> > 100uS direction switching is not very probable, to put it 
> mildly. (PWM
> > is ~ 30 kHz == 30us!)
> 
> Yikes!  That's way too high for this circuit.  Your drivers 
> are slow.  I bet
> the FETs are spending a significant fraction of their time in 
> transition.
> Are they getting warm?

in normal operation, no

I can't test them for long on a loaded motor of the real system (the
mechanism would wear out quickly) so I used a bunch of lamps for a few
hours, IIRC 2A when at 100%, running at 70% PWM. IIRC the fets were
somewhat warmer too the touch, but not uncomformtably (I would guess no
more than 45 C).

> It would operate fine at a few 100Hz PWM.  
> The only reason for higher speed is to reduce audible
> whining.  There is no way to guess what frequencies will be 
> audible and objectionable with a specific motor.  You just have to try
a 
> few.  Hopefully there is a frequency you can use in the 
> 500Hz to 5KHz range that is acceptable.

I preferred an unaudible frequency :)

> Yes, but what happens when the user hits the button to change 
> direction and
> the motor is currently being driven in the other direction.

I don't think this is possible, but more from accident than from design.
But I will check.
  
>> If I use a cap there, how do I determine the value (at type 
>> of cap)? Or should it be an R-C?

(snip calculations)

thanks (peter too)

> So let's work backwards and do a sanity check.  If you're 
> switching the full
> 300mA and your FET body diodes can turn on in 5nS (just 
> picking a number,
> you're job to look it up),

5ns for the N, 13 for the P

>> (but the reverse-leakage 
>> of the diodes
>> is a thing to check, especially because they are next to 
>> the FETs and
>> could get heated).
> 
> So the FETs are getting hot? 

Not in normal operation, but while thinking about how those diodes could
have blow I did consider a short from a motor wire to ground or power.
This might have heated the FETs, heated the diodes, and caused them to
blow from leakage current. But the leakage is specified as 10 mA @ 100C,
=> 0.24W, with 80C/W (case-ambient) that does not seem like a problem to
me.

> I'm thinking now the original problem was
> caused by voltage stress on one or more componets due to high 
> dV/dt and the
> finite time for the diodes and other parts of the circuit to 
> react and clip
> the voltage.

I think all recommendations are OK, but I fear it still is not the real
problem. There are two such H bridges in my controller, IIRC (no
hardware here now) one for 300 mA stall, the other for 2.5A stall. This
failure occured in the 300 mA bridge, yet the 2.5 one is the one that is
used most. (Yes, I shall re-do all calculations for the 2.5A side!).

Wouter van Ooijen

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