How to measure BVDSS in flyback circuit

[rcbuck]

I used PI Expert to design a flyback supply, 45 volts, 1 amp. When the design finishes, I get a warning to "verify design on bench and ensure BVDSS on MOSFET is below 700V".

How do I do that? I built the supply and with a 100X probe (scope ungrounded) connected across drain and source, I measure a 348 V Pk-Pk squarewave. This is with the supply putting out 45 volts with a 40 ohm 100 watt resistor load.

Do I have the scope probe connected to the correct points?

 

[KX36]

I'd recommend that instead of removing the ground from your scope, you power the device through an isolation transformer (or use a suitable differential probe if you have one). Across the MOSFET's drain-source, you should see a square wave with an amplitude of Vin+Vout×(Np/Ns) in CCM with a spike on its rising edge cause by the flyback transformer's leakage inductance. If it's DCM, you'll get a second step on the waveform of Vin with some ringing. I think what it's telling you is to make sure the peak voltage of this waveform including the spike is less than the FET's Vds(max).

 

[rcbuck]

Thanks for the reply. I have a 200 watt isolation transformer ordered as I plan to do more testing of SMPS. This is a learning experience for me. I probably won't build anything larger than a 100 watt supply. Coming from the analog (big iron) days it is really interesting to be able to get this much power out of something so light weight.

You are correct about the spike on the rising edge. That spike is about an additional 50 volts. That means my peak voltage is around 400 volts which is below the 700 volts break down of the MOSFET.