AN-1005
Rev. 1.0 POWER MOSFET AVALANCHE
DESIGN GUIDELINESecp
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Application Note
Tim McDonald Marco Soldano Anthony Murray Teodor Avram
TABLE OF CONTENTS
Table of Figures 3
Introduction 4 Overview 4
Avalanche Mode Defined 4
Avalanche Occurrences In Industry Applications 4
Flyback Converter Example 4
Automotive Fuel Injector Coil Example 5
Avalanche Failure Mode 5
Power MOSFET Device Physics 5
Rugged MOSFETs 6
Avalanche Testing Details 8
Single Pul Unclamped Inductive Switching 8
Decoupled VDD Voltage Source 9
Avalanche Rating 10
E AS Thermal Limit Approach 10
Single Pul Example 10
Repetitive Pul 12
Statistical Approach 15
Buyer Beware 16kasimir>louis armstrong
Conclusion 16
TABLE OF FIGURES
shero是什么意思Figure 1 Flyback Converter Circuit 4 Figure 2 Flyback Converter Switch Under Avalanche Waveform 4 Figure 3 Flyback Converter Switch Under Avalanche Waveform (Detail) 4 Figure 4 Automotive Injector Coil Circuit 5 Figure 5 Automotive Injection Coil In Avalanche Waveforms 5 Figure 6 Power MOSFET Cross Section 5 Figure 7 Power MOSFET Circuit Model 5 Figure 8 Power MOSFET Cross Section Under Avalanche 6 Figure 9 Basic HEXFET Structure 6 Figure 10 Power MOSFET Random Device Failure Spots 7 Figure 11 Good Source Contact Vs. Bad Source Contact Illustration 7 Figure 12 I A at Failure vs. Test Temperature 8 Figure 13 Single Pul Unclamped Inductive Switching Test Circuit 8 Figure 14 Single Pul Unclamped Inductive Switching Test Circuit Output Waveforms 9 Figure 15 Decoupled V DD Voltage Source Test Circuit Model 9 Figure 16 Decoupled V DD Voltage Source Test Circuit Waveforms 9 Figure 17 Typical Simulated Avalanche Waveforms 10 Figure 18 IRFP450 (500V Rated) Device Avalanche Waveforms 10 Figure 19 IRFP32N50K Data Sheet Excerp
tions 11 Figure 20 Transient Thermal Impedance Plot, Junction to Ca 11 Figure 21 Maximum Avalanche Energy Vs. Temperature for Various Drain Currents 12 Figure 22 IRF7484 E AR Vs. T START for Various Duty Cycles, Single I D 13 Figure 23 IRF7484 Typical Avalanche Current Vs. Pulwidth for Various Duty Cycles 13 Figure 24 IRF7484 Data Sheet Excerptions 14 Figure 25 IRF7484 Typical Effective Transient Thermal Impedance, Junction-to-Ambient15 Figure 26 Avalanche Current Vs. Time for Various Inductor Values 15 Figure 27 Failure Energy Distribution 15
INTRODUCTION
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International Rectifier has provided rugged Power MOSFET miconductor devices for almost 20 years. To better understand and utilize IR HEXFET TM 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 ufulness and limitations in design is considered.
Avalanche Mode Defined
All miconductor devices are rated for a certain max rever voltage (BV DSS for Power MOSFETs). Operation above this threshold will cau high electric fields in reverd biad p-n junctions. Due to impact ionization, the high electric fields create electron-hole pairs that undergo a multiplication effect leading to incread current. The rever current flow through the device caus high power dissipation, associated temperature ri, and potential device destruction.
Avalanche Occurrences In Industry Applications
Flyback Converter Example
Some designers do not allow for avalanche operation; instead, a voltage derating is maintained between rated BV DSS and V DD (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 1-3.
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
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through the primary switch and may cau avalanche operation as shown in the V DS, I D, and V
GS
versus time waveforms in Figures 2 and 3. Waveform
Waveform (Detail)
Note: Red (V DS),Blue (I D), Black (V GS)
In this application, built in avalanche capability is an additional Power MOSFET feature and safeguards against unexpected voltage over-stress that may occur at the limits of circuit operation.
Automotive Fuel Injector Coil Example
Other applications, such as automotive fuel injection, are designed to experience avalanche. See the example Injector Coil circuit below.
During switch operation, energy is stored in the solenoid inductance. Following switch turnoff, the inductor discharges on the primary switch causing avalanche operation as simulated in Figure 5.
Figure 5: Automotive Injection Coil In Avalanche
Waveforms
In this application, avalanche tested and rated devices are a necessity for reliable circuit operation.
AVALANCHE FAILURE MODE
Some power miconductor 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 ont of avalanche. The difference in performance stems from particular device physics, design, and manufacturing.girlfriend什么意思
Power MOSFET Device Physics
All miconductor devices contain parasitic components intrinsic to the physical design of the device. In Power MOSFETs, the 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 n- epi-layer, and an NPN (bi-polar junction transistor henceforth called BJT) quence (BJT) formed where the n+ source contact is diffud.
See Figure 6 for Power MOSFET cross ction that incorporates the parasitic components listed above and Figure 7 for a complete circuit model of the device.
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Figure 7: Power MOSFET Circuit Model答案的英文
