IR is insane
in [Design]
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From: Archimedes' Lever on 19 Mar 2010 21:31 On Fri, 19 Mar 2010 09:09:34 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >No, no, no. Experience is the worst teacher. Something that only a total techno-idiot would declare.
From: Archimedes' Lever on 19 Mar 2010 21:31 On Fri, 19 Mar 2010 09:09:34 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > Experience must be >constantly questioned because things keep changing. Your IQ?
From: John Larkin on 19 Mar 2010 21:46 On Fri, 19 Mar 2010 18:31:51 -0700, Archimedes' Lever <OneBigLever(a)InfiniteSeries.Org> wrote: >On Fri, 19 Mar 2010 09:09:34 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >> Experience must be >>constantly questioned because things keep changing. > > > Your IQ? Don't know. I did get 800 on my math SAT (before they dumbed it down) and 720 on the verbal. How about you? John
From: Don Klipstein on 20 Mar 2010 03:24 In article <mq17q5d4r9lpv10lmo64q636d08ti64udq(a)4ax.com>, John Larkin wrote: >On Thu, 18 Mar 2010 23:32:24 -0700, Muzaffer Kal <kal(a)dspia.com> >wrote: > >>On Thu, 18 Mar 2010 15:58:08 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>Just got a short-form IR mosfet thing in the mail. >>> >>>They have a PQFN 5x6 mm package they rate at 104 amps. And a D2PAK >>>rated for 340 amps. (And, IIRC, in that article John Larkin also mentioned: http://www.irf.com/product-info/datasheets/data/irfs3006pbf.pdf) >>>John >> >>The only thing that actually matters is whether you can stay under the >>power curve of I^2*Rdson(T). At 1milliohm max 340 Amps gives you 115 >>W. You need to make sure to check the C/W rating of the package and >>cool it enough. > >Rds-on is spec'd as 2.5 mohms max at 170 amps, and will go up at >higher currents as it self-heats. 340 amps would fuse the source lead. I checked out that datasheet, and Rds(on) at 170 amps is 2.5 mohm max, 2 mohm typ at 25 C. The datasheet also says that thermal resistance is ..4 degrees C per watt, and the absolute maximum junction temperature is 175 C. If the case is held to 25 C, the temperature difference is 150 C, meaning 375 watts is flowing through the thermal resistance inside the package out the heatsinkable surface of the package. Given the thermal conductivity of copper and aluminum, I have trouble envisioning a practical or semi-practical heatsink with thermal resistance less than .4 degree per watt - unless one does something extreme, along the lines of grinding and polishing a flat region onto a copper pipe that the heatsink tab gets soldered to, and then cold water flows through the pipe. At that rate, the heatsink tab has an area around 66 mm^2, and slightly ground down copper pipe wall may be 1.5 mm thick... Thermal conductivity of copper is about .4 W/(mm-k). Thermal resistance of the pipe wall is ..056 degree C per W on paper - plus a bit for the solder joint, plus something for the water. Suppose this extreme hetasink with its solder joint adds only .1 degree C per watt of thermal resistance (which I consider optimistic) to the .4 of the package... maximum power dissipation is down to 300 watts, at 175 C junction temperature and 25 C water temperature. Fig. 4 says that normalized on resistance at 175 C junction temperature is typically slightly over double that at 25 C. Looks to me about 2.04 times as much. How about multiplying 2.04 by that maximum 25-C-figure of 2.5 milliohms. I see 5.1 milliohms. Ohm's Law says that 300 watts is dissipated in .00504 ohm when current is 244 amps. So I see ability to pass 244 amps *maybe* with a fairly extreme water cooled heatsink, and assuming that maximum on resistance at 175 C does not exceed the maximum 25 C value times what that curve in Figure 4 indicates, and assuming that things are going to be fine and dandy and life expectancy and reliability will be sufficiently good when the junction is at 175 C. I would rather plan for 125 C junction temperature. Of course, the maximum current will be only a little less for heating the junction to 125 C than to 175 C... At .5 degree C/watt (device plus a rather extreme watwer cooled heatsink), 125 C junction, 25 C water temperature - this means 200 watts. Figure 4 indicates resistance at 125 C very nearly 1.7 times that at 25 C. This indicates 4.25 milliohms. At this rate, Ohm's Law indicates 217 amps. Assuming resistance does not exceed 1.7 times the maximum 25C value (which I do not consider a safe assumption), and assuming ability to achieve a .1 degree C / watt heatsink for a D2PAK even with running water. There is still the fact that the datasheet says the bonding wire limits safe continuous current to 195 amps, when case temperature is sufficiently low to have 195 amps not causing the junction temperature to exceed 175 C. (I calculate 96.75 C based on square relation between current and temperature difference between junction and case, and 270 amps being the "silicon limit" for 25 Ccase and 175 C junction. Figure 9 makes this maximum case temperature for 195 amps continuous appear (to me) slightly lower, to my eyeballs maybe 94-95 C. As large as these figures are and as small as a D2PAK is, it feels adventurous to me to go much past the current that many of the characterizations of this device are valid for (170 amps). 