Robert Nansel wrote:

> Are you using N-Channel MOSFETs for switches in the H-bridge?
> Including the "upper" switches? If so, then you need to drive the
> gates of the two "high-side" switches with a voltage that is _higher_
> than the Drain voltage.

  That'll be the problem for sure!

> There are several ways to drive high-side MOSFETs. The most
> straightforward is to use optocouplers to do level translation so you
> can use a ground-referred logic level

  Why introduce optocouplers with speed problems?  Why not stick to
high-voltage transistors and/ or FETs?

> Another way is to use P-Channel MOSFETs for your high-side switches.
> Wire these up Common-Drain (i.e. with their Sources connected to the
> supply voltage), and their Drains connected to the motor terminals,

  Caught you too!  You meant Common-Source.

> a mirror image of the low-side N-channel MOSFETs. You can then connect
> the gates of the "left" side of the H-bridge together, and likewise
> those of the right.

  You would REALLY do that and accept commutation pulses?

> The disadvantage of this method is that P-Channel MOSFETs are more
> expensive than equivalent N-Channel devices, by about a factor of
> three.

> Finally, you can use any of several "bootstrap" methods to do level
> translation and provide the high-side drive voltage.

  What's the difference between this and your first method?

>> There is a fifth mosfet,between the bridge and ground,which is the
>> only one working at 10 kherz (doing the Pulse width modulation).

  I wonder if this also is fouling up the drive conditions on the lower
arms of the "H".  Hey, I'll bet he was using Darlington drive on the
lower arms too!

> Why do this? Why not just switch one of the low-side MOSFETs at 10
> KHz?  That would be simpler and more efficient.

  And cheaper.  I've seen this silly in a magazine project!  I couldn't
believe my eyes!  Fascinating though, they brought out a fixed "mark II"
version some time later.

> Using a single MOSFET in the way you describe is more appropriate for
> a relay H-bridge circuit where you use the relay to change motor
> direction and the MOSFET to do the PWM.

  Like model cars?

  Cheers,
        Paul B.