® LinkSwitch-TN Design Guide Application Note AN-37 Introduction • Universal input – the same power supply/product can be used worldwide • High power density – smaller size, no µF’s of X class capacitance needed • High efficiency – Full load efficiencies >75% typical for 12 V output • Excellent line and load regulation • High efficiency at light load – ON/OFF control maintains high efficiency even at light load • Extremely energy efficient – input power <100 mW at no load • Entirely manufacturable in SMD • More robust to drop test mechanical shock • Fully fault protected (overload, short circuit and thermal faults) • Scalable – LinkSwitch-TN family allows the same basic design to be used from <50 mA to 360 mA LinkSwitch-TN combines a high voltage power MOSFET switch with an ON/OFF controller in one device. It is completely selfpowered from the DRAIN pin, has a jittered switching frequency for low EMI and is fully fault protected. Auto-restart limits device and circuit dissipation during overload and output short circuit while over temperature protection disables the internal MOSFET during thermal faults. The high thermal shutdown threshold is ideal for applications where the ambient temperature is high while the large hysteresis protects the PCB and surrounding components from high average temperatures. LinkSwitch-TN is designed for any application where a nonisolated supply is required such as appliances (coffee machines, rice cookers, dishwashers, microwave ovens etc.), nightlights, emergency exit signs and LED drivers. LinkSwitch-TN can be configured in all common topologies to give a line or neutral referenced output and an inverted or non-inverted output voltage - ideal for applications using triacs for AC load control. Using a switching power supply rather than a passive dropper (capacitive or resistive) gives a number of advantages, some of which are listed below. Scope This application note is for engineers designing a non-isolated power supply using the LinkSwitch-TN family of devices. This DFB RFB CBP DIN2 LIN FB RBIAS S D L LinkSwitch-TN AC Input DIN2 CIN1 CFB BP DFW CIN2 + RF CO RPL VO PI-3764-121003 1 (a) DFB RFB CBP RBIAS DIN1 LIN FB S D AC Input LinkSwitch-TN DIN2 CIN1 CFB BP CIN2 1 (b) DFW L CO RPL VO + RF PI-3765-121003 Figure 1 (a). Basic Configuration using LinkSwitch-TN in a Buck Converter. Figure 1 (b) Basic Configuration using LinkSwitch-TN in a Buck-Boost Converter. January 2004 AN-37 Quick Start document describes the design procedure for buck and buckboost converters using the LinkSwitch-TN family of integrated off-line switchers. The objective of this document is to provide power supply engineers with guidelines in order to enable them to quickly build efficient and low cost buck or buck-boost converter based power supplies using low cost off-the-shelf inductors. Complete design equations are provided for the selection of the converter’s key components. Since the power MOSFET and controller are integrated into a single IC the design process is greatly simplified, the circuit configuration has few parts and no transformer is required. Therefore a quick start section is provided that allows off-the-shelf components to be selected for common output voltages and currents. Readers wanting to start immediately can use the following information to quickly select the components for a new design, using Figure 1 and Tables 1 and 2 as references. 1) For AC input designs select the input stage (Table 9). 2) Select the topology (Tables 1 and 2). - If better than ±10% output regulation is required, then use opto coupler feedback with suitable reference. 3) Select the LinkSwitch-TN device, L, RFB or VZ, RBIAS, CFB, RZ and the reverse recovery time for DFW (Table 3: Buck, table 4:Buck-Boost). 4) Select freewheeling diode to meet trr determined in step 3 (Table 5). 5) For direct feedback designs, if the minimum load < 3 mA then calculate RPL = VO / 3 mA. 6) Select CO as 100 µF, 1.25 • VO, low ESR type. 7) Construct prototype and verify design. In addition to this application note a design spreadsheet is available within the PIXls tool in the PI Expert design software suite. The reader may also find the LinkSwitch-TN DAK engineering prototype board useful as an example of a working supply. Further details of support tools and updates to this document can be found at www.powerint.com. TOPOLOGY High-Side Buck – Direct Feedback BASIC CIRCUIT SCHEMATIC FB + BP S D LinkSwitch-TN VIN KEY FEATURES + 1) 2) 3) 4) Output referenced to input Positive output (VO) with respect to -VIN Step down – VO < VIN Low cost direct feedback (±10% typ.) VO PI-3751-121003 High-Side Buck Boost – Direct Feedback FB + VIN D 1) 2) 3) 4) 5) BP S LinkSwitch-TN VO + PI-3794-121503 Output referenced to input Negative output (VO) with respect to -VIN Step down – VO > VIN or VO < VIN Low cost direct feedback (± 10% typ.) Fail-safe – output is not subjected to input voltage if the internal MOSFET fails 6) Ideal for driving LEDs – better accuracy and temperature stability than low-side Buck constant current LED driver Notes 1. Low cost, directly sensed feedback typically achieves overall regulation tolerance of ± 10%. 