Heatsink for DC electronic load - Page 1
Forum section: electronics / projects

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 12:24:38 pm
content: I was looking around for a heatsink for a modular DC load I'm currently designing and ran acrossthis.How much power do you think it can handle before my 4 TO-220 MOSFETs attached to it blow? I'm guessing more than the 65W it's supposedly rated for since the MOSFETs can be allowed to run hotter than a CPU.The nice thing about this cooler is its price - US $4 on sale so I ordered 5 of them.

author: tszaboo
author_rank: Super Contributor
timestamp: June 30, 2015, 12:33:21 pm
content: Depends on the junction to case thermal resistance of the MOSFETs. You have somewhere in the ballpark of 0.5K/W to 2K/W for a TO220 power MOSFET plus insulation material. The heatsink itself will heat up. Mounting can be less than ideal.On the other hand, if all conditions are ideal, I think 65W should be possible.

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 01:02:21 pm
content: As far as I know MOSFETs can safely work at higher temperatures than a desktop CPU so I'm guessing around 80W is probably possible. Also, the cooler on the CPU is in a case that's above ambient while this will draw air at ambient temperature so it should be able to dissipate more.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 30, 2015, 01:43:57 pm
content: The heaviest amd that it is supposed to fit has an 125W TDP.With an Socket 775 server cooler (with 3 heatpipes) I was able to fume 65 Watts of a TO247 with a few mm oxide insulator.(Thermal cutoff NTC for 90C glued with paste in the to247 hole)

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 02:38:36 pm
content: That's one transistor, I have 4 so theoretically 20W/transistor should be ok. Overall thermal resistance will be lower than for 1 device.

author: nctnico
author_rank: Super Contributor
timestamp: June 30, 2015, 03:00:47 pm
content: I'd design it so the heatsink is connected to the input voltage. That way you can bolt the input terminals to the heatsink and avoid electrical insulation between the heatsink and the transistors. Electrical insulation increases the thermal resistance very quick.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: June 30, 2015, 03:07:02 pm
content: These coolers look about same size as ones I used (I think old athlon cpu coolers)to build an eload.I measured the thermal Resistance (S-A) of these it was .3 C/W with fan running. And 2.4 C/W fan off.Originally I designed to use 2 of these sinks/fans in order to mount two TO3-p case style Nfets on each sink which would have given me my aimed for ~220 Watts max of dissipation with fans and 40W without fans (But I only mounted one nfet on each sink in my prototype for testing (I haven't yet had thenecessity/inclination to mount the other 2 Nfets) and with these two I can do ~ 150W continuous with fans running but this doesnt leave much safety margin .I would say 60W each is comfortable for the fets I chose.Just do some common sense 'thermal impedance' matching similar sorta thing as you do with power transfer , (i.e theres no large advantage in increasing heatsink rating if the largest thermal bottleneck is the junction to sink resistance of cases and conversely no large benefits by adding more parrallel fets to reduce j-sink Resistance if sink-ambient rating of sink is largest bottlneck.Choose Mosfets carefully, although if only going for very low input voltages then this is no biggy. Otherwise dont choose Hi Gm, low rdson fets designed for switching, as there max dissipation rating at highger Voltages in linear mode will be very poor (often the data sheet's won't show this correctly or dont show it all). Good mosfets for linear are older generations with lower gm (especially) and higher rds on.Regards .

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 03:35:15 pm
content: Quote from: nctnico on June 30, 2015, 03:00:47 pmI was thinking about that.Quote from: Kevin.D on June 30, 2015, 03:07:02 pmThanks Kevin. I'll take that into acount. RDSis not very important here. I suppose I can get 20-25W out of a TO-220 package, the whole thing will have thermal shutdown anyway. I'll digging for suitable transistors right away.

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 03:51:22 pm
content: Looks like theIRF620might be a good candidate...What do you think?Or theIRF510?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: June 30, 2015, 05:45:39 pm
content: Old favorites irf510 and Irf530 .Yes if your satisfied with a to220 case dissipation rating then these would be good choices.Whilst higher rds(on) is good here note that it limit's the max current at LOW input Voltages (i.e if Rsense = .1 and min rds (on) = .4 so your min Output R is now .5 so if DUT is .5V max I is now 1 A ) .So more in parallel or irf530 (rds(on) .16 if higher current's at low Vins needed.In the transfer characteristics of those look at the nice slow linear rise in Id Vs Vgs over the full input Vgs range (Meaning a large variation in temp (which causes a lowering of Vgs(th)) now only causes a relatively mild increase in Id and so less chance of thermal runaway internally at high Vds ( a quicky calculation of the extra power dissipation @ max Vin due to that xtra current makes these to be almost immune to thermal runaway on a good heatsink . You could probably even get away running these in parallel without a opamp each fet if you use one xtra as safety margin )) .regards

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 05:52:40 pm
content: Quote from: AcHmed99 on June 30, 2015, 05:14:08 pmOh crap, I overlooked the thermal derating factor.Quote from: AcHmed99 on June 30, 2015, 05:14:08 pmHmm, they're quite expensive...Haven't made any bill of materials calculation yet but it might be cheaper to use less TO-247/TO3P rather than more TO-220 devices. Each MOSFET will have its own two driving op amps (no, not in parallel).IRFP150Mlooks like a good choice at $1Also, gmis gfsin the datasheets?

