MOSFETs overheating on stepper driver

Thread Starter

magiclark

Joined Jun 28, 2019
5
I'm controlling a 3v 3A bi-polar stepper motor with an Arduino, and found some 4-ch opto-isolated MOSFET drivers that I was able to use as H-Bridge drivers. (Drawing attached) I'm pretty sure I hooked them up correctly (the lines in red are my modifications); the led next to each MOSFET flashes in the correct sequence. However, with the stepper attached, MOSFETs #1 & #3 on both boards are getting very hot and the stepper isn't running. Any thoughts?

Stepper Driver Hookup.jpg
 

dendad

Joined Feb 20, 2016
3,084
You need to supply the top MOSFETs with gate volts above the supply voltage to turn them on fully. I would aim for at least an extra 5V.
Or else, use a PFET and pull the gate to 0V.
 

crutschow

Joined Mar 14, 2008
23,523
You may have hooked them correctly from a logical standpoint, but not as a sound engineering design.
As dendad noted, the top MOSFETs aren't fully turning on, which is causing the overheating.
They are acting as source followers, so their output voltage is more than one Vgs(th) voltage below the supply voltage.
The difference between the drain and source voltage is multiplied by the motor current, giving the observed high dissipation.

As the data sheet shows below, the on-resistance is tested with a gate voltage 10V above the source voltage, so you would need a gate voltage of 10V above the drain voltage to achieve that condition.
upload_2019-6-28_23-1-45.png

The easiest fix is to convert the top MOSFETs to P-channel types, since grounding the gate will apply the fully supply voltage source-to-gate, which will fully turn them on.

Note that for both the N and P MOSFETs, you will need logic-level type MOSFETs, that will fully turn on with only a 3V Vgs.
Standard MOSFETs will not work for that.
 

Thread Starter

magiclark

Joined Jun 28, 2019
5
Understood. I now also realize that these boards each have 4 N-channel MOSFETS, and for a proper H-bridge, they should have 2 N-channel and 2 P-channel MOSFETS. Thanks for your help.
 

dendad

Joined Feb 20, 2016
3,084
To turn on the PMOSFET ?
Yes. A P channel MOSFET has the gate more -ve than the source, just like an N channel MOSFET goes +ve than the source.
That is why it is a useful device to make an H bridge or high side switch with.
Remember to move the gate-source resistor to the +ve as the is the P channel MOSFET source.
And the opto now connects collector to the gate, and emitter to 0V

Draw up a revised circuit.
 

Thread Starter

magiclark

Joined Jun 28, 2019
5
Found the overheating issue. Still had a trace that was not yet cut on the original board. That's fixed. Now I'm back to the issues being discussed. Here's an updated drawing. Does this look better? The spec sheets for the IRF540 and IRF9540 seem to indicate a minimum +2v and -2v difference to switch on. Do I still need to provide a separate supply for the MOSFETs?

Thanks.

Stepper Driver Hookup-rev.jpg
 

dendad

Joined Feb 20, 2016
3,084
If you run it as above, the top FETs will be off when the indicator LEDs are on.
H-bridge.jpg
See how top PChan FETs are connected here?
Have your opto as the transistor. And an indicator LED with its resistor across the 680R resistor.
And in your circuit, the IRF540 is drawn backwards.
 

Thread Starter

magiclark

Joined Jun 28, 2019
5
I saw that with the led's too. I can live with that since it's already part of the board I found for free. I'll swap the #1 & #3 N-ch for P-ch FETs tomorrow. Of course I'll have to be creative using the existing holes by criss-crossing the source and drain pins. Also, saw I mis-drew the N-ch FETs and already corrected it. I expected that would be noticed after I posted it. Really appreciate your help.
 

Thread Starter

magiclark

Joined Jun 28, 2019
5
It works! This latest adjustment got it going. The opto circuit limits the speed a bit, but plenty fast enough for my needs. No "shoot-through" overheat issues either. Thanks all for your help!

MOSFET H-Bridge Driver.jpg
 

ScottWang

Joined Aug 23, 2012
6,834
When you draw the circuit, the better is to draw it from left (input) to right (output), and the upper position is the positive voltage, the lower position is the negative voltage (or Ground).
 
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