8825 drivers in parallel?

What if……two 8825 drivers are setup in parallel (each with identical inputs and outputs) to run one stepper motor.
I reckon it would be a fantasy to think that the drivers would share the current.
All words of discouragement, prior to running this set up, would be more than welcome.

Thanks, Howard

The chips are based on MOSFETs. These devices do not play well with putting them in parallel. The problem is that if one becomes hotter than the other, it will pull even more current its way than before and become even hotter.

Thermal runaway is what they call that.

So as long as you don’t get them hot, you can do this… But then you wouldn’t be asking this… Sorry!

On second thought, things are even worse: Both chips will be doing PWM modulation on their own. They are not commanded to do “23/256 on based on this 1MHz clock”, but decide for themselves when to go on and off. I don’t think it’s a good idea. The interactions are going to be complex. On the other hand, the interactions might work out OK. You could try, if you’re willing to blow up a couple of drivers.

Hi, Howard.

I agree with what rew said. It is not a good idea to parallel stepper motor drivers.

- Zeeshan

Just to clarify, I agree with the conclusion that you should not parallel stepper motor drivers. However, some of what rew posted is not correct.

[quote=“rew”]The chips are based on MOSFETs. These devices do not play well with putting them in parallel. The problem is that if one becomes hotter than the other, it will pull even more current its way than before and become even hotter.

Thermal runaway is what they call that.

So as long as you don’t get them hot, you can do this… But then you wouldn’t be asking this… Sorry![/quote]

This is true for BJTs, but not for MOSFETs. When using MOSFETs in parallel, if one of the MOSFETs becomes hotter than the other, the resistance of that MOSFET increases and it draws less current. They essentially balance each other out.

- Zeeshan

Rew, Zeeshan……thanks for your prompt replies.
Stepper motor drivers that, when used in parallel, share the load will be placed on my fantasy wish list.
Once again thanks and best regards, Howard

Zeeshan. Hmm. I just checked a “random” datasheet and it seems you’re right. Sorry for the confusion.

The threshold voltage DOES come down. That means that at a certain voltage the hotter it gets, the current will be larger. For example, at 2.7V VGS the PSMN7R0-30MLC will typically conduct about 10A at 25 degrees, but 25A when it’s 150 degrees. But that’s in the part of the curve that you do not want to operate in when you’re running a motor.

plus I think it would get pretty ugly due to the pwm signals not being in sync.