Fast switching in mosfet

[Murugesh_89]

Hi,
I saw a circuit attached below.



How the zener diode clamping network helps in achieving faster switching and controlling the peak reverse voltage. Also what value of zener diode to be placed there?

I know the purpose of free wheeling diode and seen many circuits using this diode. But why they have added a resistor in series with free wheeling diode?

Thanks,
Murugesh

 

[crutschow]

Normally turning a motor off rapidly is not a concern, but if you need rapid turn off, then a resistor in series with the diode clamp or a zener clamp will more rapidly dissipate the inductive and rotational energy in the motor. If you have only the diode then most of the energy is dissipated in the motor itself, which takes longer.

The value of the zener should be such that the sum of the supply voltage and the zener voltage is less than the max voltage rating of the MOSFET with perhaps a 25% margin.

The value of the resistor should also be such that the maximum motor current through the resistor gives an IR resistor voltage that, added to the supply voltage, will also be less than the MOSFET Vmax with a suitable margin.