On Wed, Aug 29, 2001 at 08:41:10PM +1200, Russell McMahon wrote:
> > After years of development ;-) I've finally have my prototype robotic
> lawnmower
> > platform to a usable state. The most valuable lesson I learned is that
> > everything in the drive path must be metal, clamped, and use lock washers.
> >
> > Next on the agenda is motor control for the drive motors. Bob Blick's
> > discrete h-bridge is a natural for the project. But of course you end up
> > with 8 power transistors, plus another one for the cutting motor.
> >
> > I wanted to get some ideas of the best way to heatsink a bunch of power
> > transistors.
>
> If power dissipation is significant you want to really DESIGN your heat
> sinking system. How hard this is depends on how close to their limits you
> want to run your transistors.

> What power dissipation are you expecting?

On the order of 10-12W each with 1/2 of the transistors on and 1/2 off at any
one time.  I'm basing this on Bob Blick's comments that the parts will drop
about 2V each under load and the motors draw 3.5 to 4A on level ground. It
rained today So I didn't get a chance to test on grass.

BTW I realize that I'm going to have to have some kind of very reliable
crossbar. The motors' stall rating is 11A each! Ouch! I need to figure out
how to build a current sensor so that when the motor stalls it'll automatically
cut out.

> What devices are you using? (part numbers or required ratings).

TIP 142/147 combo in Bob Blick's H-Bridge driver:

http://www.bobblick.com/bob/projects/hbridge/index.html

These are the 10A continouous parts.

>
> There are 3 thermal sections to the heat path.
>
> 1 - die to case
> 2 - case to heat sink
> 3 - heat sink to air
>
> If you have lots of metal for heatsink you will be able to get the HS down
> under 1 C/W (1 degree Celsius rise per watt dissipated).

So we're talking about 40C above ambient? That's hot!!

>
> If you use electrically isolated packages they can be bolted directly to the
> HS thereby eliminating path 2 above BUT the inbuilt insulation has extra
> drop. look carefully at data sheets. This extra drop is typically around 1
> C/W. Use grease.

Not isolated.

[Other suggestions trimmed]
>
> You could consider fluid heat transfer or heat pipes but that is probably
> getting excessively complicated.

I'm pretty sure that's not going to be necessary for the 50W of dissipation
required. The prototype is going to be in open air. I should be able to get
the temp down with enough metal.

I realize that I can direct couple in pairs since the collectors are shared
and are connected to the tabs.

I bought some sil-pad type thermally conductive insulators from NTE to test
with.

Thanks for the great info.

BAJ

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