VISHAY SILICONIX www.vishay.com Power MOSFETs Application Note AN-1005 Power MOSFET Avalanche Design Guidelines TABLE OF CONTENTS Page Table of Figures............................................................................................................................................................................. 2 Introduction ................................................................................................................................................................................... 3 Overview.................................................................................................................................................................................. 3 Avalanche Mode Defined ........................................................................................................................................................ 3 Avalanche Occurrences in Industry Applications.................................................................................................................... 3 Flyback Converter Example............................................................................................................................................... 3 Avalanche Failure Mode................................................................................................................................................................ 4 Power MOSFET Device Physics.............................................................................................................................................. 4 Rugged MOSFETs................................................................................................................................................................... 5 Avalanche Testing Details ............................................................................................................................................................. 7 Single Pulse Unclamped Inductive Switching ......................................................................................................................... 7 Decoupled VDD Voltage Source............................................................................................................................................... 7 Avalanche Rating .......................................................................................................................................................................... 8 EAS Thermal Limit Approach ......................................................................................................................................................... 8 Single Pulse Example .............................................................................................................................................................. 8 Repetitive Pulse..................................................................................................................................................................... 10 Buyer Beware.............................................................................................................................................................................. 12 The purpose of this note is to better understand and utilize power MOSFETs, it is important to explore the theory behind avalanche breakdown and to understand the design and rating of rugged MOSFETs. Several different avalanche ratings are explained and their usefulness and limitations in design is considered. Revision: 06-Dec-11 1 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE Conclusion .................................................................................................................................................................................. 13 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines TABLE OF FIGURES APPLICATION NOTE Page Figure 1 Flyback Converter Circuit....................................................................................................................................... 3 Figure 2 Flyback Converter Switch Under Avalanche Waveform ........................................................................................ 3 Figure 3 Flyback Converter Switch Under Avalanche Waveform (Detail) ............................................................................ 3 Figure 4 Power MOSFET Cross Section ...............................................................................................................................4 Figure 5 Power MOSFET Circuit Model ............................................................................................................................... 4 Figure 6 Power MOSFET Cross Section Under Avalanche.................................................................................................. 4 Figure 7 Basic Power MOSFET Cell Structure ..................................................................................................................... 5 Figure 8 Power MOSFET Random Device Failure Spots ..................................................................................................... 5 Figure 9 Good Source Contact vs. Bad Source Contact Illustration ................................................................................... 