF TEST TJ ≅ TA + RθJAPD ] (3.9) Where: The luminous flux varies exponentially with TJ = internal junction temperature within the LED temperature. The simplest model is shown emitter, °C below: TA = ambient temperature surrounding the LED (3.8) signal lamp, °C ΦV(TJ ) ≅ Φ V(TJ = 25°C) exp [k(TJ -25°C)] RθJA = thermal resistance, junction to ambient, Where: °C/W 19 PD = internal power dissipation within the LED emitter (IF VF),W LED signal lamps typically use several LED Where: emitters. Each LED has a slightly different Rθ JP = thermal resistance, junction to pin (LED emitter package), °C/W thermal resistance, based on the proximity of other heat sources (e.g. adjacent LED emitters, circuit and case), °C/W resistors, power transistors, bulbs, etc) and printed circuit board layout. Generally, the = thermal resistance pin to air (printed Rθ PA TP = LED cathode pin temperature on underside of printed circuit board, °C thermal resistance value used for thermal modeling is the highest thermal resistance, RθJA, of any of the LED emitters within the LED These equations are especially useful since the lamp assembly. Experience has shown that the thermal resistance junction to pin, RθJP, is LED emitter with the highest thermal resistance specified on the product data sheet and the LED is usually either one of the emitters in the center pin temperature can be measured directly by of the LED lamp assembly for an x-y attaching a thermocouple on the cathode lead of arrangement of emitters, the middle emitter in a the LED emitter on the underside of the printed single row of emitters, or one of the emitters circuit board. Thus the junction temperature can adjacent to other heat sources (e.g. resistors, be estimated based on a measurement of the pin power transistors, bulbs, etc). temperature of the LED emitter (please refer to AB20-4). The thermal modeling equation can be further broken down by separately considering the Usually thermal resistance measurements are thermal resistance of each of the elements of done at thermal equilibrium. For an LED signal the LED signal lamp as shown below: lamp, thermal equilibrium usually occurs after 30 minutes of continuous operation. In some TJ ≅ TA + (Rθ JP + RθPA) PD TP ≅ TA + (Rθ PA) PD cases, it is important to calculate the junction (3.10) temperature under a transient condition (e.g. 2 minutes at 24 V). Table 3.1 Values of k for AlInGaP SuperFlux LED Emitters Coefficient of Dominant Family LED Material Wavelength ΦV, ( T ), k HPWA-xHOO HPWA-xLOO HPWT-xDOO HPWT-xHOO HPWT-xLOO AS AlInGaP AS AlInGaP TS AlInGaP TS AlInGaP TS AlInGaP 618 592 630 620 594 20 nm nm nm nm nm -0.0106 -0.0175 -0.0106 -0.0106 -0.0175 This can be done by further subdividing the die will be the first element of the model to heat- thermal modeling equation as shown below: up, followed by the LED emitter package, and then the rest of the LED signal light. TJ ≅ TA + (RθJ LF + RθLF P + RθP A) PD (3.11) This section discussed the key concepts of Where: modeling the electrical, optical, and thermal Rθ J LF = thermal resistance, junction to lead performance of LED signal lights. Equations #3.3 frame (LED die), °C/W and #3.7 can be used to model the operation of Rθ LF P = thermal resistance, lead frame to pin an LED emitter at room temperature, ignoring (LED package excluding die), °C/W the effects of self-heating. Equations #3.6, #3.7, #3.8, and #3.9 can be used together to model Please note that each thermal resistance (Rθ J LF, the effects of self-heating of an LED emitter at Rθ LF P, and Rθ P A) has a different heating time room temperature as well as to model the constant. The time constant associated with operation of an LED emitter over temperature. heating of the LED die is in the order of one Equations #3.10 and #3.11 show the various millisecond. The time constant associated with components of the overall thermal resistance, the heating of the LED emitter package is in the RθJA , which can be useful in the thermal order of one minute. The time constant modeling of an LED signal lamp assembly and associated with the heating of the complete LED the thermal modeling of transient power signal lamp is in the order of 10 to 30 minutes. conditions. Thus, for a transient heating condition, the LED Applications Resistive Current Limiting As discussed previously in the section “Key if the LED signal light will be subjected to Concepts for Electrical Design of LED Signal automotive EMC transients. Lamps,” the choice of the number of LED For a resistive current-limited circuit the electrical emitters per series-string has a large effect on design process consists mainly of picking the the forward current regulation and the overall proper value(s) for the current limiting resistor(s). electrical power consumption of the LED signal The key principles of worst-case design are lamp. Most 12V designs commonly use either shown in Figure 3.19. The figure shows the three or four emitters per series-string, which is forward current through one LED string of four a good balance of current regulation and emitters as a function of input voltage. The electrical power consumption. Then, the choice equation for this graph (Equation #3.12) is equal of circuit topology (Figure 3.1 circuits) and the to Equation #3.2 solved for IF: design current determine the variation in forward currents for the LED emitters in the Equation #3.2, from “Key Concepts for Electrical array. Finally, protection circuitry can be added, Design of LED Signal Lamps.” 