On Fri, 8 Feb 2002, Mitch Miller wrote:
> Perhaps I missed part of this thread ... what does doing the following
> acomplish?

When you are doing PWM with an H-bridge, you turn on a positive transistor
and a negative transistor, then turn them off, then repeat.

Your motor has energy stored because of inductance, it releases that when
the transistors turn off. That energy must be sent back to the battery(or
else wasted).

In a MOSFET, it has a backward diode and that conducts the energy back to
the battery. That's great, except it's a typical .6 to 1 volt loss.

At 50% duty cycle, there's a lot of loss because of those diodes.

Spehro suggested turning the opposite pair of transistors on during the
off period. Unfortunately in reality it leads to more loss than letting
the diodes do their job. Unless you get really fancy and know just how
long to turn them on for. Which varies depending on the motor load.

Perhaps there would be a benefit of doing it even for a tiny speck of
time, when you know the current will be greatest, and then switching them
off and letting the diodes do the rest. Maybe I'll try that some time.

You can also just toggle from one polarity on to the other polarity on,
but from zero to low speed the losses are gigantic, huge, undesirable.

Cheers,

Bob

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