[princeton]

I thought these o-scope images might be interesting for some of you (especially those whose mosfets keep burning up).

The first image (img_5162#2.jpg) shows my PWM circuit running at 20KHZ running an IRF240 N-channel mosfet through an inductive load (coil of wire wrapped around an iron donut). I also have a resistive load of 4 ohms in series with the inductive coil. Power supply is a 12 volt car battery. The O-scope lead is placed on the drain of the mosfet. The peak voltage of the square wave is 12 volts but the peak voltage caused by inductive feedback and oscillation is close to 90 volts. The Mosfet is rated at 100 volts, so it's not blown.

For the next image (img_5263#2.jpg), I took a capacitor and 10 ohm resistor and connected them in series. Then I took this combination and placed them in parallel with the inductive load (from drain to V+). I tried simply using a capacitor alone in parallel with the inductive load but this caused too much oscillation. Notice how the peak voltage of the inductive feedback at the drain is blunted but significant oscillations remain.

For the next image (img_5264#2.jpg), I took a diode (reverse polarity) used as a "flyback diode" and placed it in parallel with the inductive load (drain to V+). Notice a mild reduction in the peak voltage from the inductive feedback. Also notice the mild reduction in the duration of oscillation.

For the last image (img_5265#2.jpg), I used both the flyback diode and the capacitor/resistor placed in parallel to the inductive load (drain to V+). Notice how almost all of the oscillations have been resolved using the combination of both.

If there's someone that could test this combination on their PWM controller pushing an actual motor (not a simple inductive load), it might help us to find a combination that allows us to use higher voltage batteries while preventing mosfet failure.