Why did my drive blow up?

@z34g2000cwc.googlegroups.com:


Seems unlikely that this is a flyback diode problem. Did you try driving
the motor in one direction only (no flyback involved)?

Why do you think that the MOSFETs are rated for the current that will be
drawn by the motor you are using?  You could try testing with a smaller
motor.

Are you sure you didn't have both diodes on one side on at the same time?

Mitch

Flyback voltage occurs no matter what direction the motor is running.
It occurs as the circuit attempts to dissipate the energy of the
collapsing magnetic field in the motor when the current is interrupted
(when the MOSFETs shut off the current as part of the PWM switching).
Cool - my engineering degree came in handy on that one.

Good question about the current. I'm sure I'm not exceeding the current
or heat handling capability of my MOSFETs. They are rated at 80A and I
have the circuit fused at 30A. I also have a beefy aluminum heat sink
attached to each one. I did a heat transfer calculation and it should
be able to handle 80A.

The last question is also good. The HIP4081A prevents this from
happening. Even if it did happen, my 30A fuse would have blown.

Thanks for the reply.

BRW

Flyback voltage occurs no matter what direction the motor is running.
It occurs as the circuit attempts to dissipate the energy of the
collapsing magnetic field in the motor when the current is interrupted
(when the MOSFETs shut off the current as part of the PWM switching).
Cool - my engineering degree came in handy on that one.

Good question about the current. I'm sure I'm not exceeding the current
or heat handling capability of my MOSFETs. They are rated at 80A and I
have the circuit fused at 30A. I also have a beefy aluminum heat sink
attached to each one. I did a heat transfer calculation and it should
be able to handle 80A.

The last question is also good. The HIP4081A prevents this from
happening. Even if it did happen, my 30A fuse would have blown.

Thanks for the reply.

BRW

Yes.  But if you just turn the appropriate MOSFETs on and never turn them off
(i.e., don't pulse them), there is no flyback voltage.

If you have a small motor, it might be worthwhile to debug using that first.  
You could also try an experiment with operating the circuit with no motor.  
This would reveal if there is a problem with sequencing the MOSFETs.

Mitch

In general, it is best to test new motor drivers using non-reactive
loads of decreasing resistance.  Also, does the motor have brush-noise
suppression capacitors on it and do you not have an inductor in series
with the motor?

chris

I do have brush noise suppression caps of 0.1 uF on the motor. I do not
have an inductor in series with the motor.

BRW

That issue got me once (on a higher voltage, lower current, probably
higher frequency driver and some caps I didn't know were there) but
with those values you should be alright.

chris

I ALWAYS have a bridge rectifier across my H-Bridge drivers. One of the
problems with MOSFETs is that the internal diode, while very fast, often
can not manage the current. A good high current h-bridge may help and never
hurts. Also, a small ceramic cap, about .1 uf, at the motor may help too.
It forms an L/C filter with the wires from your amp to the motor terminal.

    

Thanks for the reply. I suspected that I was introducing too much
flyback current for the MOSFETs to handle.

BRW

Didn't do my trick from a few years back ?
driving a largish motor, fine when taking the load
but when the load drove the motor
the control board (not the driver board)
started smoking and the chip I was using (8051) melted.

Feedback path was the ground link between the boards
and local chasis ground in the box I had them in.

Driver board still works.

Just waiting on some new high current h bridges from bdmicro.com

Brian hasn't got them listed on his site yet
but you can see them here
http://www.bdmicro.com/include/display_image.php?img=rx50-4_sm.jpg
and here http://www.bdmicro.com/darpa-gc/

specs are

    * 5 to 24 Volts (30V absolute max)

    * Current: 8 to 10 Amps no heat sinks
               40 to 50 Amp w/good heat sinks
               (upper end not yet determined)

    * mounting holes for standard 60mm CPU-type fan for extra cooling
      capacity

    * header to power fan (uses motor supply voltage)

    * PWM freq over 100 kHz

    * adaptive non-overlapping gate drive - eliminates shoot-thru

    * built-in ATO style fuse

    * locked anti-phase control native, sign magnitude / synchronous
      rectification possible with adapter logic board (included)

    * 5 LED indicators

    * solid 4-layer board w/large power and ground planes

    * 2.5 x 2.6 inches

    * easy to use FAST-ON blade type connectors for power

    * convenient screw terminals for logic