I have built a circuit, which consists of a 9V AC transformer, feeding a bridge rectifier and the current to the load (a length of resistance wire ~3Ω) controlled by a MOSFET. The gate of the MOSFET receives a PWM signal from an Arduino control module which allows the duty cycle to be varied from 0% - 100%.
NOTE The MOSFET is intended to chop the full wave rectified AC voltage.
The circuit works, and has been in use for a year.
I decided to add a circuit, consisting of a diode and filter capacitor to derive a DC power for the Arduino, but several electrolytics went up in smoke, and the output voltage considerably exceeded the 13V I was expecting (so it ended up being removed).
I recently bought a BitScope and was using this to circuit.
When at 0% to voltage is the expected full wave rectified DC.
It appears that when the MOSFET turns off the falling current generates a spike. As there is no other component with any inductance, this seems to be coming from the transformer itself. This was unexpected, as I am working well below the current rating of the transformer, at a much higher frequency than it would normally run. I modified the control circuit to produce 4kHz PWM, which reduced the amplitude of the spikes, but they are still of concern.
I had expected some ripple, but not the massive spikes, which I am concerned may exceed the 60V rating of the MOSFET.
My question is, can anyone suggest some way of suppressing these. None of the normal techniques for suppressing inductive pulses from the load seem applicable.