2. To ensure output regulation a pre-load may be required to maintain a minimum load current of 3 mA (Buck and Buck-Boost only). 3. Boost topology (step up) also possible but not shown. Table 1. LinkSwitch-TN Circuit Configurations Using Directly Sensed Feedback. 2 A 1/04 AN-37 TOPOLOGY High-Side Buck – Optocoupler Feedback BASIC CIRCUIT SCHEMATIC BP FB + KEY FEATURES D S + RZ LinkSwitch-TN VO VIN VZ PI-3796-121903 Low-Side Buck – Optocoupler Feedback + + RZ LinkSwitch-TN VO VIN 1) 2) 3) 4) Output referenced to input Positive output (VO) with respect to -VIN Step down – VO < VIN Optocoupler feedback - Accuracy only limited by reference choice - Low cost non-safety rated optocoupler - No pre-load required 5) Minimum no-load consumption 1) 2) 3) 4) VZ BP S Low-Side Buck Boost – Optocoupler Feedback FB D PI-3797-121903 + VZ LinkSwitch-TN VO VIN RZ BP S FB D + PI-3798-121903 Output referenced to input Negative output (VO) with respect to +VIN Step down – VO < VIN Optocoupler feedback - Accuracy only limited by reference choice - Low cost non-safety rated optocoupler - No pre-load required 1) 2) 3) 4) Output referenced to input Positive output (VO) with respect to +VIN Step up/down – VO > VIN or VO < VIN Optocoupler feedback - Accuracy only limited by reference choice - Low cost non-safety rated optocoupler - No pre-load required 5) Fail-safe – output is not subjected to input voltage if the internal MOSFET fails 6) Minimum no-load consumption Notes 1. Performance of opto feedback only limited by accuracy of reference (Zener or IC). 2. Optocoupler does not need to be safety approved. 3. Reference bias current provides minimum load. The value of RZ is determined by Zener test current or reference IC bias current. Typically 470 Ω to 2 kΩ, 1/8 W, 5% 4. Boost topology (step-up) is also possible but not shown. 5. Optocoupler feedback provides lowest no-load consumption. Table 2. LinkSwitch-TN Circuit Configurations Using Optocoupler Feedback. A 1/04 3 AN-37 INDUCTOR TOKIN COILCRAFT 220 230 320 340 440 430 SBC2-681-211 SBC2-681-211 SBC3-681-211 SBC4-681-211 SBC4-681-211 SBC4-681-211 RFB0807-681 RFB0807-681 RFB0810-681 RFB0810-681 RFB0810-681 RFB0810-681 680 1000 1500 680 1000 680 1500 180 230 320 340 440 430 400 SBC2-681-211 SBC3-102-281 SBC3-152-251 SBC3-681-361 SBC4-102-291 SBC4-681-431 SBC6-152-451 RFB0807-681 RFB0807-102 LNK304 RFB0810-152 RFB0810-681 LNK305 RFB0810-102 RFB0810-681 LNK306 RFB1010-152 ≤70 120 160 175 225 280 360 680 1200 1800 820 1200 820 1500 160 210 210 310 310 390 390 SBC2-681-211 SBC6-152-451 RFB0807-681 RFB0807-122 RFB0810-182 RFB0810-821 RFB1010-122 RFB1010-821 RFB1010-152 ≤50 120 160 175 225 280 360 680 1500 2200 1200 1500 1200 2200 130 190 180 280 280 350 360 SBC2-681-211 SBC4-152-221 SBC4-222-211 SBC6-152-451 SBC6-222-351 RFB0807-681 RFB0810-152 RFB0810-222 RFB0810-122 RFB1010-152 RFB1010-122 - VOUT IOUT(MAX) 5 ≤120 160 175 225 280 360 680 680 680 680 680 680 12 ≤85 120 160 175 225 280 360 15 24 µH IRMS (mA) Other Standard Components RBIAS: 2 kΩ, 1%, 1/8 W CBP: 0.1 µF, 50 V Ceramic CFB: 10 µF, 1.25 • VO DFB: 1N4005GP RZ: 470 Ω to 2 kΩ, 1/8 W, 5% Table 3. Components Quick Select for Buck Converters. 4 A 1/04 LNK30X LNK304 LNK305 LNK306 LNK304 LNK305 LNK306 LNK304 LNK305 LNK306 MODE DIODE trr RFB VZ MDCM CCM MDCM CCM MDCM CCM MDCM MDCM CCM MDCM CCM MDCM CCM ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns 3.84 kΩ 3.9 V ≤75 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns 11.86 kΩ 11 V MDCM MDCM CCM MDCM CCM MDCM CCM MDCM MDCM CCM MDCM CCM MDCM CCM ≤75 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns 15.29 kΩ 13 V ≤75 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns 25.6 kΩ 22 V AN-37 INDUCTOR TOKIN COILCRAFT 220 230 340 320 440 430 SBC2-681-211 SBC2-681-211 SBC3-681-361 SBC4-681-431 SBC4-681-431 SBC4-681-431 RFB0807-681 RFB0807-681 RFB0810-681 RFB0810-681 RFB0810-681 RFB0810-681 680 1200 1800 820 1200 820 1800 180 220 210 320 310 410 410 SBC2-681-211 - RFB0807-681 RFB1010-122 RFB0807-182 RFB0807-821 RFB0810-122 RFB0810-821 RFB1010-182 ≤50 120 160 175 225 280 360 680 1500 2200 1000 1500 1200 2200 180 220 220 320 320 400 410 SBC2-681-211 SBC3-152-251 SBC4-222-211 SBC4-102-291 SBC4-152-251 SBC6-222-351 RFB0807-681 RFB0807-152 RFB0810-222 RFB0810-102 RFB0810-152 RFB0810-122 RFB1010-222 ≤35 120 160 175 225 280 360 680 2200 3300 1800 2200 1800 3300 180 210 210 300 290 370 410 SBC2-681-211 SBC3-222-191 