Electronic – Why do the push-pull driver drains ring so much

mosfetpowerswitch-mode-power-supply

I have read What is killing my MOSFETs which seems to present a similar circuit to mine (my secondary is center tapped as well and has 2 high-speed diodes rectifying into a 10R / 400uF load)

The transformer is 12:1, my power supply voltage is between 10v and 25v at ~300mA.

The transistors are heating due to what I believe is avalanche breakdown. I have used 50V devices and the scope shot shows ~200V devices. In each case, DS voltage rings up to breakdown (if there is sufficient energy in the circuit). I would like to push 10 and ideally 100W through this circuit. I realize the breadboard is not feasible for a 100W design, but it should do 10.

The ringing is at 2.x MHz. The power supply input capacitors are not low-esr or particularly high valued.

Schematic
Photo
Scope shot

Best Answer

It is because of the center tap. Look at the left part of the transformer only.

You have two inductors in series. When you pull one inductor to ground a current starts to flow and the other (magnetically coupled) inductor will try to induce the same current, pushing the other transistor's drain voltage up until it breaks down.