[sdiy] OT BLDC motor controller - a MOSFET question

[Harry Bissell]

here's the link...
   Thank you for requesting this free resource.  Please click the link
below.

http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_integrated_gate_drive_optocoupler/acpl-k342-000e/

Thank you for your time,
----- Original Message -----
From: Tom Adam <tom.adam at thebigear.be>
To: Harry Bissell <harrybissell at wowway.com>
Cc: synth-diy at dropmix.xs4all.nl >> Synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Wed, 22 Dec 2010 15:29:24 -0500 (EST)
Subject: Re: [sdiy] OT BLDC motor controller - a MOSFET question

Harry,
Thanks for taking the time to answer.

I'll talk to my nephew and we'll start to do some math first.  This 
seems more complicated than I expected tho...

Off to read some more on MOSFET's first...

Cheers,
ToAd



Op 22-12-2010 18:26, Harry Bissell schreef:
> 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