21 #3.12, for the same value of external current limiting resistor, R. The maximum forward current through the LED Where: string is determined by the effects of thermal R = external current limiting resistor resistance of the LED signal lamp, RθJA, and VIN = input voltage applied to the circuit maximum ambient temperature, TA MAX, as shown VD = voltage drop across optional reverse in Figure 4 of the SuperFlux LED Data Sheet. transient EMC protection diode IF = design forward current through the circuit So, the worst-case design procedure is to VO = turn-on voltage of the linear forward determine the maximum forward current, IF MAX, voltage model RS = series resistance of the linear forward voltage model based on parameters RθJA and TA MAX from Figure 4 of the SuperFlux LED Data Sheet. Then the value of the external resistor, R, is determined y = number of series connected LED emitters with Equation #3.2, using values of IF MAX, VIN MAX, x = number of paralleled strings used with and LED linear model parameters (VO LL, RS LL). external current limiting resistor The nominal design current, IF DES, which occurs at the design voltage, VIN DES, can be calculated with Equation #3.12 using values of VIN DES, R, and LED emitters with a typical forward voltage [i.e. with linear model parameters (VO NOM, RS NOM). In practice, the LED signal lamp needs to be designed to accommodate a range of forward voltage categories and luminous flux categories. In most cases, the overall goal is to design an Figure 3.19 Worst-Case Variations in Forward Current Through Several LED Strings as a Function of the Applied Voltage. LED signal lamp with a fixed light output. For LED emitters from the same luminous flux category, this can be accomplished by calculating different values of external resistor, R, using Equation #3.2, with the same design Note, for the example shown in this section: current and voltage, IF DES and VIN DES, and using x = 1 and y = 4. the nominal forward voltage models (VO NOM, RS NOM ) for each forward voltage category. Figure 3.19 shows how the variation in forward voltage of the LED emitters affects the forward For LED emitters with higher luminous flux current through the string. These curves are categories, the design current would be reduced generated by substituting the worst-case linear to keep the light output constant. First, the forward voltage model parameters [(VO NOM, RS minimum luminous flux would be calculated for NOM ), (VO LL, RS LL), or (VO HH, RS HH)] into Equation the lowest expected luminous flux category at 22 the nominal design current. Then Figure 3 from design. Lumileds Lighting recommends that the the SuperFlux LED Data Sheet could be used to designer use realistic assumptions for these calculate the other design currents for higher parameters. It is very easy to overly guard-band luminous flux categories. Then values of these assumptions, which results in an excessive external resistor, R, can be calculated with estimate of the number of LED emitters needed. Equation #3.2 at the reduced design current using the appropriate nominal forward voltage The luminous efficiency of AlInGaP technology models for each forward voltage category. has significantly improved over the past few years. Please consult with your Lumileds Lighting After all of these “ideal” values of R are or Agilent Technologies Field Sales Engineer for computed, the designer would need to choose the recommended minimum luminous flux the closest standard resistor values. In many categories of SuperFlux LED emitters for given cases, the designer can use the same resistor future production dates. value for multiple LED emitter categories provided that the maximum forward current is Many LED signal lamp requirements also include not exceeded under worst-case conditions. operation at higher voltages for a limited duration (i.e. 24 volts for two minutes). In analyzing the Experience has shown that the worst-case performance of an LED signal lamp under these design occurs with the lowest expected conditions, it is important to analyze the transient luminous flux category and the highest forward heating effects. Under these conditions the LED voltage category. Thus, the lowest value of emitters don’t reach thermal equilibrium so the external resistor, R, and the highest design junction temperatures are lower than indicated current, IF DES, would be determined for this by Equation #3.9. Equation #3.11 can be used particular category combination. For LED to estimate the maximum junction temperature emitters with the lowest expected luminous flux using the appropriate time constants for RθJ LF, category and lower forward voltage categories, RθLF P, and RθP A. In addition, many test Equation #3.2 will generate higher values of R. specifications allow a different operating For LED emitters with higher luminous flux temperature for these tests. categories, the values of R will be even larger since the