Re: Need Advice on PWM and other stuff

Joe wrote:
My question is, what frequency PWM?. All the above tests were
conducted with the motor connected directly to the battery. I have
heard that anywhere from 60 Hz to 100Khz is acceptable, but I tried
first (on the bench, ie no load) 1Khz and the power mosfet was
dissipating too much heat (like finger burning heat). At 10Khz it is
much better (ie, much less heat dissipation).

The frequency isn't the problem, if increasing the frequency
reduces heat dissipation. It's the waveform. You're probably driving the
MOSFETS with a waveform that has too long a rise time. If
you're getting overheating like that, you're spending too much
time in the linear range of the device. Put a scope on the inputs
and measure the risetime of the MOSFET's control signal. It should
be well below 1us. Commercial MOSFET gate drivers have rise
times like 35ns. See

http://ww1.microchip.com/downloads/en/DeviceDoc/21933a.pdf

for some commercial drivers.

A 555 ought to have an output rise time of about 125ns, so you
should be seeing rise times like that at the MOSFET gate.

Also note that the output of a 555 is not rail to rail. Are you
getting enough voltage swing at the MOSFET gate?

There are also issues with driving an inductive load like a motor,
but that's less likely to be the problem here. That's a problem
which becomes worse as the switching frequency increases.

The real issues with switching frequency is that if it's too low,
the motor vibrates, and if it's too high, the motor's inductance
limits current into the motor.

John Nagle


.

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