SBC4-332-161 SBC4-222-211 - RFB0807-681 RFB0810-222 LNK304 RFB0810-332 RFB0810-182 LNK305 RFB1010-222 RFB1010-182 LNK306 - VOUT IOUT(MAX) 5 ≤120 160 175 225 280 360 680 680 680 680 680 680 ≤70 120 160 175 225 280 360 12 15 24 µH IRMS (mA) LNK30X LNK304 LNK305 LNK306 LNK304 LNK305 LNK306 LNK304 LNK305 LNK306 MODE DIODE trr RFB VZ MDCM CCM MDCM CCM MDCM CCM ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns 3.84 kΩ 3.9 V MDCM MDCM CCM MDCM CCM MDCM CCM ≤75 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns 11.86 kΩ 11 V MDCM MDCM CCM MDCM CCM MDCM CCM ≤75 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns MDCM MDCM CCM MDCM CCM MDCM CCM ≤75 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns ≤75 ns ≤35 ns 15.29 kΩ 13 V 25.6 kΩ 22 V Other Standard Components RBIAS: 2 kΩ, 1%, 1/8 W CBP: 0.1 µF, 50 V Ceramic CFB: 10 µF, 1.25 • VO DFB: 1N4005GP RZ: 470 Ω to 2 kΩ, 1/8 W, 5% Table 4. Components Quick Select for Buck-Boost Converters. PART NO. VRRM (V) IF (A) t rr (ns) MUR160 600 1 50 Leaded Vishay UF4005 600 1 75 Leaded Vishay BYV26C 600 1 30 Leaded Vishay/Philips FE1A 600 1 35 Leaded Vishay STTA10 6 600 1 20 Leaded ST Microelectronics STTA10 6U 600 1 20 SMD ST Microelectronics US1J 600 1 75 SMD Vishay PACKAGE MANUFACTURER Table 5. List of Ultra-Fast Diodes Suitable for use as the Freewheeling Diode. A 1/04 5 AN-37 LinkSwitch-TN Circuit Design To regulate the output, an ON/OFF control scheme is used as illustrated in Table 6. As the decision to switch is made on a cycle by cycle basis, the resultant power supply has extremely good transient response and removes the need for control loop compensation components. If no feedback is received for 50 ms then the supply enters auto restart. LinkSwitch-TN Operation The basic circuit configuration for a Buck converter using LinkSwitch-TN is shown in Figure 1a. Reference Schematic and Key FB S D + = MOSFET Enabled BP + LinkSwitch-TN VIN VO = MOSFET Disabled Cycle Skipped PI-3784-121603 ID Is IFB >49 µA? No At beginning of each cycle the FEEDBACK (FB) pin is sampled. • If IFB < 49 µA then next cycle occurs • If IFB > 49 µA then next switching cycle is skipped No Yes No No Yes Yes No Normal Operation High load – few cycles skipped Low load – many cycles skipped PI-3767-121903 IFB < 49 µA, > 50 ms = Auto Restart Auto Restart 50 ms 800 ms Auto Restart = 50 ms ON / 800 ms OFF PI-3768-121603 Table 6. LinkSwitch-TN Operation. 6 A 1/04 If no feedback (IFB < 49 µA) for > 50 ms then output switching is disabled for approximately 800 ms. AN-37 To allow direct sensing of the output voltage, without the need for a reference (Zener diode or reference IC), the FB pin voltage is tightly toleranced over the entire operating temperature range. For example, this allows a 12 V design with an overall output tolerance of ±10%. For higher performance, an optocoupler can be used with a reference as shown in table 2. Since the optocoupler just provides level shifting, it does not need to be safety rated or approved. The use of an optocoupler also allows flexibility in the location of the device, for example it allows a buck converter configuration with the LinkSwitch-TN in the low side return rail, reducing EMI as the SOURCE pins and connected components are no longer part of the switching node. freewheeling diode, and the average current through the output inductor are slightly lower in the Buck topology as compared to the Buck-Boost topology. Selecting the Operating Mode – MDCM and CCM Operation Selecting the Topology At the start of a design, select between mostly discontinuous conduction mode (MDCM) and continuous conduction mode (CCM) as this decides the selection of the LinkSwitch-TN device, freewheeling diode and inductor. For maximum output current select CCM, for all other cases MDCM is recommended. Overall, select the operating mode and components to give the lowest overall solution cost. Table 7 summarizes the trade-offs between the two operating modes. If possible use the Buck topology, the Buck topology maximizes the available output power from a given LinkSwitch-TN and inductor value. Also, the voltage stress on the power switch and Additional differences between CCM and MDCM include better transient response for DCM and lower output ripple (for same capacitor ESR) for CCM. However these differences, at COMPARISON OF CCM AND MDCM OPERATING MODES OPERATING MODE MDCM CCM IL IL IO Operating Description IO t tON tOFF tIDLE PI-3769-121803 t tON tOFF PI-3770-121503 Inductor current falls to zero during tOFF, Borderline between MDCM and CCM when tIDLE = 0. Current flows continuously in the inductor for the entire duration of a switching cycle. Lower Cost Lower value, smaller size. Higher Cost Higher value, larger size. Freewheeling Diode Lower Cost 75 ns ultra-fast reverse recovery