At 100 volts peak or 200volts PeakPeak, or 200/2.828 ~~ 60 volts RMS, for 600 watts you have 10amps RMS or 30 amps PeakPeak. The risetime of 100 nanoseconds provides a slewrate of 300 amps per microsecond. This produces intense magnetic field all around your PCB and +100v power leads (I see no bypass caps, to stabilize that +100 volts) and output "sin" waveform (through those 2 thick black wires running to the big capacitors).

Can you trust what the scope shows you? 

Vinduce = [MUo * MUr * Area/(2 * pi * Distance)] * dI/dT

which we re-arrange to find

Vinduce = 2e-7 * Area/Distance * dI/dT

Now assume loop area is 0.1meter * 0.1 meter (4" by 4"); have lots of those in your circuit.

Assume distance is 0.1meter (either between wires and loops in your circuit, or between wires and the loop of your scopeprobe/GNDwire).

What voltage is induced? I don't know yet. We must run the numbers.

Vinduce = 2e-7 * 0.1m * 0.1m / 0.1m * 300 e+6

Vinduce = 2e-7 * 0.1 * 300 e+6 = 60 e-7 e+6 = 60e-1 = 6 volts.

Thus your current surges induce 6 volts into any 4" by 4" region of your PCB+wiring. Or perhaps 12v, because of "S" shape of typical risetime waveforms.

You should expect a 12 volt artifact imposed on any waveform, unless your probing methods are real-good, real low-area. Note this is at 4" distance.

And that high-frequency gate ringing? At 20MHz. What path supports that? For 20MHz, the Luh*Cpf is 25,330/(20 * 20) = 25,330/400 = 60 Luh*Cpf. At 10nH, C will be 6,000pF.