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I intend to use this high side switch in a PWM application with a brushed 12V motor drawing 8A steady state max: https://www.nxp.com/docs/en/data-sheet/MC33981.pdf

The switch allows for a half bridge configuration, where an external low side MOSFET is placed across the load as show in the diagram below:

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The document states the following:

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I do not understand why decreased power dissipation is desirable in this application, other than for thermal reasons.

Using a diode, the energy stored in the inductive load would be dissipated more quickly when the high side is removed. I'm not sure how having residual current flowing through the inductor before the next on cycle of the PWM would affect the system, but I assumed it was not desirable.

So why, in this case, would a mosfet be a desirable substitute?

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In a switching converter this is desirable because it means that it lets the current in the inductor never fall to zero which lets you run in continuous mode instead of discontinuous mode which produces less ripple and noise (I think...it's something like that).

In a motor this is desirable because you don't throw away all the energy put into building up the magnetic field at the end of every PWM cycle only to use more energy to build it back up again the next PWM cycle. It's a bit like constantly stopping and accelerating your car for no reason to maintain an average speed. It wastes gas. It's more efficient just to go at a constant speed in between the two extremes because you're not throwing away energy.

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You typically choose a PWM frequency high enough to limit the ripple in the current through the motor to prevent torque oscillation and audible noise, and so the current during the off period won't decay to zero except at very low duty cycles. At 50% duty cycle, the motor will be running at about half full speed, and the current through the freewheeling diode will be equal to the motor current for half the time - so if you're passing 8A, you'd dissipate 3-4W with a 0.8 - 1.0V forward drop across the diode - enough to want to reduce the dissipation.

That 3-4W represents energy being dissipated from the field in the motor decaying, and if you can substantially reduce that, you reduce the rate of decay of current through the off period, so the on period can be shorter to regain the loss and maintain the same average speed - so the efficiency of the motor and drive improves. This improvement is most noticeable at low speeds (low PWM duty cycles) and reduces to zero at full speed. For battery powered equipment this gain in efficiency is worthwhile.

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