TO220--thin leads--high current--How?
in [Design]
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From: Don Klipstein on 2 May 2010 22:10 In <t4hrt5dacufuntdfcsnb0sogplcvoouab2(a)4ax.com>, Spehro Pefhany wrote: >On Sun, 2 May 2010 12:58:24 -0500, the renowned "Tim Williams" ><tmoranwms(a)charter.net> wrote: <SNIP sopme to edit for space> >>The only thing you need to know about MOSFETs is use them at half rated >>current. As an added bonus, the Rds(on) is relatively small, so you need >>even less heatsinking. > >So, in the case of a D2Pak rated at 429A (asterisk) continuous or 1640A >pulsed (160A package limit) would you run it at 820A, 214.5A or 80A? With decent heatsinking, it appears to me that power MOSFETs should reliably take more than half the true maximum current indicated by careful reading of the datasheet (continuous silicon limit or continuous package limit, whichever is lower). I usually think 75%, though I think more like 50-60% for the extreme example that John Larkin mentioned. Given the extreme current figures being mentioned lately, I would like to say 60% instead of 75% when the numbers are eye-catching big ones. In the above example, I would say 60% of 160A, or 96A. Then again, I am having trouble imagining a D2PAK's leads handling 96 amps continuous without some significant heatsinking or parallelling with additional copper. 80 amps sounds like a need to solder thick copper wire to the leads close to the package. The heatsink needed will probably be very substantial, with power dssipation likely in the 10's of watts. One more thing about MOSFETs, especially used for switching: I would design for maximum junction temperature of 125C. Consider the high positive temperature coefficient of Rds(on). Maximum current to avoid exceeding the maximum rated junction temperature is only a little more than the maximum current to avoid exceeding 125 C in any given usual heatsinking arrangement and ambient temperature. - Don Klipstein (don(a)misty.com)
From: Tim Williams on 2 May 2010 22:46 "Don Klipstein" <don(a)manx.misty.com> wrote in message news:slrnhtsc53.25c.don(a)manx.misty.com... > One more thing about MOSFETs, especially used for switching: I would > design for maximum junction temperature of 125C. Consider the high > positive temperature coefficient of Rds(on). Maximum current to avoid > exceeding the maximum rated junction temperature is only a little more > than the maximum current to avoid exceeding 125 C in any given usual > heatsinking arrangement and ambient temperature. Yup, I always design for double Rds(on) too. That covers thermal and manufacturing variability. Funny how datasheets seem to over/understate things by almost exactly a factor of two! Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Larkin on 2 May 2010 23:28 On Sun, 2 May 2010 21:46:19 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"Don Klipstein" <don(a)manx.misty.com> wrote in message >news:slrnhtsc53.25c.don(a)manx.misty.com... >> One more thing about MOSFETs, especially used for switching: I would >> design for maximum junction temperature of 125C. Consider the high >> positive temperature coefficient of Rds(on). Maximum current to avoid >> exceeding the maximum rated junction temperature is only a little more >> than the maximum current to avoid exceeding 125 C in any given usual >> heatsinking arrangement and ambient temperature. > >Yup, I always design for double Rds(on) too. That covers thermal and >manufacturing variability. > >Funny how datasheets seem to over/understate things by almost exactly a >factor of two! The sil-pad folks use a factor of 2 or 3 for theta. John
From: Don Klipstein on 3 May 2010 01:13 In article <hrlddv$1bn$1(a)news.eternal-september.org>, Tim Williams wrote: >"Don Klipstein" <don(a)manx.misty.com> wrote in message >news:slrnhtsc53.25c.don(a)manx.misty.com... >> One more thing about MOSFETs, especially used for switching: I would >> design for maximum junction temperature of 125C. Consider the high >> positive temperature coefficient of Rds(on). Maximum current to avoid >> exceeding the maximum rated junction temperature is only a little more >> than the maximum current to avoid exceeding 125 C in any given usual >> heatsinking arrangement and ambient temperature. > >Yup, I always design for double Rds(on) too. That covers thermal and >manufacturing variability. I would consider that sufficient when designing for 125 C maximum junction temperature. (I have noticed how "they" like to state typical and not worst-case variation of Rds(on) with junction temperature.) That means beyond doubling Rds(on), allow for the worst ambient temperature where you would rather avoid getting a phone call for failing to handle as opposed to saying that the customer was wrong, and include sufficient heatsinking for junction temperature to not exceed 125 C (or at least neither exceed 125 C by a lot nor spend a lot of time exceeding 125 C). >Funny how datasheets seem to over/understate things by almost exactly a >factor of two! > >Tim I probably said enough at this late hour in the day... -- - Don Klipstein (don(a)misty.com)
From: Don Klipstein on 3 May 2010 01:41
In article <dkgst55hfm9lferc26csvosb6c30nqjd5e(a)4ax.com>, John Larkin wrote: >On Sun, 2 May 2010 21:46:19 -0500, "Tim Williams" ><tmoranwms(a)charter.net> wrote: > >>"Don Klipstein" <don(a)manx.misty.com> wrote in message >>news:slrnhtsc53.25c.don(a)manx.misty.com... >>> One more thing about MOSFETs, especially used for switching: I would >>> design for maximum junction temperature of 125C. Consider the high >>> positive temperature coefficient of Rds(on). Maximum current to avoid >>> exceeding the maximum rated junction temperature is only a little more >>> than the maximum current to avoid exceeding 125 C in any given usual >>> heatsinking arrangement and ambient temperature. >> >>Yup, I always design for double Rds(on) too. That covers thermal and >>manufacturing variability. >> >>Funny how datasheets seem to over/understate things by almost exactly a >>factor of two! > >The sil-pad folks use a factor of 2 or 3 for theta. My father told me many times that he likes to divide (or multiply if appropriately making the story worse) datasheet figures for power handling, and all figures said by salesmen, by pi. (I have a preference over pi in favor of 3.2, 50/16, or square root of 10, whichever of these makes easier for me to do calculations / "calculations" with my "Brainiac" personal calculator that is with me everywhere I go when I am awake. As for ambient temperature - I seem to think that Newark NJ USA gets some "hot flashes" and even now has some of its population lacking A/C, meaning 40 degrees C in homes to deal with occaisionally, even when not on top-floor of a multi-story residential building. The record high in Phoenix AZ USA appears to me to be about 50 degrees C, and even-lower elevations in S and SW AZ and SE CA get a few degrees hotter still for alltime high temperature record. Automotive electronics appear to me to have extra need to survive heat, although it may be "somewhat unreasonable" for someone to drive a sun-baked hot car in a hot area on a hot day with A/C malfunctioning and the car's interior temperature in the 60's C (with electronics actually operating). Probably "enough said" at this late hour... -- - Don Klipstein (don(a)misty.com) |