> I'm ready to power up an H-bridge made with 4 N-ch FETS
> for motor control. Any advantage, recommendation or advice
> as to whether the PWM is on the top or the bottom or does it
> not matter ?


As a rule the low side drivers are liable to be better for PWMing.
This is because the high side is more liable to have limited bandwidth due
to whatever you use for isolation purposes. If you use opto couplers (as
your circuit does) then the high side is ver likely to be appreciably
slower. Whether this matters depends in part on the PWM frequency.

You can check this easily by running the circuit with a light resistive load
and applying PWM to high and low successively and DC drive to the other
half. Look at the FET gate drive waveforms as you increase the PWM
frequency. The 3 transistor FET drive circuit that you are using will
provide pulses down to around 1 uS in length without too much design effort.
The opto driven high side version us not liable to get anywhere near this
but YMMV.

        RM

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