design current is reduced. The assumptions used for maximum ambient temperature, RθJA, maximum steady-state input voltage, and the worst-case SuperFlux LED categories (minimum expected luminous flux and maximum expected forward voltage) have a large effect on the nominal design current and thus the luminous flux output. Thus these parameters have a large effect on the number of LED emitters needed for a given signal lamp 23 CHMSL Design Example This worst-case design procedure will be For HPWT-MH00, forward voltage category 6: illustrated with an example. Let suppose that an LED array is being designed using 4-LED VO LL = 1.85 V, RS LL = 14.4 ohm strings of HPWT-MH00 from luminous flux categories F through L and forward voltage Then: categories 2 through 6. For this example RθJA = 350° C/W, TA MAX = 70° C, VIN MAX = 15.0 V, and VIN DES = 12.8 V. In addition, a silicon diode with a forward voltage of 0.8 V is connected in series 3. Determine nominal design current, IF DES: with the circuit in order to provide protection against negative EMC transients. Then, the The nominal forward current through the LED design steps are shown below: emitters at the design voltage is determined with Equation #3.12. The equation should use the 1. Determine IF MAX: value of R from the Step 2 at the design voltage and the nominal forward voltage of the SuperFlux The maximum allowable DC forward current LED emitters using the same forward voltage through the SuperFlux LED emitters is category used in Step 2. determined from the maximum ambient temperature, TA MAX, estimated overall thermal For HPWT-MH00, forward voltage category 6: resistance, RθJA, of the LED signal lamp, and Figure 4 of the Data Sheet. VO NOM = 2.03 V, RS NOM = 12.4 ohm For TA MAX = 70° C, and RθJA = 350° C/W: Then: IF MAX = 55 mA, from HPWT-MH00 DataSheet, Figure 4 2. Determine minimum value of current limiting resistor, R: The minimum value of R is determined with Equation #3.2 at the maximum input voltage and maximum forward current from Step 1 for the maximum forward voltage category Super Flux LED to be used in the assembly. 24 4. Determine value of external current limiting The linear forward voltage models for the other resistor, R, for each forward voltage category: HPWT-MH00 forward voltage categories are shown below: For SuperFlux LED emitters from lower forward voltage categories, the value of the external resistor, R, will need to be increased (using Equation #3.2) in order to maintain the same nominal forward current. HPWTMHOO VO RS NOM NOM Voltage Category 2 Voltage Category 3 Voltage Category 4 Voltage Category 5 Voltage Category 6 1.87 V 8.2 ohm 1.91 V 9.2 ohm 1.94 V 10.5 ohm 1.96 V 11.6 ohm 2.03 V 12.4 ohm = = For VIN DES = 12.8 V, VD = 0.8 V, IF DES = 33.6 mA, x = 1, y = 4: HPWTMHOO Voltage Category 2 Voltage Category 3 Voltage Category 4 Voltage Category 5 Voltage Category 6 R= 102 ohm 93 ohm 84 ohm 77 ohm 66 ohm 5. Determine minimum thermally stabilized Design currents for SuperFlux LED emitters at luminous flux: higher luminous flux categories can be determined using Figure 3 from the SuperFlux The thermally stabilized luminous flux of the LED Data Sheet. This can be done by computing SuperFlux LED emitters from the lowest a new “relative luminous flux” equal to the expected luminous flux category can be desired luminous flux divided by the minimum determined using Figure 3 from the SuperFlux luminous flux category bin limit and then reading LED Data Sheet. a new value of forward current from Figure 3. For HPWT-MH00, luminous flux category F (3.0 The same minimum luminous flux obtained from lm minimum) and IF DES = 33.6 mA: a HPWT-MH00 luminous flux category F (3.0 lm) driven at 33.6 mA can be obtained from a Then: ΦV MIN = (3.0)(0.54) = 1.62 lm, (i.e. “relative HPWT-MH00 from the following luminous flux luminous flux” = 0.54) from HPWT-MH00 Data categories when driven at the specified forward Sheet, Figure 3 current: 6. Determine design currents for brighter SuperFlux LED emitters: 25 From Figure 3, HPWT-MH00 Data Sheet: Luminous Flux Category Minimum Luminous Flux Desired “relative luminous flux” Design Current, from Figure 3 F G H J 3.0 lm 0.54 33.6 mA 3.5 lm 0.46 29 mA 4.0 lm 0.41 25 mA 5.0 lm 0.32 19 mA L 6.0 lm 0.27 15 mA Note: Due to the variations in forward voltage at categories, or to accept possible visible light low currents, there is a practical limit to the use output mismatch within the array. of higher and higher luminous flux categories at correspondingly lower dc drive currents. For the 7. Determine values of R for expected luminous series string circuit shown in Figure 3.1a, at flux and forward voltage categories: drive currents less than 20 mA, the “worstcase” ratio of forward currents between two Values of R can be determined (using Equation strings of LED emitters can vary by over 2:1. As #3.2) for each SuperFlux LED emitter forward shown by this example, in order to achieve the voltage category and luminous flux category at same light output for all CHMSL arrays, designs the appropriate design current as calculated in using HPWT-MH00 emitters from luminous flux Step 6. categories J and L would require drive currents less than 20 mA. Thus, the designer needs