type (≤35 ns for ambient >70 °C). Higher Cost 35 ns ultra-fast recovery type required. LinkSwitch-TN Potentially Higher Cost May require larger device to deliver required output current–depends on required output current. Potentially Lowest Cost May allow smaller device to deliver required output current–depends on required output current. Efficiency Higher Efficiency Lower switching losses. Lower Efficiency Higher switching losses. Overall Typically Lower Cost Typically Higher Cost Inductor Table 7. Comparison of Mostly Discontinuous Conduction (MDCM) and Continuous Conduction (CCM) Modes of Operation. A 1/04 7 AN-37 the low output currents of LinkSwitch-TN applications, are normally not significant. Output Power, PO: in Watts. Power supply efficiency, η: 0.7 for a 12 V output, 0.55 for a 5 V output if no better reference data available. The conduction mode CCM or MDCM of a Buck or BuckBoost converter primarily depends on input voltage, output voltage, output current and device current limit. The input voltage, output voltage and output current are fixed design parameters therefore the LinkSwitch-TN (current limit) is the only design parameter that sets the conduction mode. Total Capacitance CIN(TOTAL) µF/POUT (CIN1 + CIN2) AC Input Voltage (VAC) 100/115 230 Universal The phrase “mostly discontinuous” is used as with on-off control, since a few switching cycles may exhibit continuous inductor current, the majority of the switching cycles will be in the discontinuous conduction mode. A design can be made fully discontinuous but that will limit the available output current, making the design less cost effective. Half Wave Rectification 6-8 1-2 6-8 Full Wave Rectification 3-4 1 3-4 Table 10. Suggested Total Input Capacitance Values for Different Input Voltage Ranges. Step 2. Determine AC Input Stage Step-by-Step Design Procedure The input stage comprises fusible resistor(s) input rectification diodes and line filter network. The fusible resistor should be chosen as flame proof and depending on the differential line input surge requirements, a wire wound type may be required. The fusible resistor(s) provides fuse safety, inrush current limiting and differential mode noise attenuation. Step 1. Determine System Requirements VACMIN, VACMAX, PO, VO, fL, η Determine the input voltage range from Table 8. Input (VAC) 100/115 230 Universal VACMIN 85 195 85 VACMAX 132 265 265 For designs ≤1 W it is lower cost to use half-wave rectification, >1 W full wave rectification (smaller input capacitors). The EMI performance of half wave rectified designs is improved by adding a second diode in the lower return rail. This provides EMI gating (EMI currents only flow when the diode is conducting) and also doubles differential surge withstand as the surge voltage is shared across two diodes. Table 9 shows the recommended input stage based on output power for a universal input design while Table 10 shows how to adjust the input capacitance for other input voltage ranges. Table 8. Standard Worldwide Input Line Voltage Ranges. Line Frequency, fL: 50 or 60 Hz, for half-wave rectification use fL/2. Output Voltage, VO: in Volts. ≤ 0.25 W POUT 0.25-1 W >1W DIN1-4 RF1 DIN1 * DIN2* + ** CIN AC IN RF2 85-265 VAC Input Stage Comments RF1 DIN1 AC IN ** CIN1 A 1/04 + CIN2 DIN2* RF1, RF2: 100-470 Ω, 0.5 W, Fusible CIN: ≥2.2 µF, 400 V DIN1, DIN2: 1N4007, 1 A, 1000 V RF1 DIN1 AC IN LIN ** CIN1 + RF1 CIN2 AC IN LIN ** CIN1 + CIN2 DIN2* PI-3772-121603 PI-3771-121603 RF1: 8.2 Ω, 1 W Fusible RF2: 100 Ω, 0.5 W, Flame proof CIN1, CIN2: ≥3.3 µF, 400 V each DIN1, DIN2: 1N4007, 1 A, 1000 V PI-3773-121603 RF1: 8.2 Ω, 1 W Fusible LIN: 470 µH-2.2 mH, 0.05 A-0.3 A CIN1, CIN2: ≥4 µF/WOUT, 400 V each DIN1, DIN2: 1N4007, 1 A, 1000 V PI-3774-121603 RF1: 8.2 Ω, 1 W Fusible LIN: 470 µH-2.2 mH, 0.05 A-0.3 A CIN1, CIN2: ≥2 µF/WOUT, 400 V each DIN1, DIN2: 1N4005, 1 A, 600 V *Optional for improved EMI and line surge performance. Remove for designs requiring no impedance in return rail. **Increase value to meet required differential line surge performance. Table 9. Recommended AC Input Stages For Universal Input. 