The MOSFET is on the power board due to the bus voltage, transformer reflection voltage, leakage inductance spike voltage and other system voltages superimposed between the MOSFET drain source, resulting in a failure mode, that is, MOS tube avalanche failure. In short, it is a common failure mode caused by the voltage of the MOSFET drain source exceeding its specified voltage value and reaching a certain energy limit.

Maybe we often ask the device manufacturer to perform failure analysis on the MOSFET on our power board. Most manufacturers only give a conclusion such as EAS.EOS. So how do we distinguish whether it is avalanche failure? The picture below shows the equivalent schematic of the avalanche test. As a power supply engineer, you can easily understand.

So, how to prevent MOS tube avalanche failure?

Avalanche failure is ultimately a voltage failure, so prevent us from focusing on voltage. Specifically, it can be handled in the following manner.

1: Reasonable transformer reflection voltage.

2: Reasonable RCD and TVS absorption circuit design.

3: Reasonable derating, the current derating in the industry generally selects 80%-95% derating, the specific situation is selected according to the company's warranty terms and circuit concerns.

4: Large current wiring should adopt coarse and short layout structure as much as possible to minimize the parasitic inductance of the wiring.

5: Select a reasonable gate resistance Rg.

6: In high-power power supply, RC damping or Zener diode can be added as needed to absorb.