[sdiy] OT BLDC motor controller - a MOSFET question

[Harry Bissell]

Two things to consider...

What is the on resistance (or voltage drop)of the MOSFETs at the operating current ?

Multiply that current by the voltage drop, then use the thermal resistance figures
(Tj-c = junction to case)to see how much the junction temperature will rise above
the case. Then ask yourself how cool you can keep the case (what is the highest
ambient temperature, can you fan cool it etc...)

You also need to calculate switching losses. This is harder to do. At low frequencies
they are not usually substantial, but at higher frequencies they can dominate the
losses (heat generated). During turn-on and turn-off, the current is still flowing
through the device, but the voltage is rising. 

Add the static and the switching losses together and see what the junction temperature
does.

if the devices run at some duty cycle, you may reduce the wattage generated by
multiplying by the square root of the duty cycle.

If the pulses are far enough apart, you might consider transient thermal impedance
curves. My guess is that in your motor application, that won't help you.

The data sheets seem to indicate that the devices are big enough (on paper) to handle
the 22A load, but thermally my guess would be that you won't be able to keep them cool
without a really good thermal management.

Consider using devices in parallel, this usually is not too hard with MOSFETs...

Or you might look for some MOSFET power modules designed for higher power applications.
These often include multiple packages (like 6 devices in a package) often enough for the
entire drive.

BTW, often motor drives are done with all one type of device, like all NMOS parts. The
high side gating gets a little more complex but there are more devices available in
NMOS than PMOS...

H^) harry (power electronics engineer in a past life :^)







----- Original Message -----
From: Tom Adam <tom.adam at thebigear.be>
To: synth-diy at dropmix.xs4all.nl >> Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Wed, 22 Dec 2010 10:15:48 -0500 (EST)
Subject: [sdiy] OT BLDC motor controller - a MOSFET question

Sorry for this OT question...

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).
Currently  he's using the IRF1205(N) & IRF5305 (P) MOSFET. And we're not 
sure these MOSFETS can handle the 800W motor.
So how does one select a MOSFET? Any pointers appreciated...

Thanks
ToAd


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-- 
Harry Bissell & Nora Abdullah 4eva