6 Figure 10 IA at Failure vs. Test Temperature .......................................................................................................................... 6 Figure 11 Single Pulse Unclamped Inductive Switching Test Circuit..................................................................................... 7 Figure 12 Single Pulse Unclamped Inductive Switching Test Circuit Output Waveforms ..................................................... 7 Figure 13 Decoupled VDD Voltage Source Test Circuit Model ............................................................................................... 7 Figure 14 Decoupled VDD Voltage Source Test Circuit Waveforms ....................................................................................... 7 Figure 15 Typical Simulated Avalanche Waveforms .............................................................................................................. 8 Figure 16 IRFP450, SiHFP450 (500 V Rated) Device Avalanche Waveforms ........................................................................ 8 Figure 17 IRFP32N50K, SiHFP32N50K Datasheet Excerptions ............................................................................................ 8 Figure 18 Transient Thermal Impedance Plot, Junction-to-Case .......................................................................................... 9 Figure 19 Maximum Avalanche Energy vs. Temperature for Various Drain Currents ............................................................ 9 Figure 20 EAR vs. Tstart for Various Duty Cycles, Single ID ................................................................................................... 10 Figure 21 Typical Avalanche Current vs. Pulsewidth for Various Duty Cycles..................................................................... 11 Figure 22 Specification of 40 V/14 A MOSFET Datasheet Excerptions ............................................................................... 11 Figure 23 Typical Effective Transient Thermal Impedance, Junction-to-Ambient ............................................................... 12 Revision: 06-Dec-11 2 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines INTRODUCTION Overview To better understand and utilize power MOSFETs, it is important to explore the theory behind avalanche breakdown and to understand the design and rating of rugged MOSFETs. Several different avalanche ratings are explained and their usefulness and limitations in design is considered. VDS Avalanche Mode Defined All semiconductor devices are rated for a certain max. reverse voltage (BVDSS for power MOSFETs). Operation above this threshold will cause high electric fields in reversed biased p-n junctions. Due to impact ionization, the high electric fields create electron-hole pairs that undergo a multiplication effect leading to increased current. The reverse current flow through the device causes high power dissipation, associated temperature rise, and potential device destruction. VGS ID Fig. 2 - Flyback Converter Switch Under Avalanche Waveform Avalanche Occurrences In Industry Applications Flyback Converter Circuit Some designers do not allow for avalanche operation; instead, a voltage derating is maintained between rated BVDSS and VDD (typically 90 % or less). In such instances, however, it is not uncommon that greater than planned for voltage spikes can occur, so even the best designs may encounter an infrequent avalanche event. One such example, a flyback converter, is shown in figures 1 to 3. Avalanche Operation VGS VDS ID L leakage Fig. 3 - Flyback Converter Switch Under Avalanche Waveform (Detail) APPLICATION NOTE Note • Red (VDS), Blue (ID), Black (VGS) In this application, built in avalanche capability is an additional power MOSFET feature and safeguards against unexpected voltage over-stresses that may occur at the limits of circuit operation. Fig. 1 - Flyback Converter Circuit During MOSFET operation of the flyback converter, energy is stored in the leakage inductor. If the inductor is not properly clamped, during MOSFET turnoff the leakage inductance discharges through the primary switch and may cause avalanche operation as shown in the VDS, ID, and VGS vs. time waveforms in figures 2 and 3. Revision: 06-Dec-11 3 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines AVALANCHE FAILURE MODE In avalanche, the p-n junction acting as a diode no longer blocks voltage. With higher applied voltage a critical field is reached where impact ionization tends to infinity and carrier concentration increases due to avalanche multiplication. Due to the radial field component, the electric field inside the device is most intense at the point where the junction bends. This strong electric field causes maximum current flow in close proximity to the parasitic BJT, as depicted in figure 6 below. The power dissipation increases temperature, thus increasing RB, since silicon resistivity increases with temperature. From Ohm’s Law we know that increasing resistance at constant current creates an increasing voltage drop across the resistor. When the voltage drop is sufficient to forward bias the parasitic BJT, it will turn on with potentially catastrophic results, as control of the switch is lost. Some power semiconductor devices are designed to withstand a certain amount of avalanche