to Note: Since the design current of the HPWT- establish whether it is better to limit the forward MH00 LED array is less than 32 mA for designs current to 20 mA and allow the light output to using luminous flux bins G through K, a “low increase for these brighter luminous flux current” linear forward voltage model (8 mA ≤ IF ≤?32 mA) was used. This model is shown below: HPWTMHOO VO RS NOM NOM = = Voltage Category 2 Voltage Category 3 Voltage Category 4 Voltage Category 5 Voltage Category 6 1.80 V 10.4 ohm 1.83 V 11.7 ohm 1.85 V 13.2 ohm 1.88 V 14.3 ohm 1.94 V 15.1 ohm For VIN DES = 12.8 V, VD = 0.8 V, IF DES from Step 6, VO NOM, RS NOM, x = 1, y = 4: HPWTMHOO Design Current Flux, F Flux, G Flux, H Flux, J Flux, K 33.6 mA 29 mA 25 mA 19 mA[1] 15 mA[1] Voltage Category 2 102 124 151 212 279 ohm ohm ohm ohm ohm Voltage Category 3 Voltage Category 4 Voltage Category 5 Voltage Category 6 93 ohm 115 ohm 140 ohm 199 ohm 265 ohm 84 ohm 106 ohm 131 ohm 189 ohm 253 ohm 77 ohm 97 ohm 122 ohm 179 ohm 242 ohm 66 ohm 86 ohm 109 ohm 162 ohm 222 ohm Note 1: Operation at dc drive currents below 20 mA can cause noticeable light output differences within the LED array. 26 8. Select “standard” resistor values: Standard 5% Tolerance Resistors HPWTMHOO Design Current Flux, F Flux, G Flux, H Flux, J Flux, K 33.6 mA 29 mA 25 mA 19 mA[1] 15 mA[1] Voltage Category 2 100 120 150 220 270 Voltage Category 3 Voltage Category 4 Voltage Category 5 Voltage Category 6 91 ohm 110 ohm 150 ohm 200 ohm 270 ohm 82 ohm 110 ohm 130 ohm 180 ohm 240 ohm 75 ohm 100 ohm 120 ohm 180 ohm 240 ohm 68 ohm 91 ohm 110 ohm 160 ohm 220 ohm ohm ohm ohm ohm ohm Note 1: Operation at dc drive currents below 20 mA can cause noticeable light output differences within the LED array. 9. Group “standard” adjacent cells in resistor matrix in Step 8 as desired: Standard 5% Tolerance Resistors HPWTMHOO Design Current Flux, F Flux, G Flux, H Flux, J Flux, K 33.6 mA 29 mA 25 mA 19 mA[1] 15 mA[1] Voltage Category 2 100 120 150 220 270 Voltage Category 3 ohm ohm ohm ohm ohm Voltage Category 4 Voltage Category 5 Voltage Category 6 82 ohm 110 ohm 130 ohm 180 ohm 240 ohm Note 1: Operation at dc drive currents below 20 mA can cause noticeable light output differences within the LED array. 10. Perform “worst-case” analysis to ensure varies slightly over temperature as shown in that maximum forward current is not exceeded Equation #3.6. This thermal effect can be over temperature. included in Equation #3.12 as shown below: Calculate maximum forward current (using Equation #3.12) at “worst-case” conditions— i.e. maximum input voltage, minimum resistor values, and minimum forward voltages for each SuperFlux LED emitter forward voltage category. The forward voltage of LED emitters EMC Transient Protection Circuits designed for the automotive electrical tolerate a number of different types of electrical environment must be able to operate over a transients. These worst-case voltage ranges and wide range of input voltages and be able to electrical transients have been characterized and 27 are defined in different automotive specifications are turned off within the vehicle, switching such as: transients of electronic circuitry, alternator field decay, or a fully discharged battery being DIN 40839 Part 1 disconnected while the alternator is operating at “Electromagnetic Compatibility (EMC) in Motor rated load. These transients consist of both Vehicles; interferences conducted along supply positive and negative pulses with different lines in 12 V onboard system” amplitudes and decay times. DIN 40839 Part 2 Limited reliability testing has been done with “Electromagnetic Compatibility (EMC) in Motor AlInGaP LED emitters connected in typical LED Vehicles; interferences conducted along supply signal lamp configurations. lines in 24 V onboard system” High-voltage negative transients in excess of the ISO 7647-1 reverse breakdown voltage can permanently “Road Vehicles—Electrical Disturbance Caused damage AlInGaP LED emitters. Sufficient energy by Conduction and Coupling; passenger cars can be dissipated within the AlInGaP LED die to and light commercial vehicles with nominal 12 V cause localized damage to the p-n diode supply voltage” structure. This damage can result in reduced breakdown voltages, and degraded low-current ISO 7647-2 performance. Under extreme conditions, high “Road Vehicles—Electrical Disturbance Caused voltage negative transients can even destroy the by Conduction and Coupling; commercial p-n junction, resulting in a short between anode vehicles with nominal 24 V supply voltage” and cathode. SAE J1113 Adding a high-voltage silicon diode in series with “Electromagnetic Susceptibility Measurement the LED signal light array, such as previously Procedures for Vehicle Components (except shown in Figure 3.6, can prevent potential Aircraft)” damage to high voltage negative transients. The silicon diode should have a higher reverse SAE J1211 breakdown voltage than the amplitude of the “Recommended Environmental Practices for worst-case negative transient, which can be as Electronic Equipment Design” large as –300 V (–600 V for heavy trucks). Table 3.2 shows several recommended silicon diodes SAE J1812 for different LED