Dave Tweed wrote:
> But the premise was that the battery couldn't supply the peak current
> directly, so you'll need a VERY hefty capacitor in parallel with it
> (or better yet, an L-C low-pass filter between the battery and the
> switch) in order to handle the ripple current.

Yes, but that depends on whether he meant instantaneous peak, like a few
microseconds, or peak as in max current the motor could draw under load.

If the battery and possibly cap in parallel with it can't supply the
instantaneous peak currents required for each switching pulse, you're
screwed either way.  Whether you drive the motor directly or put a
intermediate buck converter in there, the pulses will look about the same t=
o
the battery.  A good portion of the current out the buck converter or thru
the motor windings will come from ground via the catch diode.  The average
current from the battery will therefore be lower, but the peak output can't
exceed the peak input current per pulse.

Either way, we are making the assumption that the battery/cap combination
can produce the peak output current for a few microseconds at a time during
the on phase of the buck pulse.  The average current over a whole buck puls=
e
will be significantly less, which is all the battery will see over any
period of a few 10s of microseconds or more.


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