8 RF2 AN-37 Step 3. Determine Minimum and Maximum DC Input Voltages VMIN and VMAX Based on AC Input Voltage Step 5. Select the Output Inductor Tables 3 and 4 provide inductor values and RMS current ratings for common output voltages and currents based on the calculations in the design spreadsheet. Select the next nearest higher voltage and/or current above the required output specification. Alternatively the PIXls spreadsheet tool in the PI Expert software design suite or Appendix A can be used to calculate the exact inductor value (Eq. A7) and RMS current rating (Eq. A20). Calculate VMAX as (1) VMAX = 2 ⋅ VACMAX Assuming that the value of input fusible resistor is small, the voltage drop across it can be ignored. Assume bridge diode conduction time of tc = 3 ms if no other data available. It is recommended that the value of inductor chosen should be closer to LTYP rather than 1.5 • LTYP due to lower DC resistance and higher RMS rating. The lower limit of 680 µH limits the maximum di/dt to prevent very high peak current values. Tables 3 and 4 provide reference part numbers for standard inductors from two suppliers. Derive minimum input voltage VMIN VMIN = (2 ⋅ VACMIN 2 ) 1 2 ⋅ PO − tC 2 ⋅ fL η ⋅ CIN ( TOTAL ) (2) 680 µH < LTYP < L < 1.5 ⋅ LTYP If VMIN is ≤70 V then increase value of CIN(TOTAL). For LinkSwitch-TN designs the mode of operation is not dependent on the inductor value. The mode of operation is a function of load current and current limit of the chosen device, the inductor value merely sets the average switching frequency. Step 4. Select LinkSwitch-TN Device Based on Output Current and Current Limit Decide on operating mode - refer to Table 7. Figure 2 shows a typical standard inductor manufacturer’s data sheet. The value of off-the-shelf “drum core / dog bone / I core” inductors will drop up to 20% in value as the current increases. The constant KL_TOL in equation (A7) and the design spreadsheet adjusts for both this drop and the initial inductance value tolerance. For MDCM operation, the output current (IO) should be less than or equal to half the value of the minimum current limit of the chosen device from the data sheet. (3) I LIMIT _ MIN > 2 ⋅ IO For example if a 680 µH, 360 mA inductor is required, referring to Figure 2, the tolerance is 10% and an estimated 9.5% for the reduction in inductance at the operating current (approximately [0.36/0.38] • 10). Therefore the value of KL_TOL = 1.195 (19.5%). For CCM operation, the device should be chosen such that the output current IO, is more than 50%, but less than 80% of the minimum current limit ILIMIT_MIN. If no data is available assume a KL_TOL of 1.15 (15%). 0.5 ⋅ I LIMIT _ MIN < IO < 0.8 ⋅ I LIMIT _ MIN (4) Not all the energy stored in the inductor is delivered to the load, due to losses in the inductor itself. To compensate for this a loss Please see data sheet for LinkSwitch-TN current limit values. Inductance and Tolerance SBC3 Series (SBC3- - Current Rating for 20 °C Rise Current Rating for Value -10% Current Rating for 40 °C Rise ) Inductance L(mH/ at 10 kHz Rdc (W) max. Rated Current (A) ∆T = 20 °C 680±10% 1000±10% 1500±10% 2200±10% 3300±10% 1.62 2.37 3.64 5.62 7.66 0.36 0.28 0.25 0.19 0.15 Model 681-361 102-281 152-251 222-191 332-151 (5) Figure 2. Example of Standard Inductor Data Sheet. Current (Reference Value) (A) ∆T = 40 °C L change rate -10% 0.50 0.39 0.35 0.26 0.21 0.38 0.31 0.26 0.21 0.17 PI-3783-121003 A 1/04 9 AN-37 factor KLOSS is used. This has a recommended value of between 50% and 66% of the total supply losses as given by equation (5). For example, a design with an overall efficiency (η) of 0.75 would have a KLOSS value of between 0.875 and 0.833. (1 − η) 2(1 − η) K LOSS = 1 − to 1 − 2 3 (6) Let the value of RBIAS = 2 kΩ; this biases the feedback network at a current of ∼0.8 mA. Hence the value of RFB is given by RFB = (VO − VFB ) ⋅ RBIAS = (VO − 1.65 V) ⋅ 2 kΩ VO − VFB = VFB 1.748 V + I FB VFB + ( I FB ⋅ RBIAS ) RBIAS (10) Step 9. Select the Feedback Diode and Capacitor Step 6. Select Freewheeling Diode For MDCM operation at tAMB ≤70 °C, select an ultra-fast diode with trr ≤75 ns. At tAMB >70 °C, trr ≤ 35 ns. For the feedback capacitor, use a 10 µF general purpose electrolytic capacitor with a voltage rating ≥1.25 • VO. For CCM operation, select an ultra-fast diode with trr ≤35 ns. For the feedback diode, use a glass passivated 1N4005GP or 1N4937GP device with a voltage rating of ≥1.25 • VMAX. Allowing 25% design margin for the freewheeling diode, Step 10. Select Bypass Capacitor VPIV > 1.25 ⋅ VMAX (7) The diode must be able to conduct the full load current. Thus I F > 1.25 ⋅ IO (8) Table 5 lists common freewheeling diode choices. Use 0.1 µF, 50 V ceramic capacitor. Step 11. Select Pre-load Resistor For direct feedback designs if the minimum load <3 mA then calculate RPL = VO / 3 mA. Other information Step 7. Select Output Capacitor Startup Into Non-Resistive Loads The output capacitor should be chosen based on the output voltage ripple requirement. Typically the output voltage ripple is dominated by the capacitor ESR and can be estimated as: ESRMAX = VRIPPLE I LIMIT (9) where VRIPPLE is the maximum output ripple specification and ILIMIT is the LinkSwitch-TN current limit. The capacitor ESR value should be specified approximately at the switching frequency of 66 kHz. Capacitor values above 100 µF are not recommended as they can prevent the output voltage from reaching regulation during the 