Re: Why did my drive blow up?


Mitch Berkson wrote:
"BRW" <bennet@xxxxxxxxxxxxxxxxxx> wrote in news:1141150436.659494.57660
@z34g2000cwc.googlegroups.com:


The only thing I can think that would have caused this is flyback
voltages that the MOSFET zener diodes did not suppress. I thought that
the zener diodes built into the MOSFETs would protect the circuit from
flyback voltages (so did the author of the Nuts & Volts article). Maybe
these built-in diodes are only effective for small loads.

Am I right about the cause for explosion? If I need to add diodes for
flyback suppression, how should I size them? I'm building this drive to
handle 20A.

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

.

Relevant Pages

  • Re: Why did my drive blow up?
    ... The only thing I can think that would have caused this is flyback ... voltages that the MOSFET zener diodes did not suppress. ... the motor in one direction only? ... Why do you think that the MOSFETs are rated for the current that will be ...
    (comp.robotics.misc)
  • Re: Why did my drive blow up?
    ... the MOSFETs literally exploded. ... since my friend built a drive from the same design and tested it ... voltages that the MOSFET zener diodes did not suppress. ... Also, a small ceramic cap, about .1 uf, at the motor may help too. ...
    (comp.robotics.misc)
  • Re: Why did my drive blow up?
    ... the MOSFETs literally exploded. ... since my friend built a drive from the same design and tested it ... voltages that the MOSFET zener diodes did not suppress. ... Also, a small ceramic cap, about .1 uf, at the motor may help too. ...
    (comp.robotics.misc)
  • Re: Why did my drive blow up?
    ... The only thing I can think that would have caused this is flyback ... voltages that the MOSFET zener diodes did not suppress. ... Why do you think that the MOSFETs are rated for the current that will be ...
    (comp.robotics.misc)
  • Re: Return of "What is JS214A?" (prev MOSFET Testing)
    ... > Where these three ICs are located, ... you are saying that the symbols of two opposing diodes are not the ... commonplace to have three MOSFETs in parallel, ... common cathode. ...
    (sci.electronics.components)