[ebarker]

At risk of showing my age.....

Think of a MOSFET as being a Voltage Controlled Resistor.

The MOSFET's 'transfer function' sez you apply a voltage to the gate. That will control the resistance of the MOSFET, somewhere between completely open and completely closed.

BTW, a vacuum tube has a transfer function a lot like a MOSFET.

In your application you are operating this thing complete like an on/off switch and just varying the duty cycle. When the switch is complete on, the resistance doesn't go completely to zero, but a very low value. The better the MOSFET, the lower the value.

If you motor stalled or shorted, or (even if it isn't shorted) there is a voltage drop, and I squared R loss across the device, that turns into heat. It you can dissipate the heat and stay within the operating limits of the device then it will be fine. You just have to move the heat off the silicon.

That is pretty much the trick for reliability. Keep all you signals, spikes, heat loads, etc well withing the device parameters. Just like you would design a building or a bridge (no pun intended).

FYI,

MOSFETs have a 'transfer function'. It is a mathematical thing (often times graphed) that shows the relation between the input and output of the device.

If you operate this thing over it's 'linear region' you can build an amplifier. Nearly all guitar, PA, stereo amplifiers are built around mosfets. Broadcast transmitters now use mosfets in all stages on most transmitters or drive final tube stages on the largest transmitters.

Years ago mosfets (the gates failed) were pretty fragile but modern mosfets are amazingly tough. These broadcast transmitters have to be because they get really banged around with those big lightning seeking iron sticks in the backyard.

Ed