Den s�ndag den 25. januar 2015 kl. 12.37.15 UTC+1 skrev Chris Jones:> On 25/01/2015 04:40, John Larkin wrote: > > On Sat, 24 Jan 2015 23:57:24 +1100, Chris Jones > > <lugnut808@spam.yahoo.com> wrote: > > > >> On 23/01/2015 23:31, Alain Coste wrote: > >>> Thank you John for the interesting information. The use of mosfets in linear > >>> mode is not very common, and it's more difficult to find data than for > >>> switch mode. > >>> > >>>> It often makes sense to use more fets, spread out over the heatsink > >>>> surface, especially if the baseplate part of the heat sink is thin, > >>>> namely has high thermal spreading resistance. > >>>> > >>>> This uses copper heat spreaders to transfer the heat into the aluminum > >>>> sink. > >>>> > >>>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Amp.jpg > >>>> > >>> > >>> Now I see what to use _more_ fets means... > >>> For my electronic load I could have used more transistors, but this > >>> increases the number of current sense resistors and operational amplifiers > >>> to control them. For the power I wanted (400 .. 420W), I thought that two > >>> mosfets was a good compromise. > >>> [...] > >> > >> > >> I am building an electronic load using a single IXTN60N50L2. It uses an > >> unconventional control scheme which in my case was easier to do with a > >> single large mosfet than with many small ones. I am cooling the MOSFET > >> with liquid. I was very surprised that nobody seems to sell water blocks > >> already drilled for SOT-227 packages. I expect that if I drilled the > >> required mounting holes on any of the widely available CPU or GPU water > >> blocks, the drill would hit a water channel and it would leak. Therefore > >> I expect I'll have to make my own water block. > >> > >> Chris > > > > McMaster sells cold plates, cheap compared to most others. No problem > > drilling this kind. > > > > https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Coldplate.jpg > > > > https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Water_Cooled_Pulser.jpg > > > > https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Copper.jpg > > > > > > Thanks. Since the "cooling" fluid that I will get arrives already at > about 70 deg C, and I want to dissipate a lot of power, I want very low > thermal resistance, so I think I will use something made from copper. I > also need it to be not much bigger than the SOT-227 package because of > the mechanical constraints. I would like to find something already made > for this package, but otherwise I might as well silver-braze some pipes > into a block of copper - either into grooves made with a ball-end mill, > or drill long holes right through the copper (yuck!), drill manifolds in > the other direction, and braze plugs into the unnecessary holes and > braze on inlet and outlet pipes, then mill the mounting surface flat. I > only need a couple of them. > > If I could buy a small version of this CP25 thing with threaded inserts > in the right place for SOT-227 then I would be tempted: > http://www.amstechnologies.com/products/thermal-management/liquid-cooling/cold-plates/vacuum-brazed-flat-tube-pin-fin-cold-plates/view/extended-surface-iiTM-cp25/ > > Chrisnot exactly cheap but if you only need a few http://www.customthermoelectric.com/Water_blocks.html WBA-1.62-0.55-CU-01 drill and tap CPT-2.25-1.62-0.25-AL for the sot-227 or you could just take on of the cheap GPU coolers and reflow solder a ~10mm plate of copper on top drilled and tapped for the sot-227 http://i01.i.aliimg.com/wsphoto/v0/2027141455_2/New-Water-cooling-Copper-Water-Block-40-x-40-x-10-mm-free-shipping.jpg -Lasse
estimating junction temperature of a power mosfet
Started by ●January 21, 2015
Reply by ●January 25, 20152015-01-25
Reply by ●January 26, 20152015-01-26
On 26/01/2015 05:21, Lasse Langwadt Christensen wrote:> Den s�ndag den 25. januar 2015 kl. 12.37.15 UTC+1 skrev Chris Jones: >> On 25/01/2015 04:40, John Larkin wrote: >>> On Sat, 24 Jan 2015 23:57:24 +1100, Chris Jones >>> <lugnut808@spam.yahoo.com> wrote: >>> >>>> On 23/01/2015 23:31, Alain Coste wrote: >>>>> Thank you John for the interesting information. The use of mosfets in linear >>>>> mode is not very common, and it's more difficult to find data than for >>>>> switch mode. >>>>> >>>>>> It often makes sense to use more fets, spread out over the heatsink >>>>>> surface, especially if the baseplate part of the heat sink is thin, >>>>>> namely has high thermal spreading resistance. >>>>>> >>>>>> This uses copper heat spreaders to transfer the heat into the aluminum >>>>>> sink. >>>>>> >>>>>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Amp.jpg >>>>>> >>>>> >>>>> Now I see what to use _more_ fets means... >>>>> For my electronic load I could have used more transistors, but this >>>>> increases the number of current sense resistors and operational amplifiers >>>>> to control them. For the power I wanted (400 .. 