Electronics > Projects, Designs, and Technical Stuff
Driving a 250W motor using PWM
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JoeP:

--- Quote from: rstofer on October 31, 2018, 03:52:08 pm ---You didn't say which MOSFET you were using or how you were driving the gate.  It matters...

If the gate is slow to turn on and off (like a standard MOSFET driven by a uC), the MOSFET heats up during the turn-on and turn-off interval.  Think of current as a trapazoid, slow turn-on, slow turn-off.  When your PWM frequency gets high enough, the MOSFET never actually turns on, it spends all its time in the transition region where it acts as a heat generating resistor.

Ordinarily, you would want the PWM frequency to be above the hearing level of 20 kHz.  The MOSFET may not like this high value - think trapazoid.  On the low end, 1 kHz might work well.  10 Hz is toggle switch range and the MOSFET sees full inrush at every turn-on.

There is a reason that they invented MOSFET drivers.  You really want to be able to dump AMPS into the gate if you want to switch in a short period of time.  Be sure to read the datasheet if you buy 'logic level' MOSFETs.  There are some gotcha's related to Vgs and it isn't necessarily true that a uC can drive the gate in a short period of time.  It's no big deal if the MOSFET is an on-off switch, it matter a lot if the application is PWM.


--- End quote ---

The mosfet in question is this one: https://www.rapidonline.com/stp55nf06-mosfet-n-60v-55a-47-0530
I'm driving it with a BJT, to boost the voltage up from the MCU logic output to 8-9V. I don't think the MOSFET is spending much time in the transition region, as it never got hot when in use. That said, I was using a 10Hz signal, so I can't be sure. I'll double check with an oscilloscope when I get round to replacing the broken mosfet with a better one (for which I'm thinking this will work well: https://docs-emea.rs-online.com/webdocs/0dcb/0900766b80dcb3bb.pdf).
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