[sdiy] OT BLDC motor controller - a MOSFET question

[Tom Adam]

Harry,

This is interesting... I'm finally getting a pictures of what I need to 
look into.
I hope my nephew is going to stick to his original idea of not buying a 
controller. This looks like so much fun to try out. I'll keep you 
informed, if you like, of our progress.

Thanks again, free resource of mine ;-)

Cheers,
ToAd

Op 22-12-2010 21:53, Harry Bissell schreef:
> 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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>