Avalanche phenomenon in the flyback converter

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Avalanche phenomenon in the flyback converter

Basically, the application designers don't allow the avalanche operation in a MOSFET

device; instead, the voltage across the drain-source is maintained at around 80-90% of

the flyback converter.

Figure 2. Flyback circuit schematic

In the flyback converter, the basic issue is the presence of various leakage inductance

points in the transformer. If the inductor energy is not properly clamped, during MOSFET

absolute maximum rating of the MOSFET device. In particular when the MOSFET turns off,

the magnetic fields collapse and the voltage across the inductances reverses because the

current cannot interrupt suddenly. Since the leakage inductance doesn't participate in this

energy transfer, it cannot find a circulating path and thus generates a large positive spike on

avalanche energy needs to be monitored.

Below is a typical waveform of the avalanche phenomenon occurring in a flyback converter

during the start up phase.