A very nice explanation of driving DC motors with PWM.

My question is - in the high frequency case (snipped below) - what
direction is the current flowing through the short circuit? As I read it,
if the current is a result of motor inductance, rather than back emf,  a
diode should be able to handle it without the danger of entering the
breaking region. (I.e. it becomes a simple buck regulator using the motor
inductance).

Any current resulting from back emf would be in the opposide direction and
therefore not pass through the diode.

A separate breaking transistor could then be used if this mode is required
- but special switching arrangements would not be needed provided drive &
break were not applied together.

Richard P





It isn't very relevant to running a little tiny 10mA motor but IMHO, it is
a mistake to assume that 400 to 2000Hz is a good typical PWM frequency.

(WARNING: here comes a long explanation :-)

snip....

The other mode of PWM operation is high speed mode, where your PWM period
is so short that the RL time constant tends to keep the motor current
constant during each PWM cycle.  Here, you connect the motor to the supply
during the ON time but SHORT it during the OFF time.

This rubs a lot of people the wrong way because they realize that normally,
shorting a motor causes breaking (the back EMF acts to put current through
in the direction that causes torque opposite the direction of rotation).
However, what they don't realize is that when you suddenly short a motor,
you actually get positive torque for a very short period of time (due to
the stored energy in the inductance being used very efficiently to drive
the motor) followed by breaking. If you are using a high enough PWM
frequency, then the breaking never happens (unless you are at a lower PWM
duty than you should be for the current speed and load torque).

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