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Driving a 250W motor using PWM
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JoeP:
I've been trying to drive a 250W (24V, 13A) motor from a pwm signal using a mosfet. So far, every time I turn on the circuit, the pwm starts with a low duty cycle, then I slowly turn it up, and then the mosfet seems to blow and the motor whizzes out of control. In the most recent attempt, it worked perfectly when the motor had little load, however when the load was applied, the same thing happened. I'm using a mosfet rated for 50A, and a flyback diode rated for a constant 3A or a 150A spike. My current hypothesis is that the diode isn't properly getting rid of the voltage spike, caused by the fact that I'm using a very low frequency pwm (around 10Hz). In my next attempt, I'm planning to using a frequency of 1kHz.
What I want to know, is whether this is a likely cause of the problem, and is there anything I should be adding to my circuit (such as a smoothing cap) to stop it happening again.
Wolfram:
At very low PWM frequencies, the current is not averaged by the inductance of the motor, so the motor will potentially draw its full stall current when starting from zero speed. A higher PWM frequency will help with this, but the main problem is that you're not controlling the current to the motor. If you want to make a solution that doesn't kill the MOSFET upon a mechanical overload, you need an active current limiting circuit. This can be as simple as a current sensing circuit that terminates the present PWM pulse when the MOSFET current reaches the maximum safe value.

In addition, you need to have a reasonably low impedance loop between the filtering capacitors, the freewheeling diode and the MOSFET. The inductance of this loop determines how much energy the MOSFET has to absorb in the form of turn-off spikes. A picture of your setup will answer more questions than a text description of what you're doing. Also note that the freewheeling diode can end up carrying a considerable fraction of the motor current (it will see an average current of Imotor*(1-(duty cycle)) if the switching frequency is high enough to keep the motor current continous.

Pinkus:
For a quick and dirty solution I would:
1) I would add a bidirectional suppressor diode with 25.6 volt just to make sure that there are no transients etc. killing the Mosfet. For the first test add one directly at the motor and another at the Mosfet.
2) Then a low cost protection variant would be to add a 0.5 or 1 Ohm power resistor into power cable. This will limit the maximum current to a e.g. 24V/0.5 Ohm = 48A as a maximum. There might be a minimal power loss, but you probably will not recognize.
Nice side effect: you can then use the resistor to measure the voltage across it with an oscilloscope - then you know how much power the motor draws.
3) what about cooling of the Mosfet?
chemelec:
What is the Voltage Rating of you Mosfet?
schmitt trigger:
An oscilloscope here is your best friend, and if you have a current probe, an even better.

Monitoring Vds and Ids simultaneously will provide you with extremely useful insight on what really is going on, instead of speculation.

To ensure solid waveforms, trigger from the gate drive output.
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