signal lamp applications. “Function Performance Status for EMC Susceptibility Testing of Automotive Electronic In addition, high-voltage positive transients can and Electrical Devices” permanently damage AlInGaP LED emitters. Sufficient energy can be dissipated within the These specifications define several electrical AlInGaP LED die to cause permanent damage to transient pulses that occur when inductive loads the p-n diode structure and cause epoxy 28 delamination between the LED and surrounding limiting properties of the LED drive circuit, epoxy. Under extreme conditions, the epoxy determine the maximum peak current through surrounding the LED die can be charred and the LED array. For best results, the breakdown the gold bond wire and LED die can be voltage of the transient suppressor should fall destroyed, resulting in an open circuit. The within the following range: AlInGaP LED die can tolerate non-recurring peak current transients of several hundred 24 V < VBR < 45 V milliamperes for short time periods (t << 1 ms) with minimal permanent effects. However, Note that the 24 V restriction is determined by longer transients can cause sufficient localized the “Jump Start” voltage condition. The 45 V heating to cause the various effects listed restriction is determined by the ability of the LED earlier. The “Load Dump” transient pulse can be array to withstand the peak current imposed by especially damaging since the pulse duration the transient voltage. Since the 45 V limit can be up to 400 ms. depends on the circuit topography of the LED array and the maximum “Load Dump” transient The effects of “Load Dump” transients can be pulse duration, this voltage limit should be minimized by putting a surge-suppressor or established by reliability testing. silicon transient suppressor in parallel with the LED array as previously shown in Figure 3.6. Note that the breakdown voltage of the transient suppressor, as well as the current- Table 3.2 Diode Part Number 1N4005 1N5396 1N5406 Silicon Diodes for EMC Negative Transient Protection Maximum Continuous Reverse Breakdown Forward Current, IO Voltage, VRRM Applications 1.0 A 1.5 A 3.0 A 600 V 600 V 600 V CHMSL Rear Combination Lamp Rear Combination lamp, & Front Turn Signal Special Considerations for Dual Luminous Intensity Operation Some applications, such as a Stop/Tail signal ratios for Stop/Tail signals are 7:1 to 15:1. lamp require two discrete levels of light output. Generally, the LED emitters should appear These applications require some additional matched in luminous flux at both drive design considerations. In most cases, the ratios conditions. This implies that the forward currents in light output at the two signal conditions are for the LED emitters in the array should be determined by the various signal lamp matched at both drive currents. SuperFlux and specifications or regulations. Typical dimming SnapLED 70 emitters are categorized for 29 luminous flux and forward voltage at 70 mA Figure 3.20 shows a PWM Tail circuit. When the (150 mA for the SnapLED 150). Thus, their input voltage is applied to the Stop pin, the matching is best when driven at high forward cross-connected paralleled string circuit is currents. As discussed in the section “Electrical, operated at a dc forward current determined Optical, and Thermal Properties of an LED external resistor, RSTOP. The value for RSTOP can be Emitter,” Figure 3.11 shows the expected range determined using Equation #3.2. When the input in light output for HPWT-xH00 emitters that are voltage is applied to the Tail pin, the NE555 matched at 70 mA. Note that the light output oscillator is energized. Resistors, R1 and R2, and varies by approximately 2:1 at a 20 mA forward capacitor, C1, determine the frequency and duty current. Note that since the SnapLED 150 is cycle of the oscillator. The values shown matched at 150 mA, then the light output would generate a 2K Hz frequency and a 10% duty vary by about 2:1 at 40 mA. Thus, even if a cycle (output low). When the output of the well-controlled constant current circuit drives NE555 (pin 3) is low, a high-current switching the LED emitters, the light output matching transistor is turned-on, which supplies current to might be unacceptable for the Tail function if the LED array. External resistor, R TAIL, determines driven by a low DC forward current. the peak current of the Tail circuit. The value for R TAIL can be determined using a similar equation A PWM circuit is recommended for best as used previously for RSTOP and including the luminous intensity matching for the Tail function. extra voltage drop across the high-current This circuit could drive the LED array at a high switching transistor. Thus for the same value of forward current for the Stop function and drive peak forward current, R TAIL < RSTOP. Diodes D1 the LED array at the same peak forward current and D2 protect the circuit from negative EMC but at a low duty cycle for the Tail function. With transients. Zener diode D 3 protects the NE555 this approach, the matching of the LED array from positive EMC transients. will be the same regardless of whether the signal is operating in Stop or Tail mode. Figure 3.20 Stop/Tail LED Signal Lamp Circuit that Uses a PWM Scheme to Generate the Reduced Light Output of the Tail Signal. 