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Avoiding avalanches

How to determine if a power mosfet can stand the heat. By Sungmo Young.

Power mosfets are popular switching devices in modern power supplies. Responding to demands for smaller devices with higher power densities, manufacturers are increasing mosfet operation frequencies. This feature causes high di/dt, intensifies the negative effect from parasitic inductances and results in high voltage spikes between the power mosfet drain and source during device turn off.
The spike is worst at ‘power on’ due to the bulk capacitors being empty and because of the transformer’s primary side inductance almost reaches the level of leakage inductance. Fortunately, the power mosfet is equipped to withstand a certain level of stress, eliminating the need for expensive protection circuits. So how can you determine the applicability of a power mosfet in switching power applications?
Firstly, let’s clarify the meaning of ‘avalanche breakdown’. Over voltage in actual applications fall into two groups. One group is when the power mosfet’s drain-source voltage exceeds the specified absolute maximum rating, but is still short of the device’s breakdown voltage.This is not an avalanche breakdown and the device’s feasibility can be determined through junction temperature analysis.

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Author
Vanessa Knivett

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