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[DPRG] Sigh, my speed controller...

Subject: [DPRG] Sigh, my speed controller...
From: Bryan Andersen bryan at visi.com
Date: Thu Oct 11 13:03:26 CDT 2001

Shaun Parsons wrote:
> 
> These FETs will be stressed beyond their limits.
> 
> Search for:
> '*** Begin Shaun ***' in body of text below.

Hum, I haven't run the numbers, but if he switches them to IRL1004 he 
should be much closer to back in the running.  Still won't have a good 
safety margin.  The main issue I see with the switch is if any surge 
voltages will get above 40VDC at the FETs.  The current part is a 
60VDC part, but the application is for 24VDC.  A 40VDC part can't get 
a nice 2X overrating for voltage.  The IRL1004 has much higher current 
and nearly half the RDSon.  The wattage dissipation is still roughly 
the same.  It is a logic level part, but that means little as they 
both can be driven the same.  Going to nearly half the RDSon helps 
allot.  Personally I'd also add more FETS to get the FETs below 50% of 
their maximum ratings.  This is due to the fact that some of the FETs 
will be carrying more of the load than others.  The rule I've heard is 
derate to 2/3 when paralleling.  The three minute surge to 300A from 
the FETs standpoint is in the continious opperation range.  If it was 
only 10 seconds, then it maybe could be considered surge from the FETs 
viewpoint.


> -----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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-- 
|  Bryan Andersen   |   bryan at visi.com   |   http://www.nerdvest.com   |
| Buzzwords are like annoying little flies that deserve to be swatted. |
|   -Bryan Andersen                                                    |

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