30 Current and Voltage Regulator Circuits This section will discuss active circuits that are and compare the load voltage with a reference designed to drive the LED emitter array at a voltage. Current regulator circuits usually constant voltage or constant current despite measure the current through the load by input voltage or load variations. These circuits measuring the voltage drop across a “sense” are called voltage or current regulator circuits resistor in series with the load. Then the voltage because they are designed to regulate the input across the sense resistor is compared with a voltage to generate either a fixed output voltage reference voltage. or current. Since most LED signal lamp designs consist of The use of voltage or current regulation several LED emitters, they are normally arranged improves the operation of the LED signal lamp. in one or more series-connected strings, such as Since the drive current of the LED array remains shown previously in Figure 3.1. While it is constant despite variations in the supply possible to use one voltage or current regulator voltage, the light output is not affected by input per string, due to cost considerations, most voltage variations. Since the drive current practical designs use a single voltage or current doesn’t increase due to over voltage conditions, regulator for the entire LED array. Note that when the LED emitters can be driven at a higher only a single regulator is used for the entire array forward current at the design voltage without it is possible to encounter the same type of exceeding the maximum allowable forward forward current variations as described earlier in current at the maximum input voltage. In the section “Key Concepts for the Electrical addition, if the circuit is located outside of the Design of LED Signal Lamps.” Since LED LED signal lamp case, the voltage or current emitters are current-controlled devices, regulator circuit can improve the thermal voltage regulator circuits should use current- properties of the signal lamp by reducing the limiting resistors in series with each string of power consumption within the LED signal lamp. LED emitters (Figure 3.1a circuit), paralleled string of LED emitters (Figure 3.1b circuit), or Block diagrams of typical voltage and current cross-connected paralleled string of LED regulator circuits are shown in Figure 3.21. The emitters (Figure 3.1c circuit). For voltage basic elements of all of these circuits consist of regulator circuits, Equation #3.2 can be used to a high gain amplifier and feedback circuit, which calculate the value of the external current-limiting vary the dynamic load of a power circuit that is resistor(s) if the regulated output voltage, VOUT, is either in series or parallel with the LED emitter substituted into the equation for VIN. For current array. The regulator circuit modulates the regulator circuits, external current limiting dynamic load so as to provide either a constant resistors are not required but their use can voltage or current to the LED emitter array reduce forward current variations within the LED independent of input voltage or load variations array. (over some specified range). Voltage regulator circuits measure the voltage across the load 31 As shown in Figure 3.21, there are three basic due to the large variations in input voltage and types of regulator circuits. The circuits shown in will not be covered further in this section. Figures 3.21a and 3.21d are called “shunt” regulators. They use a dynamic load in parallel The circuits shown in Figures 3.21b and 3.21e with the load being regulated that shunts some are called “series-pass” regulators. They use a of the supply current around the load. Shunt dynamic load in series with the load being regulators also have a power resistor in series regulated. At minimum input voltages the voltage with both loads. The value of the power resistor drop across the dynamic load goes to a has been selected such that at the minimum minimum value. This minimum voltage drop is input voltage and maximum load condition, the called the “drop-out” voltage. At higher input current through the dynamic load goes to zero. voltages, the voltage drop across the dynamic At higher input voltages or smaller loads, the load increases so as to maintain either a fixed current through the dynamic load is increased, current or voltage across the load. At voltages which increases the voltage drop across the below the drop-out voltage, the dynamic load power resistor to keep the load current or can no longer regulate the output voltage or voltage constant. In this way, the shunt current. Thus, for proper voltage or current regulator maintains either a fixed current or regulation, the input voltage needs to be higher voltage across the load. Shunt regulators are than the sum of the drop-out voltage, the voltage not very practical for automotive signal lamps across the load, and the voltage drop across the sense resistor (if applicable). Figure 3.21 Block Diagrams of Several Active Drive Circuits for LED Signal Lamps. 