420W), I thought that two >>>>> mosfets was a good compromise. >>>>> [...] >>>> >>>> >>>> I am building an electronic load using a single IXTN60N50L2. It uses an >>>> unconventional control scheme which in my case was easier to do with a >>>> single large mosfet than with many small ones. I am cooling the MOSFET >>>> with liquid. I was very surprised that nobody seems to sell water blocks >>>> already drilled for SOT-227 packages. I expect that if I drilled the >>>> required mounting holes on any of the widely available CPU or GPU water >>>> blocks, the drill would hit a water channel and it would leak. Therefore >>>> I expect I'll have to make my own water block. >>>> >>>> Chris >>> >>> McMaster sells cold plates, cheap compared to most others. No problem >>> drilling this kind. >>> >>> https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Coldplate.jpg >>> >>> https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Water_Cooled_Pulser.jpg >>> >>> https://dl.dropboxusercontent.com/u/53724080/Thermal/T222_Copper.jpg >>> >>> >> >> Thanks. Since the "cooling" fluid that I will get arrives already at >> about 70 deg C, and I want to dissipate a lot of power, I want very low >> thermal resistance, so I think I will use something made from copper. I >> also need it to be not much bigger than the SOT-227 package because of >> the mechanical constraints. I would like to find something already made >> for this package, but otherwise I might as well silver-braze some pipes >> into a block of copper - either into grooves made with a ball-end mill, >> or drill long holes right through the copper (yuck!), drill manifolds in >> the other direction, and braze plugs into the unnecessary holes and >> braze on inlet and outlet pipes, then mill the mounting surface flat. I >> only need a couple of them. >> >> If I could buy a small version of this CP25 thing with threaded inserts >> in the right place for SOT-227 then I would be tempted: >> http://www.amstechnologies.com/products/thermal-management/liquid-cooling/cold-plates/vacuum-brazed-flat-tube-pin-fin-cold-plates/view/extended-surface-iiTM-cp25/ >> >> Chris > > not exactly cheap but if you only need a few > > http://www.customthermoelectric.com/Water_blocks.html > > WBA-1.62-0.55-CU-01 drill and tap CPT-2.25-1.62-0.25-AL for the sot-227 > > or you could just take on of the cheap GPU coolers and reflow > solder a ~10mm plate of copper on top drilled and tapped for the sot-227 > > http://i01.i.aliimg.com/wsphoto/v0/2027141455_2/New-Water-cooling-Copper-Water-Block-40-x-40-x-10-mm-free-shipping.jpg > > -Lasse >Thanks for those links. They are quite interesting.
Reply by ●January 29, 20152015-01-29
Thanks to all those who responded. I decided initially to limit the power of the electronic load to 300W (two fets in //), which seems safe. Later I will make an experimental setup to measure the parasitic diode drop at various temperatures, and see if I can safely dissipate 400 to 450W. For now, I am fighting against parasitic thermo-electric effect of sense resistors, which seriously compromises the precision of intensity measurement and regulation athigh power dissipation. -- Alain Coste "Alain Coste" <coste@irit.fr> a �crit dans le message de news: m9oq8a$sfu$1@dont-email.me...> Hello > > I design an electronic load, using IXTK200N10L2 power mosfets (T0264 > package). > I try to estimate the junction temperature of the mosfets, using an IR > camera to measure accessible temperatures. > Mosfets are directly mounted on a air-forced heatsink (Theta-sa around > 0.08�C/W), with thermal grease. > With the camera, I can access the temperatures of : > - the heatsink near the mosfet (Ts) > - the package top of the mosfet (Tt) > - but not to the case temperature (Tc), as the metallic part of the > TO264 package is not visible. > Ts is in line with the calculated value, but the measured Tt is much > higher > than the calculated Tc (from Theta-jc, Theta-cs of the data-sheet, and > Theta-sa of the heat-sink). > I suppose this is normal, as much of the heat is evacuated through the > heatsink, and few watts flow from the package top to ambiant. So Tt is > probably closer to Tj than to Tc, but how much ? > Is there a way to estimate Tj from Tt. I am looking for some parameter as > Psi-jt, but nothing of the sort appears in the data-sheet....Or perhaps a > rule of thumb saying that, for a TO264 package mounted on a "serious" > heatsink, Tt is never less than Tj - 5�.... > I found this kind of information for smt IC packages, but not for TO264 or > TO220. > > NB : I could also measure the lead (drain) temperature, which seems to be > close to Tt. This reinforces my idea that the measured Tt is not too far > from Tj, but I would be sure before jeopardising such expensive devices as > IXTK200N10L2 ! > > Thanks in advance > > > -- > Alain Coste > > > --- > L'absence de virus dans ce courrier electronique a ete verifiee par le > logiciel antivirus Avast. > http://www.avast.com >--- L'absence de virus dans ce courrier electronique a ete verifiee par le logiciel antivirus Avast. http://www.avast.com
Reply by ●January 29, 20152015-01-29
On Thu, 29 Jan 2015 13:31:57 +0100, "Alain Coste" <coste@irit.fr> wrote:>Thanks to all those who responded. >I decided initially to limit the power of the electronic load to 300W (two >fets in //), which seems safe. >Later I will make an experimental setup to measure the parasitic diode drop >at various temperatures, and see if I can safely dissipate 400 to 450W. >For now, I am fighting against parasitic thermo-electric effect of sense >resistors, which seriously compromises the precision of intensity >measurement and regulation athigh power dissipation.Manganin has a low tc and a low thermoelectric potential against copper. Zeranin is even better. We make our own manganin shunts, which we epoxy to an anodized aluminum block which is itself thermally regulated to sit at the flat spot of the manganin's parabolic resistance/temperature curve. That's extreme, but heat sinking a strip of manganin can make an awfully good shunt resistor. https://dl.dropboxusercontent.com/u/53724080/Parts/Shunts/Manganin_Bits.JPG What's important is thermal symmetry of the two shunt-to-copper junctions. With modern chopper opamps, you don't need a lot of voltage to work with, which helps keep the shunt heating down. Interesting geometries are possible. You can buy thru-hole and surface mount low TC shunt resistors, which are fine if the power dissipation is low. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
