Transient Suppression Results

This experiment shows that a small RC snubber connected across the mosfet switching device can eliminate the turn off ringing. The oscilloscope trace for an unsuppressed clamped coil B is shown in fig 2.

Fig 2. Unsuppressed turn off transient.

Clearly the ringing is very pronounced and could interfere with circuitry. The effect of the addition of a RC snubber can be seen in fig 3.

Fig 3. Turn off ringing almost eliminated.

Reducing the size of the resistor and increasing the value of the capacitor completely removes the ringing as shown in fig 4.

Fig 4. Ringing eliminated by snubber.

Although this small snubber can remove the ringing we are still left with the large overshoot. In fact the snubber appears to have no significant effect on the shape of the overshoot. In order to reduce the overshoot we would require a much larger snubber and this could impact the switching time for the mosfet. The overshoot is most likely a consequence of the slow recovery of the rectifier type commutating diode. Using a 'fast' diode may improve the situation.

This overshoot is very short and is well within the avalanche capability of the mosfet module, however, the waveform still contains sharp voltage transitions which could be capacitively coupled into sensitive circuitry.