I'm designing a speed controller for a mid size dc motor. I've got the

Mosfet(s) selected and power generation worked out. The flyback diode

I'm trying to size will be on the same heat sink as the mosfet.

I need some help with the following:

How can I choose the right size diode? (current rating)

Power dissipated by the diode?

Details:

Low side drive (mosfet sorce to ground)

24 Volt

40 amp max current

Details for the motor or unknown at this time.

thanks

jim

So...I suppose the "right" answer will depend not only on the motor

but on the load on the motor as well. Assuming an electrical model

with a moderate amount of inductance, whatever the inductor peak

current is, is what the diode peak current will be, not accounting for

high dv/dt and charge being swept out of the diode as it reverses.

With an infinite inductance, the current is constant at whatever the

motor draws, and the average diode current will just be that current

times the percentage of time the diode is conducting. If you're

supplying 24V to the motor, the diode conducts zero percent of the

time, but if you are driving a heavy load at low speed, so the motor

current is 40 amps when the motor voltage is 1 volt, then the diode

will be conducting most of the time. Diode dissipation is just the

average over one cycle of the instantaneous diode current times the

instantaneous diode voltage drop. LTSpice can simulate this sort of

stuff quite nicely for you. And you may find that you can save

considerable power if you use a half-H-bridge, with a mosfet to the +

side, too, though you need a very low on-resistance mosfet to make

that work at 40 amps. Oh, and beware of conditions of regenerative

braking, where the motor delivers power back to the controller.

Cheers,

Tom