195 amps squared times .005 ohms (high-side but short of guaranteeable- maximum resistance at junction temp. of 125 C) is 190 watts. At heat discharge point temperature of 25 C and junction temp. of 175 C, this means thermal resistance of .79 degre C per watt. .4 of that is in the device itself, leaving .39 degree C per watt of thermal resistance maximum combined for the heatsink and the bonding of the device to the heatsink. That sounds to me like something quite on the beefy side and maybe requiring a little optimism even as far as fan-cooled heatsinks go. If figuring on 150 amps absolute maximum, things get easier. At junction temperature of 175 C with resistance .005 ohm high-side (but not guaranteed maximum), I find 112.5 watts. If heatsink temperature is 50 C, then thermal resistance is 1.11 degree C per watt - minus .4 for in the device itself, leaves .71 for the heatsink and the bonding of the device to the heatsink. It sounds to me like a more agressive modern CPU heatsink and fan can do that or almost can, assuming good bonding such as some sort of solder joint between the heatsink tab and the typically aluminum heatsink (gallium alloy, while using 600 grit sandpaper to sand the contact area of the aluminum heatsink while that area is immersed in in gallium alloy?) This is still with the heatsink being fan-cooled or something of more monstrous size as far as audio power amp heatsinks go. >There's no practical way to heatsink a D2PAK to dissipate 375 watts. I agree here. I seem to think semi-practically optimistically 300 watts, and realistically practically 112.5 watts is sounding to me like quite a notably big number. One thing my father tells me is that engineering is an economic science. I seem to think that there are alternatives more economical than water-cooling a D2PAK, including likely using larger or paralleled devices to avoid resorting to water-cooling. The gate charge of just one of these beasts is very considerable... 200 typical, 300 maximum nanocoulombs for gate delta V of 10 volts, and drain delta V of 30 volts. (To whatever extent this matters, the current being switched here is 170 amps.) 37 typ of this 200 typ nanocoulombs is in gate-source capacitance alone, indicating 3.7 nanofarads gate-source capacitance. *Typical* 60 of those nanocoulombs are "Miller", indicating 1.5 nanofarads of "Miller capacitance" from 40 volt change in D-G voltage. I wonder how the remaining 103 typ nanocoulombs is accounted for here. 200 typical, 300 maximum nanocoulombs gate charge with gate voltage change of 10 volts means that the gate looks like a 20 typ 30 max nanofarad capacitor *as averaged through voltage swing* between 0 and 10 volts. If the gate has voltage changed through this range in 1/2 microsecond, then the average current over that .5 microsecond is 400 mA typ, 600 mA max, with peak being noticeably higher. It appears to me that this is a big MOSFET in a small package. This MOSFET appears to me to have many of the requirements typical of MOSFETs of similar voltage and current ratings and larger package sizes. Any comments at this point, anyone? >John -- - Don Klipstein (don(a)misty.com)
From: Don Klipstein on 20 Mar 2010 03:59
In article <QKGdnSW78tT5rz7WnZ2dnUVZ_uIAAAAA(a)posted.localnet>, Robert Baer wrote: >John Larkin wrote: >> Just got a short-form IR mosfet thing in the mail. >> >> They have a PQFN 5x6 mm package they rate at 104 amps. And a D2PAK >> rated for 340 amps. >> >> John >> >...in that case, i have some #30 wire rated at 1,000 amps and will >guarantee it or double your money back! #30 wire sent back for refund under the warranty achieves refund at sold cents/foot for portions of the purchase that remain discernably defective wire after verifyingly endured 1,000 amps RMS for 1 continuous second! (Warranty does not apply for lower currents.) ==================== Warranty on the XZXZFUFYOU13 xenon flashtube, secondary safe operating area thereof, and the flashtube is a linear one having overall diameter less than 3.5 mm and overall length less than 25 mm: Secondary warranty on this flashtube guarantees minimum of 10,000 flashes provided anode-cathode voltage is at least 1,000 volts and less than self-firing voltage, ratio of flash frequency to flash energy does not exceed 1.6 watts, and flash energy is at least 100 megajoules. Warranty provides for refund of purchase price of the flashtube if the flashtube fails to successfully flash in these parameters. Warranty is only valid if the flashtube is actually flashed within these parameters and fails to repeat successful attempt at a flash within these parameters. Warranty furthermore requires that the failing flashtube be shipped to the engineering department of the supplier (ADDRESS BELOW) in order to qualify for refund of purchase price. Warranty covers nothing other than refund of purchase price of the flashtube. Warranty does not cover damages to property other than the flashtube in question, or injuries (whether physical or mental or both) (whether to human beings or other life forms or both) or killing any life form with lack of detectable injury (including but not limited to vaporizing-from-existence any life form or property in question). Warranty does not cover anything else, such as fire/blast damage to property other than the waranteed flashtube, or impairment of sexual or other physical or psychological or even partially psychological performance of a human being or other organism exposed to any damaging effects of a flashtube failing in a way qualifying for this warranty, or "the like". - Don Klipstein (don(a)misty.com) |