Why did my drive blow up?

My first attempt at building a high-current H-bridge failed miserably. I was trying to drive a pair of windshield wiper motors on a robot with
a new H-bridge that I built using discrete MOSFETs (HRF3205) with HIP4081A drivers. I got the design from a Nuts & Volts article. One of the MOSFETs literally exploded. I don't think it was a build problem since my friend built a drive from the same design and tested it completely with a small motor. Two of his MOSFETs exploded when connected to the big motors.
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.
Thanks,
BRW
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mitch Berkson wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mitch Berkson wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@gmail.com wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
BRW wrote:

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
mlw wrote:

Thanks for the reply. I suspected that I was introducing too much flyback current for the MOSFETs to handle.
BRW
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.