50 ms period prior to auto-restart. If more capacitance is required, then a soft-start capacitor should be added (see Other Information section). If the total system capacitance is >100 µF or the output voltage is >12 V, then the output may fail to reach regulation during start-up. This may also be true when the load is not resistive, for example the output is supplying a motor or fan. To increase the startup time, a soft-start capacitor can be added across the feedback resistor, as shown in Figure 3. The value of this soft-start capacitor is typically in the range of 0.47 µF to 47 µF with a voltage rating of 1.25 • VO. Figure 4 shows the effect of CSS used on a 12 V, 150 mA design driving a motor load. CSS RFB Step 8. Select the Feedback Resistors FB The values of RFB and RBIAS are selected such that at the regulated output voltage, the voltage on the FEEDBACK pin (VFB) is 1.65 V. This voltage is specified for a FEEDBACK pin current (IFB) of 49 µA. + VIN D BP S LinkSwitch-TN + VO PI-3775-121003 Figure 3. Example Schematic Showing Placement of Soft-Start Capacitor. 10 A 1/04 AN-37 12 No soft-start capacitor. Output never reaches regulation (in auto-restart). 10 Voltage (V) PI-3785-010504 14 FB + +7 V LinkSwitch-TN VIN 8 BP S D 6V8 RTN 6 5V1 4 -5 V PI-3776-121003 2 0 Figure 5. Example Circuit – Generating Dual Output Voltages. -2 0 2.5 5 Generating Negative and Positive Outputs Time (s) PI-3786-010504 14 12 Voltage (V) 10 8 6 4 Soft-start capacitor value too small – output still fails to reach regulation before auto-restart. 2 In appliance applications there is often a requirement to generate both an AC line referenced positive and negative output. This can be accomplished using the circuit in Figure 5. The two zener diodes have a voltage rating close to the required output voltage for each rail and ensure that regulation is maintained when one rail is lightly and the other heavily loaded. The LinkSwitch-TN circuit is designed as if it were a single output voltage with an output current equal to the sum of both outputs. The magnitude sum of the output voltages in this example being 12 V. Constant Current Circuit Configuration (LED Driver) 0 -2 0 2.5 5 Time (s) PI-3787-010503 14 The circuit shown in Figure 5 is ideal for driving constant current loads such as LEDs. It uses the tight tolerance and temperature stable FEEDBACK pin of LinkSwitch-TN as the reference to provide an accurate output current. 12 RFB 300 Ω Voltage (V) 10 Correct value of soft-start capacitor – output reaches regulation before auto-restart. 8 FB 6 + D BP RBIAS 2 kΩ VRFB DFB RSENSE S VIN IO DFW LinkSwitch-TN 4 Optional See Text L CSENSE CO 2 0 PI-3795-122403 -2 0 2.5 5 Time (s) Figure 4. Example of Using Soft-Start Capacitor to Enable Driving a 12 V, 0.15 A Motor Load. All Measurements were made at 85 VAC (worst case condition). Figure 6. High-Side Buck-Boost Constant Current Output Configuration. To generate a constant current output, the average output current is converted to a voltage by resistor RSENSE and capacitor CSENSE and fed into the FEEDBACK pin via RFB and RBIAS. With the values of RBIAS and RFB as shown, the value of RSENSE should be chosen to generate a voltage drop of 2 V at the A 1/04 11 AN-37 required output current. Capacitor CSENSE filters the voltage across RSENSE, which is modulated by inductor ripple current. The value of CSENSE should be large enough to minimize the ripple voltage, especially in MDCM designs. A value of CSENSE is selected such that the time constant (t) of RSENSE and CSENSE is greater than 20 times that of the switching period (15 µs). The peak voltage seen by CSENSE is equal to RSENSE • ILIMIT(MAX). The output capacitor is optional; however with no output capacitor the load will see the full peak current (ILIMIT) of the selected LinkSwitch-TN. Increase the value of CO (typically in the range of 100 nF to 10 uF) to reduce the peak current to an acceptable level for the load. If the load is disconnected, feedback is lost and the large output voltage which results may cause circuit failure. To prevent this, a second voltage control loop, DFB and VRFB, can be added as shown if Figure 6. This also requires that CO is fitted. The voltage of the Zener is selected as the next standard value above the maximum voltage across the LED string when it is in constant current operation. Recommended Layout Considerations Traces carrying high currents should be as short in length and thick in width, as possible. These are the traces which connect the input capacitor, LinkSwitch-TN, inductor, freewheeling diode and the output capacitor. Most off-the-shelf inductors are drum core inductors or dogbone inductors. These inductors do not have a good closed magnetic path, and are a source of significant magnetic coupling. They are a source of differential mode noise and for this reason, they should be placed as far away as possible from the AC input lines. Appendix A Calculations for Inductor Value for Buck and BuckBoost Topologies There is a minimum value of inductance that is required to deliver the specified output power, regardless of line voltage and operating mode. The same design equations / design spreadsheet can be used as for a standard buck-boost design, with the following additional considerations. 