On 20 Feb 2005 at 14:05, Wouter van Ooijen wrote:
> I want to drive a modest (1A 24V) motor. The circuit must be potted, so
> heat must be minimal, so I want to use FETs. For simplicity I will use
> N/P fets.

The voltage drop when the FET is "on" is less than the drop across 
the body diode. During the "off" part of the PWM cycle, the freewheel 
current is pushed through the body diodes.

Consequently, the biggest dissipation at 50% duty cycle will be the 
body diodes during the "off" part of the cycle. 

If you reverse the H-bridge for a small percentage(very small!) at 
the beginning of the "off" cycle you can gain back some of that loss, 
because your reverse conduction is through the FET instead of its 
diode. Don't reverse the bridge for very long, and be sure to have 
some "off" time before the reverse, to allow for delays in your H-
bridge and prevent shoot-through.

On another topic, if you use P-channel MOSFETs and run at 24 volts, 
insure your drive voltage is lower than the maximum gate voltage - 
you can find MOSFETs with 24 volt gates, but they are not typical. 
You can add extra parts to clamp the gates, of course.

Cheerful regards,

Bob

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