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MOS Fets-probably a stupid question


Charles Jean

Jan 1, 1970
This just occurred to me, and it's probably a stupid question, but
I've got to ask:

I've seen lots of circuits for DC motor speed control using the PWM
principle and a MOS FET to control the DC current through a motor.
The gate of the FET is modulated by a square wave of variable duty
cycle. All of the diagrams I have seen about the construction of a
MOS FET seem to indicate a device that is symmetric on the source and
drain ends about the gate electrode. If that is really the case,
could one switch source and drain connection ends in a such a circuit
and still have the same operational characteristics?
If so, could the same arrangement be used to control an AC current
through a load?

Jerry Greenberg

Jan 1, 1970
This is not a stupid question, but a learning one. I would have a hard
time to explain this simply in an email.

To be very simplistic, the device has to be properly heatsinked.
Since the drain is on the thermo coupling side to the heatsink, and
the source is not, if you reverse the device, the device will overheat
during high current conditions.

If you reverse the source drain, in any case the gate bais will have
to follow accordingly, and so will any of the polarized associated
components in the circuit. This is why you cannot use AC directly on
the FET device. Also, any caps that are used, will have to follow very
quickly, if they are in any type of bypass or decoupling

The stupid question is the one you did not ask. Just because a lot of
people may know an answer, this does not mean that everyone knows it!

Jerry Greenberg