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[DPRG] Sigh, my speed controller...

Subject: [DPRG] Sigh, my speed controller...
From: Shaun Parsons Shaunp at cashpower.co.za
Date: Thu Oct 11 07:18:47 CDT 2001

These FETs will be stressed beyond their limits.

Search for:
'*** Begin Shaun ***' in body of text below.

Regards
Shaun



-----Original Message-----
>From: Chuck McManis [mailto:cmcmanis at mcmanis.com]
Sent: Thursday, October 11, 2001 8:14 AM
To: DPRG
Subject: [DPRG] Sigh, my speed controller...


The design spec calls for 200A continuous operation at 24V (yes a 5KW speed 
controller). However, as I got into this process I realized that building 
the controller was trivial, getting rid of the dissipated heat was the 
tricky bit.

As it currently stands I'm using 16 IRF1010E's in four groups of four for 
each leg of the bridge. The 1010 has a 12mOhm RDS(on) at nominal 
temperatures (100 dC)

*** Begin Shaun ***
12mOhm @ Tj of 25DegC
At Tj of 120DegC, Rds(on) is 150%(Rds(on)@25DegC)
*** End Shaun ***

, I've got Rev B printed circuit boards en route so my 
theory should getting reduced to practice quite quickly (not to mention 
that Nov 2nd is the start of BBTI 2.0)

At 200A, the FETs (8 of them representing half the bridge) are dissipating 
about 240 watts. Given the datasheet values of .75 dC/Watt R(theta)jc and a 
R(theta)cs of 0.5 (with Mica insulators/grease) dividing 240/8 has each of 
the FETs pushing approximately 30 watts into the thermal system and holding 
a 37.5 dC temperature differential between Junction and sink.

The FETs are speced to operate up to a Junction temp of 175 dC so the sink 
could, in theory get up to 137.5 dC before damage is done to the FETs.

*** Begin Shaun ***
The FETs need to be de-rated based on Tc.
At Tc of 175DegC the FET can carry 0Amps.
At Tc of 137,5DegC the FET can carry 41Amps
At Tc of 55DegC the FET can carry 75Amps
*** End Shaun ***
 
However, my design spec was to be able to surge to 300A for up to 3 minutes 
and at 300A we jump to 540 watts and a 84 dC junction to sink offset so the 
sink really can't get up beyond 90 dC if we want to keep our FETs alive in 
that situation.

*** Begin Shaun ***
In this situation the heat sink must not exceed 55DegC as the FETs are now
at their max rated limit of 75Amps at Tc of 55DegC.
It must be remembered that at Tj of 175DegC that Rds(on) is around 190% of
Rds(on)@25DegC.
Dissipation in the FETs will be in the order of 1Kw.(Probably not do-able)
*** End Shaun ***

So I've also designed a fairly complex thermal management system to get the 
heat away from the FETs and one of the questions I've been trying to answer 
is this:
	"Is it enough?"
And that answer is, "Yes, if the R(theta)sa value of the sink is less than 
about .1 dC/Watt."

So our main bot, the B, is a heavy weight and does have quite a bit of 
empty space in the middle of it. However, its a pain to lay 300Amp motor 
lines long distances so we want to keep the leads to the motors short. That 
makes life interesting. My solution is a dual cooling system that combines 
a temperature feedback circuit, a forced air heat sink, and a circulating 
water cooling system. The latter being not well treated in the literature 
at all. (except sometimes in jest, as in "...well I suppose you could pump 
water through it! HA HA HA") However, it turns out we can use a water 
system and we are going to try it, although I have not yet figured out how 
to compute it R(theta)sa equivalent yet.

I'm hoping to have enough of it together to talk about it at the October 
24th meeting so if you SF Bay Area readers are listening that would be the 
time to attend :-)

--Chuck

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