I am switching a 1 Ohm load with an N-channel MOSFET, which is driven by a TC4420 driver. The load is three Arcol HS50 330 mOhm resistors in series, the power supply is a 3S LiPo battery. The switching frequency is 1kHz with 10% duty.
I am trying to understand what is happening in the circuit when the FET is switching off. I started with this:
This is the waveform in this case. Channel 2 (blue) is just the signal generator output, Channel 1 (yellow) is Vds, channel 4 (green) is the battery voltage, measured on the main battery wires (if I measure it through the balancer connector, the shape of the waveform is still the same, with smaller amplitude):
First I thought the huge peak around 55V is the avalanche breakdown of the FET, due to the load and the wiring having some inductance, followed by some ringing for the same reason plus the FET output capacitance. But then why is there that big rise on the battery voltage? This implies to me that current is flowing into the battery, which I really do not understand.
Then I tried adding a flyback diode:
Which resulted in a bigger battery voltage rise, but not much otherwise:
To eliminate the ringing, I added a snubber circuit (without the previous flywheel diode):
This was quite effective regarding the ringing:
So I tried with both the snubber and the diode:
With the same additional results as before (bigger battery voltage rise, and nothing else):
So my question is basically what is happening here? More specifically:
- What causes the voltage rise on the battery, when I do not expect the current flowing back into the battery?
- Why does the avalance happen regardless the presence of the flywheel diode? (Maybe this is some other phenomenon, not avalanche?)