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[DPRG] Cheap high power H-bridge

Subject: [DPRG] Cheap high power H-bridge
From: Chuck McManis cmcmanis at mcmanis.com
Date: Mon Nov 26 17:08:48 CST 2001

At 02:34 PM 11/26/01, Clay Timmons wrote:

Interesting idea. Having been in the throes of designing a FET based 
h-bridge I thought I'd check it out.

The bad news is that it has "issues"

They claim: 15A without additional heat sinks
Given a 22mOhm FET, that's a power dissipation of 5 watts from two of the 4 
FETs. To keep the junction temperature at < 175 dC they have to keep the 
case temperature at less than 171 dC (.75 dc/Watt R(t)cj). That means their 
copper heat sinks on the PCB have to have an R(theta)sa of 34 dC/Watt or 
better. Assuming they have .75 x 1" copper pads under the FET for a sink 
(and their board is 2.5" on a side and the FETs are .4" x .6" in standard 
TO-220s so this is a good, possibly generous guess), even at 2oz copper 
(which the board does not appear to have) they would get at best a  49 
dc/Watt value. So it will probably burn up if you run it at 15amps 
continuously. :-)

Then there is the issue of the "you can't go faster than 1Khz" PWM issue. 
What's up with that? Probably has to do with their high side booster. If 
they were using a switched capacitor I'd expect to read "not less than 
1Khz" but that doesn't seem to be the case.

If anyone gets one of these I'd be interested in a review of its "real 
world" capabilities.

I'm guessing 10Amps tops.


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