current for a limited time and can, therefore, be avalanche rated. Others will fail very quickly after the onset of avalanche. The difference in performance stems from particular device physics, design, and manufacturing. Power MOSFET Device Physics All semiconductor devices contain parasitic components intrinsic to the physical design of the device. In power MOSFETs, these components include capacitors due to displaced charge in the junction between p and n regions, resistors associated with material resistivity, a body diode formed where the p+ body diffusion is made into the nepi-layer, and an NPN (bi-polar junction transistor henceforth called BJT) sequence (BJT) formed where the n+ source contact is diffused. See figure 4 for power MOSFET cross section that incorporates the parasitic components listed above and figure 5 for a complete circuit model of the device. Increasing RB Parasitic BJT being F.B. Max. Current Path E Field Fig. 6 - Power MOSFET Cross Section Under Avalanche Typical modern power MOSFETs have millions of identical trenches, cells or many strips in parallel to form one device, as shown in figure 7. For robust designs, then, avalanche current must be shared among many cells/strips evenly. Failure will then occur randomly in a single cell, at a high temperature. In weak designs, the voltage required to reach breakdown electric field is lower for one device region (group of cells) than for others, so critical temperature will be reached more easily causing the device to fail in one specific area. APPLICATION NOTE Fig. 4 - Power MOSFET Cross Section Fig. 5 - Power MOSFET Circuit Model Revision: 06-Dec-11 4 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines Fig. 7 - Basic Power MOSFET Cell Structure Rugged MOSFETs First introduced in the middle 1980’s, avalanche rugged MOSFETs are designed to avoid turning on the parasitic BJT until very high temperature and/or very high avalanche current occur. This is achieved by: • Reducing the p+ region resistance with higher doping diffusion • Optimizing cell/line layout to minimize the “length” of RB The net effect is a reduction of RB, and thus the voltage drop necessary to forward bias the parasitic BJT will occur at higher current and temperature. APPLICATION NOTE Avalanche rugged MOSFETs are designed to contain no single consistently weak spot, so avalanche occurs uniformly across the device surface until failure occurs randomly in the active area. Utilizing the parallel design of cells, avalanche current is shared among many cells and failure will occur at higher current than for designs with a single weak spot. A power MOSFET which is well designed for ruggedness will only fail when the temperature substantially exceeds rated TJ (max.). Fig. 8 - Power MOSFET Random Device Failure Spots The risk of manufacturing process or fabrication induced “weak cell” parts is always present. The SEM cross-section micrograph on the top shows one such example. The source metal contacts the n+ layer at the near surface, but not the p+ layer. As a result the BJT base is floating and easily triggerable. An example of a good contact is shown on the bottom. The source metal contacts and shorts the n+ layer to the p+ layer thus supressing the parasitic BJT operation. An analysis of various MOSFET devices tested to destruction indicates that failure spots occur randomly in the active area. Some samples are shown in the figure 8: Revision: 06-Dec-11 5 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines A “three legged” approach is used during design: 1. Statistically significant samples of prospective designs are tested to failure at test conditions chosen to reach extremes in temperature and current stress. Representative parts from DOE elements are tested to assure uniform avalanche failure across expected variation of critical process steps. 2. Each design is tested to failure across temperature and inductor (time in avalanche) to assure that failure extrapolates to zero at a temperature well in excess of TJ (max.). (See sample figure 10 of “IAS at failure vs. Tstart” below.) 3. A sample of final design parts are stressed with repetitive avalanche pulses of such a value to raise junction temperature to TJ (max.). This “three legged” solution helps assure that designs are rugged and can be avalanche rated. The following factors are used to provide rugged avalanche MOSFETs: • Improved device design: - To mute the parasitic BJT by reducing RB - To eliminate the effect of weaker cells in particular positions of the layout (i.e. cells along device termination, gate bussing, etc.) • Improved manufacturing process: - To guarantee more uniform cells - To reduce incomplete or malformed cell occurrences • Improved device characterization: - To assure devices fail uniformly across wide range of ID, temperature - To assure device fails at very high (extrapolated) temperature - To assure device is capable of surviving multiple avalanche cycles at the thermal limit • 100 % avalanche stress testing Bad Contact n+ p+ n- Good Contact n+ p+ n- Fig. 9 - Good Source Contact vs. Bad Source Contact Illustration Parts with weak cells such as are shown on the top of figure 9 can be removed from the population by 100 % avalanche (EAS) stress testing during production. IAS at Failure vs. Tstart 300 Inductor Values (mH) 0.05 0.50 1.00 20.00 Linear Linear Linear Linear 200 IAS Fail APPLICATION NOTE 250 150 100 50 0 0 100 200 300 400 500 600 Temperature (°C) Fig. 10 - IAS at Failure vs. Test Temperature Revision: 06-Dec-11 6 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines AVALANCHE TESTING DETAILS Decoupled VDD Voltage Source Vishay performs avalanche stress testing on its power semiconductor devices to assure conformance of new designs with avalanche rating, to validate parts for ruggedness, and to screen production for weak devices. To surpass the limitations of the single pulse unclamped inductive switching test circuit, the decoupled VDD voltage source illustrated in figures 13 and 14 is used. Single Pulse Unclamped Inductive Switching 15 V A single pulse unclamped inductive switching test circuit for avalanche testing that is shown below in figures 11 and 12. This circuit is still referenced in older “legacy” product datasheets. Driver L VDS Rg D.U.T. + A - VDD IAS 20 V tp 0.01 Ω Fig. 13 - Decoupled VDD Voltage Source Test Circuit Model VDS tp IAS Fig. 11 - Single Pulse Unclamped Inductive Switching Test Circuit Fig. 14 - Decoupled VDD Voltage Source Test Circuit Waveforms From the figure 11 schematic we can calculate the single pulse avalanche energy (EAS) as: Here a driver FET and recirculation diode are added so that the voltage drop across the inductor during avalanche is equal to the avalanche voltage. With this circuit (neglecting the angular ESR in the inductor) the energy can be simply calculated as: 2 L x IAS V DS E AS = -------------------------- x --------------------------2 V DS - V DD (1) APPLICATION NOTE The measured energy values depend on the avalanche breakdown voltage, which tends to vary during the discharge period due to the temperature increase. Also note that for low voltage devices VDS - VDD may become quite small, limiting the use of this circuit since it introduces high-test error. 1 2 E AS = --- L x I AS (2) 2 A better and more accurate reading of the avalanche energy can be obtained by measuring instantaneous voltage and current in the device and integrating as described in the following equation: t2 E AS = V(AV)DSS t x IAS t x dt (3) t1 For further reference, figures 15 and 16, depict ideal and actual avalanche waveforms, respectively. Fig. 12 - Single Pulse Unclamped Inductive Switching Test Circuit Output Waveforms Revision: 06-Dec-11 7 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines Gate Voltage IRFP450, SiHFP450 VDD = 50 V L = 7 mH IAS = 14 A TJ = 25 °C Inductor Charging Ipk VAV at 50 μs/div. ID Drain VAV Avalanche Phase Drain Voltage Fig. 15 - Typical Simulated Avalanche Waveforms Fig. 16 - IRFP450, SiHFP450 (500 V Rated) Device Avalanche Waveforms Note that the peak avalanche voltage VAV can be approximated as 1.3 times the device rating, or 650 V. Further note that V(BR)DSS and VAV are used interchangeably in this text. AVALANCHE RATING EAS THERMAL LIMIT APPROACH Generally, there are three approaches to avalanche rating devices: Single Pulse The single pulse avalanche rating (EAS) is based on the assumption that the device is rugged enough to sustain avalanche operation under a wide set of conditions subjection only to not exceeding the maximum allowed junction temperature. Typically, the avalanche rating on the datasheet is the value of the energy that increases the junction temperature from 25 °C to TJ (max.), assuming a constant case temperature of 25 °C and assuming a specified value of ID (usually set at 60 % of ID (25 °C). 1. Thermal Limit Approach: The device is rated to the value(s) of energy, EAS, that causes an increase in junction temperature up to TJ (max.). EAS avalanche rated MOSFETs are rated in this manner. 2. Statistical Approach: Devices are tested up to the failure point. The rating is given using statistical tools (e.g., average (EAS) - 6 ) applied to the failure distribution. Some parts are rated this way and indicated as EAS (tested), generally in addition to the thermally limited rating. However, some MOSFET suppliers provide only this rating on their datasheets. For example, consider the 500 V/32 A device as excerpted from the datasheet below, APPLICATION NOTE 3. No rating at all. While the first two approaches provide a value for avalanche energy, the designer must take care to know the important differences that are outlined below. Fig. 17 - IRFP32N50K, SiHFP32N50K Datasheet Excerptions Revision: 06-Dec-11 8 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines with the following initial conditions: • Single pulse avalanche current: IAS = ID = 32 A • Starting temperature: Tstart = 25 °C • Inductor value: L = 0.87 mH To calculate the temperature increase due to the avalanche power dissipation we utilize a thermal model with Ohm’s Law equivalence. The resulting equation follows: T = ZTH x P AV Now from equation 2 we can calculate 1 2 2 E AS = --- L x I AS = 0.5 x 0.87 mH x 32 = 445 mJ 2 which agrees with the datasheet value within rounding of the least significant digit. The duration of the avalanche power pulse can be calculated, assuming the inductor is discharging with a constant voltage applied to it, as (4) I pk 32 A t AV L x --------- = 0.87 mH x --------------- 43 μs V AV 650 V The average power dissipated during avalanche can be calculated as 1 V AV x I AS