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High Current H-bridge circuit using mosfets

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Damnusername

New Member
Hello everyone
I have made an H-bridge circuit using p-type Mosfets (IRF9540) and n-type mosfets (IRF540). My circuit works fine in one direction and drives motors with a reasonable RPM but on reversing the direction, RPM sufficiently decreases to a point where motor is almost not working, mosfets and wires start to heat up. Sparking on battery terminals is also observed.

I m using two BJTS ( C546 and C556B) along with a single optoisolator( 4n25) to drive each mosfet. There are four such units in my h-bridge

Please help me out with this
 

Sceadwian

Banned
We can't help you with anything if you don't post a complete schematic of your h-bridge circuit.
 

smanches

New Member
You are using P and N mosfets in the same H-bridge? Shouldn't they all be the same type, preferrably n-type using a positive voltage rail?
 

Sceadwian

Banned
No smatches, the low side of the h-bridge are driven by N fets, and the high side is driven by P fets. You can use all N fet's but the gate voltages for the high side fets need to be isolated or the circuit doesn't work.
 

smanches

New Member
Oh, ok. I've been working off just one H-bridge design, which is all Nfets with a floating "common" between the high and low side. Everything is isolated.

This is actually the class info that most of my information has come from (No, I'm not going to school.) It was one of the only sources that showed how to do it practically. Too much theory on the web, not enough practical examples.

http://users.ece.utexas.edu/~grady/EE362L_Fall08.html
 

Sceadwian

Banned
For h-bridges? You haven't looked enough!
What are you doing for isolation for your Nfets?
 

smanches

New Member
All the firing circuits are isolated using logic level optoisolaters. I was using the class suggestion and trying transistor outputs, but trying to work within the linear range was not ideal. Changed to logic level and everything is much cleaner.

Small 12v power transformers isolate the low voltage rails from the mains. This isolates the LV rails from the mains on both the controller and power sides.

I have not buitl a schematic for it yet. Will do that shortly.
 

Sceadwian

Banned
opto isolaters aren't as effcient as a good direct drive. Optos have pretty 'course' turn on and off times.
 

smanches

New Member
I really wanted the safety of them though, as tht PIC that I'm using for the PFC/H-Bridge needs to talk to the MCP (Master Control Pic :)) which is also talking to a number of other small modules. Also just like to push the safety envelope for the sake of practice/learning.

Here are the logic opto's I'm using. They are quite nice and don't inject too much delay into the firing circuit. And now my minimum PWM width is 50ns, instead of 500ns which was the best I could get using the transistor outputs.

http://www.electro-tech-online.com/custompdfs/2009/03/6n137vo2.pdf

Just realized we hijacked this thread. OP, post a schematic. :p
 

Sceadwian

Banned
Watch your frequency, optos's have pretty soft on/off limits. If you increase the frequency too much the opto forces the FET's into their linear range and they self destruct.
 
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