32 The circuits shown in Figures 3.21c and 3.21f circuit shown in Figure 3.1b or “rung” for the are called “switching” regulators. They use a circuit shown in Figure 3.1c. dynamic load that is switched ON and OFF at very high frequencies at a varying duty cycle. With a voltage regulator, the forward voltage The dynamic load supplies electrical power to applied to the LED array voltage will be an energy storage element such as a capacitor independent of supply voltage variations as long or an inductor or a combination of both. This as the voltage regulator remains in its active energy storage element then supplies power to region. However, ambient temperature variations the load. The percentage of time the dynamic and the use of different forward voltage load is ON is varied depending on the input categories can affect the forward current through voltage and load requirements. The “switching” the LED array unless provisions are made in the regulator provides the highest power efficiency design. As mentioned earlier, current limiting of the three circuits. However, it is the most resistors, R, are needed for each string of LED complex of the three regulator circuits and has emitters. With R > y ∆RS, the forward current the highest potential for creating unwanted EMI through each string will primarily be determined (due to the high-frequency switching). by the value of R. If the designer uses a voltage regulator with a fixed output voltage, then the The performance of these different types of values of these current-limiting resistors will need regulators is compared with an example shown to be varied for each of the different forward in the sidebar “Comparison of Three Constant- voltage categories in order to compensate for the Current Circuits.” different forward voltages at the design current. Alternatively, the designer could use the same The LED emitter array can be driven from either value of current-limiting resistors for all forward a voltage regulator or a current regulator circuit. voltage categories. However, in this case, the With a current regulator, the total array current regulator output voltage would need to be varied will be independent of supply voltage, slightly for each different forward voltage temperature and forward voltage category category to compensate for the different forward variations as long as the current regulator voltages at the design current. Despite these remains in its active region. If the current precautions, there will still be small variations in regulator is used with parallel-connected LED the total current through the LED array due to emitters, such as shown in Figure 3.1b or 3.1c, slightly different forward voltages of the individual there can still be similar forward current emitters. With only a small voltage drop across variations within the LED array as was the current limiting resistor, small variations in discussed in the section “Key Concepts for the the regulated voltage can cause large changes Electrical Design of LED Signal Lamps.” Note in forward current through the LED emitters. In that the forward current matching can be addition, since the forward voltage of the LED improved with the addition of a small resistor emitter varies with temperature, the forward (ROPT > RS) in series with each string for the current through the LED array will increase at elevated temperatures. However, it is possible 33 to maintain fixed current through the LED array the LED array as was discussed in the section if the output voltage of the regulator tracks “Key Concepts for Electrical Design of LED the ∆VF /∆T of the LED array (approximately Signal Lamps.” Note that the forward current –2 mV/°C times the number of emitters in each matching can be improved with the addition of a series string). Finally, if the voltage regulator is small resistor (ROPT > RS) in series with each string used with parallel-connected LED emitters, for the circuit shown in Figure 3.1b or “rung” for such as shown in Figure 3.1b or 3.1c, there can the circuit shown in Figure 3.1c. still be similar forward current variations within Comparison of Three Constant-Current Circuits SETUP: Suppose an LED signal lamp is being designed SOLUTION: using 30 HPWT-DH00 SuperFlux LED emitters For the first two designs, the value of the external from forward voltage category 3. The circuit will current limiting resistor, R, can be determined be designed to operate at 50 mA per emitter at using Equation #3.2. Note, for forward voltage a design voltage of 12.8 V. category 3, the nominal linear forward voltage model is VO NOM = 1.91 V, and RS NOM = 9.2 ohm. PROBLEM STATEMENT: Thus, for the three-LED string circuit, R = 114 How does the overall power consumption ohm. For the four-LED string circuit, R = 66 ohm. compare for the following 4 possible circuit The detailed designs are shown in Figure 3.22. designs over an input voltage range of 9 V Then over an input voltage range of 7 to 18 volts, to 18 V? the forward current through each LED string would vary as shown in Figure 3.23. 1. Resistive current limiting (Figure 3.1a circuit) with ten strings of three emitters per string. 2. Resistive current limiting (Figure 3.1a circuit) with eight strings of four emitters per string. 3. Series-pass constant-current regulator driving ten strings of three emitters per string. 