x t AV P AV = --- --------------------------------------- = 0.5 x 650 V x 32 A = 10 kW (5) t AV 2 (7) The thermal impedance (ZTH) for this pulsewidth can be read from the transient thermal impedance plot provided with the datasheet, as shown in figure 18. Avalanche voltage can be estimated as V AV 1.3 x BV DSS = 1.3 x 500 V = 650 V (6) 0.012 43 μs Fig. 18 - Transient Thermal Impedance Plot, Junction-to-Case The temperature increase due to avalanche and the final junction temperature can therefore be calculated using equation 4 T = ZTH x P AV = 0.012 x 10 kW = 120 °C (8) APPLICATION NOTE T J = T start + T = 145 °C T J (max.) = 150 °C showing that the datasheet rating is consistent with the calculated TJ (max.) within minor error due to reading ZTH from figure 18. Fig. 19 - Maximum Avalanche Energy vs. Temperature For Various Drain Currents Revision: 06-Dec-11 9 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines Figure 19 is included in datasheets for EAS rated parts and shows many values of EAS for varying starting TJ and ID. Each point along the curves shown represents the energy necessary to raise the temperature to TJ (max.). Note that this curve belies the myth of trying to compare datasheet table EAS values : by varying current and/or temperature the EAS value can vary by a range of 800 x! Specifying EAS at lower ID values results in higher EAS even though the device stress (TJ) is the same. Repetitive Pulse Historically, the repetitive pulse avalanche energy (EAR) was rated at 1/10 000 of PD (25 °C). This practice is now supplanted on newer products by an explicit rating of avalanche operation up to the TJ (max.) condition. Datasheets utilizing this newer rating also include: • EAS: the single pulse rating Fig. 20 - EAR vs. Tstart For Various Duty Cycles, Single ID • ZTH graph: ZTH vs. time for various duty cycles (example in figure 18 preceded by discussion) The IAR graph (see figure 21) shows how the avalanche current varies with the avalanche pulsewidth for various duty cycles, with a “budgeted” increase in junction temperature due to avalanche losses assumed at (T) = 25 °C. An effect similar to that in the EAR graph occurs. In repetitive pulse operation, the junction temperature does not have sufficient time to decrease to the ambient temperature between pulses. As a result, the starting temperature for subsequent pulses will be higher than the ambient temperature. Therefore, a smaller amount of avalanche energy, corresponding to smaller avalanche current, will raise the junction temperature to TJ (max.) for subsequent pulses. So for increasing duty cycles, the avalanche current required to raise the junction temperature by 25 °C will decrease. • EAS graph: EAS vs. Tstart for various ID (example in figure 19 followed by discussion) • EAR graph: EAR vs. Tstart for various duty cycles, single ID (example and discussion to follows) • IAR graph: Typical avalanche current vs. pulsewidth for various duty cycles (example and discussion to follow below) APPLICATION NOTE The EAR graph shows the avalanche energy necessary to raise the junction temperature from the starting temperature to TJ (max.) for various duty cycles, at a given current. A sample EAR graph is given in figure 20. The top curve represents single pulse behavior at 125 A, while the bottom curve represents repetitive pulse operation at 125 A, 10 % duty cycle. In repetitive pulse operation, the junction temperature does not have sufficient time between pulses to return to the ambient level. The larger the duty cycle, the higher the junction temperature will be when the next pulse arrives. Therefore, with increasing duty cycle, the avalanche energy required to raise the junction temperature to TJ (max.) will be lower. Revision: 06-Dec-11 A detailed specific example now follows to illustrate how to design for repetitive avalanche operation for the 40 V/14 A MOSFET (see figure 22). 10 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines Fig. 21 - Typical Avalanche Current vs. Pulsewidth For Various Duty Cycles Fig. 22 - Specification of 40 V/14 A MOSFET Datasheet Excerptions APPLICATION NOTE The initial conditions are: • Ambient temperature: TA = 120 °C • Solenoid inductance: L = 5 mH • Solenoid resistance: RL = 15 • Pulse frequency: f = 125 Hz • Supply voltage: VDD = 14.5 V By applying Kirchoff’s Laws to the fuel injection coil circuit we find i t = I AR e RL -t V AV - V DD - ----L + -------------------------- e -1 RL (10) I AR x R L L t AV = ------ x ln 1 + -------------------------- = RL V AV - V DD (11) 5 mH 0.966 A x 15 = -------------- x ln 1 + ----------------------------------------- = 109 μs 15 52 V - 14.5 V since avalanche voltage can be obtained from measurement (best), or estimated from the 40 V/14 A MOSFET datasheet using equation 6 as dI t V DD = L x ------------ + R L i t + V AV (9) dI Using boundary condition at t = 0, i(t) = IL = IAR, yields the general solution in the time domain: V AV 1.3 x BV DSS = 1.3 x 40 V = 52 V and avalanche current can be calculated as Solving for the avalanche pulsewidth (tav) assuming i(tav) = 0 gives Revision: 06-Dec-11 - R -----L-t L Repetitive avalanche energy can be calculated as 11 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines V DD 14.5 V I L = I AR = -------------------------------- = --------------------------------------- = 