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Parallel, redundant, Mosfets on DC motor control board

Catwell
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I was having a issue with some CNC machinery of mine, and I had to take apart the factory power systems in each. I noticed that the powering scheme has several paralleled mosfets. One of the mosfets, a D8020LD8020L, is more than enough for powering in the motor. But there were 5 in parallel. Is this a common practice for driving DC motors in machinery?
 
The above, blurry, pictures are from the machine's DC motor drive board.
 
Cabe
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    Hi Gabe,

     

    This issue here is reliability and thermal management. Let's start with thermal management, a spec sheet might say that a transistor is good for 5A or 25W of heat dissipation, however if you look carefully in the spec sheet your going to find a derating curve, which shows that as tempature increases the amount of power that can be dissipated decreases. Since these transistors are going to be running CNC motors and are inside an enclosure that probably doesn't have the most abundant air flow, the designers of the controller derated the transistors accordingly and gave a some margin for reliable operation(assuming good design practice).

     

    The general rule of thumb for reliability is that tempature and reliability tend to follow an inverse square relationship. If your tempature doubles then you can expect the product lifetime to be cut by a quarter. By paralleling the FETs each transistor handles only a portion of total current which yields a lower temperature rise, which in turn yields an increase in reliability minimizing the number of failures in the field.

     

    As an aside MOSFETs are very good for parallel applications, their on resistance has a positive temperature coefficient. What this means is that if one of the mosfets starts drawing more current, its temperature will increase. The increase in temperature will yield an increase in the on resistance, which will in turn lower the amount of current drawn by the transistor. Such behaviour acts as form of feedback to share the load current equally among FETs.

     

    Hope this answers your question.

     

    Best Regards,

    Jorge Garcia

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  • 0

    Hi Gabe,

     

    This issue here is reliability and thermal management. Let's start with thermal management, a spec sheet might say that a transistor is good for 5A or 25W of heat dissipation, however if you look carefully in the spec sheet your going to find a derating curve, which shows that as tempature increases the amount of power that can be dissipated decreases. Since these transistors are going to be running CNC motors and are inside an enclosure that probably doesn't have the most abundant air flow, the designers of the controller derated the transistors accordingly and gave a some margin for reliable operation(assuming good design practice).

     

    The general rule of thumb for reliability is that tempature and reliability tend to follow an inverse square relationship. If your tempature doubles then you can expect the product lifetime to be cut by a quarter. By paralleling the FETs each transistor handles only a portion of total current which yields a lower temperature rise, which in turn yields an increase in reliability minimizing the number of failures in the field.

     

    As an aside MOSFETs are very good for parallel applications, their on resistance has a positive temperature coefficient. What this means is that if one of the mosfets starts drawing more current, its temperature will increase. The increase in temperature will yield an increase in the on resistance, which will in turn lower the amount of current drawn by the transistor. Such behaviour acts as form of feedback to share the load current equally among FETs.

     

    Hope this answers your question.

     

    Best Regards,

    Jorge Garcia

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