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 09:04:10 pm
content: Quote from: AcHmed99 on June 30, 2015, 07:35:00 pmTwo of them should be good for 100W total then but I'll double check. They're the cheapest I could find for a TO-247 case.

author: Mr.B
author_rank: Unknown
timestamp: June 30, 2015, 09:41:17 pm
content: I used IXTH30N50L2 to build my load.Two TO-247 per CPU cooler.Although not cheap...Datasheet below.The SOA graph says it all.

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 10:42:44 pm
content: Quote from: AcHmed99 on June 30, 2015, 09:33:04 pmLooks like two won't be good enough for 100W assuming 0.25C/W heatsink-ambient and 0.5C/W case-heatsink at an ambient temperature of 40C. I simply halved the thermal resistance of 1 transistor to get the pair's combined thermal resistance.Quote from: Mr.B on June 30, 2015, 09:41:17 pmStaying away from IXYS mainly because of the price. Building a dirt cheap modular DC load using $4 CPU coolers.Quote from: Mr.B on June 30, 2015, 09:41:17 pmHow much power are they dissipating? They seem overkill.

author: Mr.B
author_rank: Unknown
timestamp: June 30, 2015, 11:00:02 pm
content: Quote from: void_error on June 30, 2015, 10:42:44 pm100W per FET.My "Power Module" has four FETs and two coolers.The power boards are designed so that they can be daisy chained using FPC cable for the control and feedback signals.The controller board is separate.Cooling is provided by Corsair H60 CPU water block cooler units with high velocity fans. One on each side of the Power Module.At 30 degrees C ambient I estimate that Tj is possibly about 115 degrees.I say 'estimate' because Corsair will not give me any details on the thermal performance of their cooler, so all I can do is measure it manually as best I can.I am fairly sure I could get more than 100W per device, but I am not convinced the coolers would perform satisfactory.

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 11:24:09 pm
content: Quote from: void_error on June 30, 2015, 10:42:44 pmTwoIRFP460s should provide an adequate safety margin.

author: Mr.B
author_rank: Unknown
timestamp: June 30, 2015, 11:32:57 pm
content: Quote from: void_error on June 30, 2015, 11:24:09 pmThat FET does not look to me to be designed for linear use.Also, Vishay have not plotted a DC curve on their SOA graph, so there is no way to determine from the datasheet how it might perform as a linear device.Edit: I am no expert by a long shot, but it has a "linear derating factor" of 2.2 W/deg... That doesn't look good.

author: fivefish
author_rank: Frequent Contributor
timestamp: July 01, 2015, 05:25:02 am
content: I built this... will do 240W dissipation.Tested it for at least 30min at 30V @ 6A, 60V @ 3A (180W) and 120V @ 2A (240W).Around 120V @ 2.5+A, the mosfet gets shorted permanently... oops!Using (2) Mosfets, one per CPU cooler.I tapped the copper core, and screwed the TO247 directly to the heatsink core.Using multi-turn pot for fine-grained control.Also a BNC jack, for connection to a scope so I can monitor DC waveform if needed.12VDC adapter.

author: Jeroen3
author_rank: Super Contributor
timestamp: July 01, 2015, 06:11:53 am
content: I used an IXTQ22N60P and 3 mm of aluminum oxide insulator plate.The problem is that you cannot make a load step from ambient temperature to theoretical maximum load. The mosfet has too much thermal inertia(is that the right term?)and it will break. You have to ramp it up slow enough for it to not create a lot of thermal delta and expansion stress.Discussing soa curves is really awkward in dutch... since soa == std.

