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MOSFET intrinsic diode vs. Schottky (PWM motor circuit)

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rhodges:
I am testing a PWM motor control circuit. The motor is DC 12V and about 260mA.

My first thought was maybe a 2N4401 could handle that. I fed the 5V PWM signal to a 510 ohm resistor to base, common emitter. I put a reversed Schottky on the motor and a 0.56 uF as a snubber. I believe I read on a thread that the motor acts like an inductor, so it should have the diode to protect the transistor (just like on a relay.)

The 2N4401 ran too hot, probably because it spent some time in linear mode due to the 0.56 uF snubber. I could see it on my scope quite clearly.

I got rid of the 2N4401 and replaced it with a logic level MOSFET (IRLZ24). Much better, but I did not like the time in linear mode. After removing the capacitor, the traces cleaned up nicely. There is some ringing, but I don't see it needs a capacitor.

First question: Since the IRLZ24 has an intrinsic diode, should I remove the Schottky? The PWM is 10khz, by the way. The datasheet states diode current of 18A at 1.5 volts, so it should be able to take everything the motor can push, and it should be fast enough.

Also, my "gut feeling" is that I should have a capacitor as a snubber, even though my scope suggests it is just fine. Maybe a very small one? Or no?

Sadly, it looks like I lost my Tek 2465 while testing this circuit. I smelled "magic smoke" and thought it was my circuit under test. By the time I decided the circuit was fine, smoke was coming out of the 2465 case :(

Thanks!

Benta:
You'll have some stray inductance, so a reverse diode across the motor is a good idea. The rest will be taken care of by the intrinsic diode.
What you normally need are noise caps on the motor due to brush fire/arcing. For a small motor like yours, I'd suggest 10...22 nF across the motor terminals, and 2.2...4.7 nF from each terminal to motor case.

I strongly suggest that you reduce the PWM frequency. At 10 kHz you'll have quite a lot of magnetic losses in the motor, limit is usually around 3 kHz.



langwadt:

--- Quote from: blueskull on July 04, 2019, 07:05:41 pm ---0.56uF at 10kHz is insane.

Your snubber is WAY too large. You are forcibly charging a cap every time you turn on your FET.

Also, keep the schottky, some random 1N5819 or PMEGxxxx should work.

Normally should shouldn't see reverse voltage on your motor due tp the inertia of the rotor keeping voltage potential on the terminals, but if your motor hits a bump and had a sudden stop, the reverse EMF can and need a path to freewheel.

--- End quote ---


also need a path to freewheel in the off part of the pwm cycle, https://www.4qd.co.uk/4qd-diags/tec/chop.gif

rhodges:
Great, thanks for the advice! The Schottky is MBR745 (7.5 volt amps). It was what I had. The IRLZ24 is what I had.

Adding 0.01 uF capacitor actually increases the ringing undershoot (switching on) from about -0.7 volts to -1.8 volts. And the ringing time increases from about 0.2 uSec to about 1 uSec.

I will leave the Schottky in and I will think about the 0.01 uF cap.

This is a personal project, by the way. Price is not an issue.

Thanks!

strawberry:
use ~10R loss resistor in series with capacitor to surpress ringing

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