[sdiy] OT BLDC motor controller - a MOSFET question

[ASSI]

On Wednesday 22 December 2010, Tom Adam wrote:
> A nephew of mine is working on his thesis and is having a problem.
> He's building a brushless DC motor controller with an arduino. He's got
> a working prototype, so far so good.
> For his prototype he's using a 25W BLDC motor, but in the end he needs
> to drive an 800W BLDC motor (36V, +/-22A).

At that voltage/power point things will be fairly interesting, at least if 
you want good efficiency (you will likely need better than 90% just for 
thermal reasons).  If the driver and control stage are not optimized 
together, the driver would have to be overdimensioned quite a bit just to 
survive the switching stress.  What's the application, an E-Bike or 
something?

> Currently  he's using the IRF1205(N) & IRF5305 (P) MOSFET. And we're not
> sure these MOSFETS can handle the 800W motor.

It is somewhat unusual to use a PMOS in such an application, as the expense 
of a high-side driving circuit (and gate boost supply) is usually outweighed 
by the much better performance of an NMOS.  Also, since you're driving an 
inductive load, you need to be really careful at how exactly you switch 
things off (that is much trickier than you might think and you will learn a 
lot about parasitics you didn't even know before).  At this point your 
controller should know something about the driver and provide suitable dead- 
and overlap-times to reduce stress.  High power drivers are usually designed 
to take at least some of the inductive energy away in a controlled breakdown 
(that's what the avalanche and repetitive avalanche energies are for in the 
data sheets), so you're really close to what physics allow you to do with 
the device and the margin of error is just tiny.  At 800W that'd literally 
mean fireworks, so you don't really want to cross the edge.

> So how does one select a MOSFET? Any pointers appreciated...

A good starting point would be the BLDC development and evaluation kits that 
are out there from practically all manufacturers -- especially if the driver 
circuitry is not explicitly a part of the thesis, I'd not really want to try 
to design and debug an almost 1kW power stage.  If you want to learn it 
anyway, there have been numerous articles on BLDC control in EE Times and 
their sister publications over the past years, some of which also discuss 
the various aspects of how to drive and select high-power MOSFET.  There's a 
ton of application notes that make a good reading, too.  Just don't expect 
to go straight from 25W to 800W and have it all working without a hitch.


Achim.
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