Re: Need Advice on PWM and other stuff

On May 26, 3:55 am, John Nagle <n...@xxxxxxxxxxx> wrote:
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


Hello John,

Thank you for the response. I should have specified, I am using the
ts555cn, which according to STMicro has a rise time of 25 ns.
Yesterday, I did just what you suggested and measured an actual rise
time of 40 ns. I didn't think too much of the discrepancy at the time.
Is this significant? Although, now that I read the data *** again,
it says 'trigger propagation delay is 100 ns" Is this what you meant?
It (the ts555) is connected to the 12V power supply the whole time.

Now you gave me an idea, my power mosfet (the IRF 520) has a rise time
of 23ns at 50V, but the number I quoted you above for the ts555 is
into a 30pf load, where the input capacitance of the mosfet is 330
pf.

BTW, the microchip link you gave me above just timed out. I did not
receive the pdf yet. Is there a part number you were referring me to?
I will try the link again, but just in case, if you do not mind,
perhaps I would have better results looking for the part number.

Oh, I am whacking the gate of the mosfet with 12 V from the ts555cn.
I noticed that using 5V resulted in unacceptable heating as well.
Experimenting showed that any voltage above about 7V resulted in
significant less heating of the mosfet. I only wanted to have 2
voltage sources for my electronics, 12 V for the eventual SONAR, and
5V for the micro and other electronics. But that is still flexible.

I'm still stuck on this but I will try the link you gave me once I log
off this forum to see what that's all about. Oh, and while I am at it,
since I did not specify enough info in my first post. I am using the
20L15T Schottky diode b tween the drain and source of the mosfet. Do I
really need something that hefty? They are expensive, but I didn't
want to take any chance on blowing mosfets due to the inductive
kickback.

Thank you again for the info,

Joe



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