4. Switching constant-current regulator driving ten strings of three emitters per string. Figure 3.22 Two LED Signal Lamp Designs Using Resistive Current Limiting. 34 Figure 3.23 Forward Current Through LED Emitters as a Function of Applied Voltage for Resistive Limited Circuits Shown in Figure 3.22. Figure 3.24 Block Diagram of LED Signal Lamp Design Using Series-Pass Constant-Current Regulator. The key elements of the series pass constantcurrent regulator are shown in Figure 3.24. For series strings of three forward voltage category 3 HPWT-DH00 emitters, the forward voltage of the string is about 7.10 V at 50 mA. Assuming a voltage drop across the sense resistor of 0.25 V, then at an input voltage of 9 V, the drop-out voltage of the regulator would be (9 V – 7.1 V – 0.25 V), or 1.65 V. Since there are 10 strings of LED emitters, the total LED array current would Figure 3.25 Forward Current Through LED Emitters as a Function of Applied Voltage for Series-Pass Constant-Current Regulator Circuit Shown in Figure 3.24. be 50 mA times 10, or 500 mA. Thus, the sense resistor would be (0.25 V / 0.500 A), or 0.5 ohms. Then over an input voltage range of 7 to 18 volts, the total load current of the circuit The key elements of the switching constant- would vary as shown in Figure 3.25. As current regulator are shown in Figure 3.26. There designed, this circuit maintains a constant are a number of different types of switching current through the LED array at input voltages regulators. Buck or Down Converters are greater than 9 V. Suppose that the minimum designed to generate a regulated output voltage compliance voltage of the circuit is designed to that is always less than the input voltage. Boost be 10 V, then an additional volt can be dropped or Up Converters are capable of generating a across the load or series pass regulator. regulated output voltage that is always higher than the input voltage. Buck/Boost or Up/Down Converters can generate a regulated output voltage using any input voltage. By comparison, 35 current-limiting resistors and series-pass input voltages, the efficiency of the switching regulators can only reduce the output voltage to regulator is better than the series-pass regulator a lower value than the input voltage. Thus for and the resistor-limited circuits. Assuming a some types of switching regulator circuits the 0.25 V drop across the sense resistor, then for number of LED emitters per string can be larger the ten-string circuit, RSENSE would be equal to than the number of LED emitters per string for a (0.25 V / 0.500 A), or 0.5 ohms. Assuming a resistor-limited or series-pass regulator circuit. power conversion efficiency of 80% and an input In general, the switching regulator converts the voltage range of 7 to 18 volts, then the input average input power (VIN times IIN) to the desired current and total load current of the circuit would output power (VLOAD times ILOAD) with a relatively vary as shown in Figure 3.27. fixed power conversion efficiency. At higher Figure 3.26 Block Diagram of LED Signal Lamp Design Using Switching Constant-Current Regulator. Figure 3.27 Forward Current Through LED Emitters as a Function of Applied Voltage for Switching Constant-Current Regulator Circuit Shown in Figure 3.26. Figure 3.28 Comparison of Total Supply Current versus Applied Voltage for Circuit Designs Shown in Figures 3.22, 3.24 and 3.26. The total power consumption for the four that at an input voltage of 18 V, both resistor different LED signal lamp designs is shown in limited circuits have an overall power consumption Figure 3.28. The series pass and switching of 15 W. The series pass current regulator circuit regulator designs provide substantial power has an overall power consumption of 9 W. The savings compared to the resistor-controlled switching current regulator has an overall power circuits during over-voltage conditions. Note consumption of 5 W. 36 Company Information Lumileds is a world-class supplier of Light Emitting Diodes (LEDs) producing billions of LEDs annually. Lumileds is a fully integrated supplier, producing core LED material in all three base colors (Red, Green, Blue) and White. Lumileds has R&D development centers in San Jose, California and Best, The Netherlands. Production capabilities in San Jose, California and Malaysia. Lumileds is pioneering the high-flux LED technology and bridging the gap between solid state LED technology and the lighting world. Lumileds is absolutely dedicated to bringing the best and brightest LED technology to enable new applications and markets in the Lighting world. LUMILEDS www.luxeon.com www.lumileds.com For technical assistance or the location of your nearest Lumileds sales office, call: Worldwide: +1 408-435-6044 US Toll free: 877-298-9455 Europe: +31 499 339 439 Fax: 408-435-6855 Email us at [email protected] 2002 Lumileds Lighting. All rights reserved. Lumileds Lighting is a joint venture between Agilent Technologies and Philips Lighting. Luxeon is a trademark of Lumileds Lighting, Inc. Product specifications are subject to change without notice. Publication No. AB20-3 (Sept2002) 37 Lumileds Lighting, LLC 370 West Trimble Road San Jose, CA 95131