author: void_error
author_rank: Frequent Contributor
timestamp: July 01, 2015, 09:24:17 am
content: Quote from: Mr.B on June 30, 2015, 11:32:57 pmHere are some calculations I've done:2xIRFP460 - Rth[j-c]=0.225C/W (0.45/2) PD=280W, 2.2C/W derating above 25C2xKAP 218- Rth[c-h]=0.27C/W - it's 0.15C/W for 1 sq in, scaled it down to the transistor's thermal pad and divided by two because there's two transistors (result is actually 0.535C/W for a kapton pad that's the exact size of the transistor's thermal pad)DEEPCOOL CK-AM209 - Rth[h-a]= 0.25C/W estimatedRth[j-a]= 0.225 + 0.270 + 0.250 = 0.745C/WFor 100W that results in a 74.5C temperature rise above ambient.For a 40C ambient (worst case) Tj= 114.5C. I don't run above 30C ambient so this won't either.Tcwill be Tj-(0.225*100)=92CApply the linear derating factor of 2.2W/C and you get 280-((92-25)*2.2)= 146W you can dissipate at a Tc=92C.If I've done this right then these MOSFETs are overkill but I've probably screwed up somewhere. Please correct me if I'm wrong.EDIT:IRFP140 - 99.6W @ Tj=133.5CIRFP250M - 120.2W @ Tj=127.0CIRFP450 - 89.5W @ Tj=124.5CIRFP150M - 99.6W @ Tj=139.5C - cheapest TO-247IRFP140N - 79.0W @ Tj=147.0C

author: void_error
author_rank: Frequent Contributor
timestamp: July 01, 2015, 03:17:14 pm
content: I have another idea which will go well with they way I plan to mount the transistors.They'll be bolted onto the heatsink using M3 screws and with the leads bent forward. The whole thing will sit on top of the PCB.I want to add an I2C temperature sensor to each of them. They'll be mounted on daughter boards in direct contact with the MOSFETs using thermal paste.TheMCP9805andMCP9843are the cheapest I could find. They have 3 external address bits so than means with two of them per DC load module I can use up to 4 DC load modules daisy-chained for a total power of 400W.They also have a pin called "Event" which can be used for hardware shutdown of the DC load module in case the case temperature of the MOSFETs exceeds a preset value.What do you think?

author: fivefish
author_rank: Frequent Contributor
timestamp: July 01, 2015, 03:53:17 pm
content: QuoteThat's what I did, tapped and threaded the heatsink, and bolted my TO247 to it. There are pads on the PCB that align with the TO247 legs if they're bent forward. But on my photo above, I just used flying wires to attached to the PCB.If you didn't use an insulator (mica, silpad, etc), the heatsink's aluminum body will be at same potential as the V+ of device under test. It's up to you if that's a big deal or not.Here's my heatsink's fin temperature after running 30 minutes at 180Watts. Later on, I realize I should have connected the temp probe at the TO247 itself. The junction temperature will be hotter than this 40C.Good luck on your project!

author: void_error
author_rank: Frequent Contributor
timestamp: July 22, 2015, 07:15:46 pm
content: Sorry for bumping this up again...I've been double-checking the MOSFETs I was planning to use and and oops... they're probably gonna blow up. SOA is not DC rated. Found an IXYS device that would do and hold up at DC dissipating the required power but it's bloody expensive so I started looking at darlington BJTs. Much cheaper. Found the BDV65B to be suitable as far as SOA, VCEand ICare concerned. Haven't done the thermal calculations yet.Is it a good idea to use BJTs instead of MOSFETs?

author: dom0
author_rank: Super Contributor
timestamp: July 22, 2015, 08:59:46 pm
content: BJTs are a very good option for a current sink, under the following conditions:- required minimal compliance voltage is >2 V- required currents are not very high (which would lead to a lot of control current and power)- note second breakdown at higher Uce. It gives similar limits to DC operation to BJTs as the Spirito effect gives to FETs.

author: TopLoser
author_rank: Unknown
timestamp: July 22, 2015, 09:06:42 pm
content: Quote from: void_error on July 22, 2015, 07:15:46 pmSee my teardown of a 1200W 240A that uses MOSFETS, posted it today in the test equipment board.

author: void_error
author_rank: Frequent Contributor
timestamp: July 22, 2015, 10:43:24 pm
content: Quote from: dom0 on July 22, 2015, 08:59:46 pm- not planning to go below 2.5-3V- up to 2A per transistor- not going to exceed SOA, MCU will adjust max current depending on the voltage to keep it within the SOA. I know more about BJTs than I know about MOSFETsAfter I do some math to see if I don't exceed the junction temperature I'll know how many devices I need for a 100W module.Quote from: TopLoser on July 22, 2015, 09:06:42 pmNice! 37.5W/MOSFET (1200W/32). Using IRFP250N which is what I was planning to use. It seems they might be able to survive 40W continuous each in my design, although 50W is kind of pushing it. Thing is there'll be one temperature sensor for each MOSFET to measure case temperature and I've got only 8 I2C addresses available which means a max of 4 modules per bus...What do you think? Some thermal calculations can be foundhere.