DPRG List

 [DPRG] Sigh, my speed controller... Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] Previous message: [DPRG] Sigh, my speed controller... Next message: [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 _______________________________________________ DPRGlist mailing list DPRGlist at dprg.org http://nimon.ncc.com/mailman/listinfo/dprglist NOTICE OF CONFIDENTIALITY The information contained in this e-mail message and in the documents attached herewith (hereinafter "the message") is intended only for the individual or the entity named above and is intended to be confidential. The reading of the message or any retention, copying, dissemination, distribution, disclosure of the existence of the message or of its contents, or any other use of the message or any part thereof, by anyone other than the intended recipient is strictly prohibited. If you received this message and you are not the intended recipient or agent responsible for the delivery of this message to the intended recipient, please refrain from reading it and notify us immediately by telephone +27 (11) 921-7900, so that we can co-ordinate with you, erasure of the message. Although this e-mail and its attachments are believed to be free of any virus or other defect, it is the responsibility of the recipient to ensure that they are virus-free, and no responsibility is accepted by this firm for any loss or damage arising from receipt or use thereof. ``` Previous message: [DPRG] Sigh, my speed controller... Next message: [DPRG] Sigh, my speed controller... Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] More information about the DPRG mailing list