Mohit (Lists) wrote:
> Hello all,
>
> In a product, we are controlling the speed of a PMDC motor (12V, 80W, 
> 8.3A) by PWM (@ ~2 kHz).
>
> In the setup, we use an IRF4905 MOSFET (International Rectifier, 
> p-channel, -74A, 0.02ohm, -55V) and a freewheeling diode MBR3045 
> (Vishay, Schottky, 30A). The MOSFET has its own internal diode as well. 
> This setup works okay.
>
> However, we would like to replace the MBR3045 with something like an 
> MUR860 (ON Semi, Ultrafast, 8A average, 16A peak repetitive @ 20kHz) 
> diode, since the MBR3045 seems to be an overkill.
>
> What are the parameters that decide the selection of a freewheeling 
> diode? If the motor is rated 8.3A @ 12V, should the FW diode be rated 
> more than this?
>
> Thanks,
> Mohit Mahajan.
>   

Generally speaking - the freewheel diode should be equally capable as
the switch. If you have 50% duty cycle and perhaps 5 amps of current
when the MOSFET is on, you have 5 amps going through the diode the other
50% of the time.

If you take a conservative calculation and say that the MOSFET may need
to handle up to 10 amps, you should assume that the diode will have to
handle the same amount of current at a high duty cycle. In practice this
means that the diode is usually similarly sized (physically) to the MOSFET.

-
Martin K


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