0.966 A R L + R DS(on) 15 + 10 m 2 P cond. = I L x R DS(on) x D = (12) (16) 2 = 0.966 A x 10 m x 0.013 = 121 μW I AR x V AV x t AV E AR = ---------------------------------------- = 2 (13) 0.966 A x 52 V x 109 μs ---------------------------------------------------------------= = 2.74 mJ 2 Average avalanche, and conduction power values can be calculated as D = t AV x f = 109 μs x 125 Hz = 0.013 E AR 2.74 mJ P AV = ---------- = --------------------- = 25.1 W, t AV 109 μs (14) T = P AV x Z TH = 25.1 W x 0.18 °C/W = 4.5 °C P ave = E AR x f = 2.74 mJ x 125 Hz = 343 mW, (15) where the thermal impedance (ZTH) is approximated from the transient thermal impedance plot provided with the datasheet, as shown in figure 23. (17) T SS = P ave + P cond. x R + T A = = 343 mW + 121 μW x 50 °C/W + 120 °C = (18) = 137.2 °C The peak rise in junction temperature due to each avalanche pulse is given by since the avalanche duty cycle can be calculated as (19) Note that TSS + T = 137.2 °C + 4.5 °C = 141.7 °C < TJ (max.) The average junction temperature can be calculated as 0.18 109 μs Fig. 23 - Typical Effective Transient Thermal Impedance, Junction-to-Ambient APPLICATION NOTE BUYER BEWARE Many suppliers rate power MOSFET avalanche capability with only a single number in the datasheet and without providing full circuit or test condition details. In such cases, buyer beware! It is not sufficient to merely compare the numeric values of avalanche energy which appear in datasheet tables. The following example will help illustrate one such pitfall. Example 1 Pulse: I AS = 32 A L = 0.87 mH Result: 1 2 E AS = --- x L x I AS = 445 mJ 2 I pk t AV = L x --------------------- 43 μs V (AV)DSS Since avalanche energy depends on the inductor value and starting current, it is possible to have two pulses with the same energy but different shape provide two different junction temperatures. This phenomenon is illustrated in the following examples: Revision: 06-Dec-11 Z TH = 0.012 °C/W T = 120 °C T J = 145 °C < T J (max.) 12 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-1005 www.vishay.com Vishay Siliconix Power MOSFET Avalanche Design Guidelines Example 2 CONCLUSION Pulse: Vishay applies 3 different classes of avalanche rating: I AS = 16 A L = 3.48 mH • The thermal approach allows single pulse and (where indicated) repetitive pulse avalanche operation as long as neither ID (max.) nor rated TJ (max.) are exceeded. Energy losses due to avalanche operation can be analyzed as any other source of power dissipation. Such thermally rated parts are indicated by Vishay with a rating of “EAS” and, more recently, with inclusion of repetitive avalanche SOA graph 9 (for example see figures 20 and 21). Result: 2 1 E AS = --- x L x I AS = 445 mJ 2 I pk t AV = L x --------------------- 86 μs V (AV)DSS Z TH = 0.02 °C/W T = 200 °C T J = 225 °C > T J (max.) • Statistically based avalanche ratings are set based on sample failure statistics. At this rating is labeled “EAS (tested)” and corresponds to a production test screening limit. While the statistical approach generally gives higher energy value, it does not provide a practical method for evaluating avalanche capability in conditions that differ from the datasheet. Since circuit designers’ conditions usually differ significantly, the statistical approach does not give a clear idea on how to design for occurrence of avalanche. Examples 1 and 2 both have the same energy, however, since the inductor varies, so does the junction temperature. While one junction temperature is within TJ (max.), the second is not. Note as well that power MOSFETs which are “EAS” rated include graphs showing constant junction temperature energy values. See for example figure 20, top curve. Which value of energy should be compared with another suppliers power MOSFET? • Some legacy products were designed without an avalanche rating. Devices without an avalanche rating on the datasheet should not be used In circuits which will see avalanche condition during any mode of operation. By special arrangement, most such designs can be avalanche guaranteed; contact factory representative for further information. APPLICATION NOTE Another common industry practice is to rate avalanche capability based on curves showing allowable time in avalanche as a trade-off with drain current. At best, such curves are backed up with test to failure data as seen in figure 10. However, sometimes these curves are based on statistically determined limits without apparent regard for junction temperature. The result is that a thermal TJ calculation (see examples 1 and 2) for the rated allowed condition may show that TJ exceeds TJ (max.), without reliability qualification data at this higher than TJ (max.) condition. Again, buyer beware. Revision: 06-Dec-11 Power MOSFET users should take care to understand differences in avalanche rating conditions between various suppliers. Devices that are not “avalanche robust” can cause unexpected and seemingly unexplained circuit failure. Some manufacturers do not rate their MOSFETs for avalanche at all. Others use a statistical rating alone which does not offer the same assurance for robust operation provided by a more complete characterization and rating. 13 Document Number: 90160 THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000