1. VO = LED VF • Number of LEDs per string 2. IO = LED IF • Number of strings 3. Lower efficiency estimate due to RSENSE losses (enter RSENSE into design spreadsheet as inductor resistance) 4. Set RBIAS = 2 kΩ and RFB = 300 Ω 5. RSENSE = 2/IO 6. CSENSE = 20 • (15 µs/RSENSE) 7. Select CO based on acceptable output ripple current through the load 8. If the load can be disconnected or for additional fault protection, add voltage feedback components DFB and VRFB, in addition to CO. VIN-VO VL t VO ILimit IL IO t tON tOFF tIDLE PI-3778-121803 Figure 7. Inductor Voltage and Inductor Current of a Buck Converter in DCM. Thermal Environment To ensure good thermal performance, the SOURCE pin temperature should be maintained below 100 °C, by providing adequate heatsinking. For applications with high ambient temperature (>50 °C), it is recommended to build and test the power supply at the maximum operating ambient temperature, and ensure that there is adequate thermal margin. The figures for maximum output current provided in the data sheet correspond to an ambient temperature of 50 °C, and may need to be thermally derated. Also, it is recommended to use ultra fast (≤35 ns) low reverse recovery diodes at higher operating temperatures (>70 °C). 12 A 1/04 As a general case, Figure 7 shows the inductor current in discontinuous conduction mode (DCM). The following expressions are valid for both CCM as well as DCM operation. There are three unique intervals in DCM as can be seen from Figure 7. Interval tON is when the LinkSwitch-TN is ON and the freewheeling diode is OFF. Current ramps up in the inductor from an initial value of zero. The peak current is the current limit ILIMIT of the device. Interval tOFF is when the LinkSwitch-TN is OFF and the freewheeling diode is ON. Current ramps down to zero during this interval. Interval tIDLE is when both the LinkSwitch-TN and freewheeling diode are OFF, and the inductor current is zero. AN-37 In CCM this idle state does not exist and thus tIDLE = 0. Neglecting the forward voltage drop of the freewheeling diode, we can express the current swing at the end of interval tON in a buck converter as ∆I (tON ) = I RIPPLE = VMIN − VDS − VO ⋅ tON LMIN ( I RIPPLE = 2 ⋅ I LIMIT _ MIN − IO ) I RIPPLE = I LIMIT _ MIN , VO ⋅ tOFF LMIN (A2) The initial current through the inductor at the beginning of each switching cycle can be expressed as I INITIAL = I LIMIT _ MIN − I RIPPLE 1 1⋅ I ⋅ LIMIT _ MIN + I INITIAL ⋅ tON + 2 2 I LIMIT _ MIN + I INITIAL ⋅ tOFF + 0 ⋅ t IDLE ( ( ( ) 2 ⋅ (VO ⋅ IO ) ⋅ (VMIN − VDS − VO ) ( I LIMIT _ MIN 2 − I INITIAL 2 ) ⋅ FSMIN ⋅ (VMIN − VDS ) (A6) This however does not account for the losses within the inductor (resistance of winding and core losses) and the freewheeling diode, which will limit the maximum power delivering capability and thus reduce the maximum output current. The minimum inductance must compensate for these losses in order to deliver specified full load power. An estimate of these losses can be made by estimating the total losses in the power supply, and then allocating part of these losses to the inductor and diode. This is done by the loss factor KLOSS which increases the size of the inductor accordingly. Furthermore, typical inductors for this type of application are bobbin core or dog bone chokes. The specified current rating refer to a temperature rise of 20 °C or 40 °C and to an inductance drop of 10%. We must incorporate an Inductance Tolerance Factor KL_TOL within the expression for minimum inductance, to account for this manufacturing tolerance. The typical inductance value thus can be expressed as (A3) The average current through the inductor over one switching cycle is equal to the output current IO. This current can be expressed as TSW _ MAX ) t IDLE > 0 ( for DCM ) Similarly, we can express the current swing at the end of interval tOFF as IO = TSW _ MAX t IDLE = 0 ( for CCM ) where IRIPPLE = Inductor Ripple Current ILIMIT_MIN = Minimum current limit VMIN = Minimum DC Bus Voltage VDS = On state Drain to Source Voltage drop VO = Output Voltage LMIN = Minimum Inductance 1 ( 1 LMIN = (A1) ∆I (tOFF ) = I RIPPLE = IO = I RIPPLE ⋅ LMIN 1⋅ I LIMIT _ MIN + I INITIAL 2 VMIN − VDS − VO I RIPPLE ⋅ LMIN + 1 ⋅ I I + INITIAL 2 LIMIT _ MIN VO (A5) ) ) LTYP = V ⋅I 2 ⋅ K L _ TOL ⋅ O O ⋅ (VMIN − VDS − VO ) K LOSS ( I LIMIT _ MIN 2 − I INITIAL 2 ) ⋅ FSMIN ⋅ (VMIN − VDS ) (A7) where KLOSS is a loss factor, which accounts for the off-state total losses of the inductor. (A4) where IO = Output Current. TSW_MAX = the switching interval corresponding to minimum switching frequency FSMIN. KL_TOL is the Inductor Tolerance Factor and can be between 1.1 and 1.2. A typical value is 1.15. With this typical inductance we can express maximum output power as 1 ⋅ LTYP ⋅ I LIMIT _ MIN 2 − I INITIAL 2 ⋅ 2 K VMIN − VDS (A8) FSMIN ⋅ ⋅ LOSS VMIN − VDS − VO K L _ TOL PO _ MAX = Substituting for tON and tOFF from equations (A1) and (A2) we have ( ) A 1/04 13 AN-37 Similarly for Buck-Boost topology the expressions for LTYP and PO_MAX are LTYP = V ⋅I 2 ⋅ K L _ TOL ⋅ O O K LOSS ( I LIMIT _ MIN 2 − I INITIAL 2 ) ⋅ FSMIN PO _ MAX = The current through the LinkSwitch-TN as a function of time is given by iSW (t ) = I INITIAL + (A9) 1 ⋅ LTYP ⋅ ( I LIMIT _ MIN 2 − I INITIAL 2 ) (A10) 2 VMIN − VDS − VO ⋅ t , 0 < t ≤ tON L iSW (t ) = 0 , tON < t ≤ tON The current through the Freewheeling diode as a function of time is given by iD (t ) = 0, Average Switching Frequency Since LinkSwitch-TN uses an on-off type of control, the frequency of switching is non-uniform due to cycle skipping. We can average this switching frequency by substituting the maximum power as the output power in equation (A8). Simplifying, we have FSAVG = 2 ⋅ VO ⋅ IO ⋅ K L _ TOL ( 2 2 ) L ⋅ I LIMIT − I INITIAL K LOSS ⋅ Similarly for Buck-Boost converter, simplifying equation (A9) we have FSAVG K 2 ⋅ VO ⋅ IO = ⋅ L _ TOL 2 2 L ⋅ I LIMIT − I INITIAL K LOSS K LOSS ( 0 < t ≤ tON VO , tON < t ≤ t SW L V I LIMIT _ MIN − O ⋅ t < 0 L iD (t ) = I LIMIT _ MIN − (A15) iD (t ) = 0, (A16) And the current through the inductor as a function of time is given by VMIN − VDS − VO VMIN − VDS (A11) (A14) iL (t ) = iSW ( t ) + iD (t ) (A17) From the definition of RMS currents we can express the RMS currents through the switch, freewheeling diode and inductor as follows t ON 1 iSW _ RMS = TAVG ) ∫ iSW (t )2 ⋅ dt (A18) 0 (A12) iD _ RMS = Calculation of RMS Currents The RMS current value through the inductor is mainly required to ensure that the inductor is appropriately sized and will not over heat. Also, RMS currents through the LinkSwitch-TN and freewheeling diode are required to estimate losses in the power supply. Assuming CCM operation, the initial current in the inductor in steady state is given by I INITIAL = I LIMIT _ MIN − VO ⋅ tOFF L For DCM operation this initial current will be zero. 14 A 1/04 (A13) iL _ RMS = t ON + t OFF 1 TAVG 1 TAVG ∫ t ON iD (t )2 ⋅ dt TAVG 2 ∫ (iSW (t ) + iD (t )) ⋅ dt 0 (A19) (A20) Since the switch and freewheeling diode currents fall to zero during the turn off and turn on intervals respectively, the RMS inductor current is simplified to iL _ RMS = iSW _ RMS 2 + iD _ RMS 2 (A21) AN-37 Table A1 lists the design equations for important parameters using the Buck and Buck-Boost topologies. PARAMETER BUCK BUCK-BOOST LTYP LTYP = FAVG iSW(t) LinkSwitch-TN Current id(t) Diode Forward Current iL(t) Inductor Current Max Drain Voltage FSTYP = V ⋅I 2 ⋅ K L ⋅ O O ⋅ (VMIN − VDS − VO ) K L _ LOSS ( I LIMIT _ MIN 2 − I INITIAL 2 ) ⋅ FSMIN ⋅ (VMIN − VDS ) V 2 ⋅ VO ⋅ IO ⋅ K L − VDS − VO ⋅ MIN 2 V L ⋅ I LIMIT − I INITIAL ⋅ K L _ LOSS MIN − VDS ( ) VMIN − VDS − VO ⋅ t , t ≤ tON L iSW (t ) = 0 , t > tON iSW (t ) = iINIT + VO ⋅ t , t > tON L V iD (t ) = 0 , I LIMIT _ MIN − O ⋅ t < 0 L iD (t ) = 0 , t ≤ tON iD (t ) = I LIMIT _ MIN − LTYP = FSAVG = V ⋅I 2 ⋅ KL ⋅ O O K L _ LOSS ( I LIMIT _ MIN 2 − I INITIAL 2 ) ⋅ FSMIN 2 ⋅ VO ⋅ IO KL ⋅ 2 2 K L ⋅ I LIMIT − I INITIAL L _ LOSS ( ) VMIN − VDS ⋅ t , t ≤ tON L iSW (t ) = 0 , t > tON iSW (t ) = iINIT + VO ⋅ t , t > tON L V iD (t ) = 0 , I LIMIT _ MIN − O ⋅ t < 0 L iD (t ) = 0 , t ≤ tON iD (t ) = I LIMIT _ MIN − iL (t ) = iSW (t ) + iD (t ) iL (t ) = iSW (t ) + iD (t ) VMAX VMAX + VO Table A1. Circuit Characteristics for Buck and Buck-Boost Topologies. A 1/04 15 AN-37 For the latest updates, visit our Web site: www.powerint.com PATENT INFORMATION Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any license under its patent rights or the rights of others. The products and applications illustrated herein (including circuits external to the products and transformer construction) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations’ patents may be found at www.powerint.com. LIFE SUPPORT POLICY POWER INTEGRATIONS' PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF POWER INTEGRATIONS. As used herein: 1. Life support devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. The PI logo, TOPSwitch, TinySwitch, LinkSwitch and EcoSmart are registered trademarks of Power Integrations. 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