Due to the forum bug I had to cut off the full title "MOSFET body diode not dissipating negative voltage?"

The output stage of the circuit I'm working on (schematic) is a Class E amplifier, with the primary coil, 47uH inductor, and drain-source cap forming an LCR tank. If I understand correctly, when the MOSFET (IRFP450) switches off, the residual voltage rings back and forth through the LC tank, at the same time being damped by the load (primary coil). When properly tuned, the first time the circuit rings down to zero, the MOSFET turns back on.

According to this page, an "underdamped" circuit which attempts to swing below zero will simply dissipate the negative portion through the MOSFET's body diode.

So I wound a toilet-paper roll full of 30AWG wire, expecting to cut off wire as needed to tune the circuit, and 8 turns for the primary. However in this setup the drain-source waveform rings back and forth several times before the MOSFET turns back on (not unexpected), but half of the ringing voltage is below zero (waveform attached)! How is this possible? I thought the body diode was supposed to keep everything at zero or above.

 

Can't read the scale on that photo, but I can say a diode will allow up to about .6 volts below zero.

 

The mosfet diode is just an artifact of the production process. It should not be counted on to be of a specific value.

 

it could also be that the diode will not respond to that high a frequency.