DC load using a CPU cooler - Page 1
Forum section: electronics / projects

author: microbug
author_rank: Frequent Contributor
timestamp: September 13, 2014, 10:09:58 am
content: I'm going to make myself a simple electronic load with optional dynamic load (pulsing) capability, based on Jay_Diddy_B's dynamic load (driving the current input with a DAC). I'm just wondering why I haven't seen any loads around that have CPU coolers (active heatsinks) instead of a lump of aluminium. From my limited research, they're fairly cheap and can have thermal characteristics of less than 1C/W (easy to dissipate 75W per cooler). Comments?

author: bktemp
author_rank: Super Contributor
timestamp: September 13, 2014, 10:47:14 am
content: I have done that, works great. Especially the better ones with heat pipes can easily dissipate >100W. The only problem: Either it is big an quiet or small and noisy.

author: mrflibble
author_rank: Super Contributor
timestamp: September 13, 2014, 11:03:23 am
content: Quote from: bktemp on September 13, 2014, 10:47:14 amDitto.QuoteYup. IMO best go for big and quiet. And to keep it cheap you can buy after market coolers for an outdated socket from the bargain bin.

author: microbug
author_rank: Frequent Contributor
timestamp: September 13, 2014, 11:18:35 am
content: Thanks for the bargain bin idea! You can get coolers fairly cheap on aliexpress as well. Good to know it works!

author: rob77
author_rank: Super Contributor
timestamp: September 13, 2014, 12:10:55 pm
content: why to buy a new cpu cooler ? i built my DC load with a old pentium II passive cooler with a second old amd sempron cooler on top of itit takes ~ 75W continuous and 100W short term... doesn't have to be noisy either - 2 PTCs (one glued near the MOSFETs and one near the power-resistor) and comparators with hysteresis controlling the fan.

author: Unknown
author_rank: Unknown
timestamp: September 13, 2014, 12:14:37 pm
content: I have an entire drawer full of various heat sinks/fans from Socket 3 upwards. Let me know if you're after something in particular.

author: microbug
author_rank: Frequent Contributor
timestamp: September 13, 2014, 12:16:21 pm
content: I would, but shipping a fairly heavy heatsink from Australia to the UK would cost a fortune! I imagine there are plenty of cheap heatsinks for outdated sockets on eBay as well.

author: Unknown
author_rank: Unknown
timestamp: September 13, 2014, 12:20:46 pm
content: Anything up to 500g from AU to UK costs AUD$10.80 via Sea Mail and 500g - 1000g is AUD$20.45 (roughly double it for air mail).

author: microbug
author_rank: Frequent Contributor
timestamp: September 13, 2014, 12:27:08 pm
content: Wow! That's not bad... I'll let you know if I need something, but the point of this is that I can choose some outdated cooler which is cheap and design the PCB to fit it.

author: max_torque
author_rank: Super Contributor
timestamp: September 13, 2014, 12:32:04 pm
content: Thing is, a CPU cooler is doing a specific job, which is to MOVE a high heat flux heat from a very small area, and then reject it to the ambient. A typical DC load, to handle a high power dissipation will need multiple parallel power devices, and so you don't have such a small area / concentrated heat source. Also, in modern computer design, the issue is packaging the heat transfer device into an ever smaller physical space, again, unlikely to be an issue with a typical bench top DC load.And ultimately, you have to reject that heat to the ambient atmosphere, which for a high power DC load requires a lot of surface area or a lot of fast moving air, as it is (should be) continuously rated. This means either a large heatsink or a powerful fan.I suspect that while CPU coolers have nice low thermal transfer co-efficient, they actually don't have amazing total heat rejection values (because the CPU is still a relatively low power device (it gets "hot" because it is so compact!)For my DC load i used a nice big heatsink, placed the power devices nicely spread out on it, and used two powerful speed controlled server cooling fans to blow a LOT of air past it. This manages pretty much 300W continuous in an 30degC ambient.

author: microbug
author_rank: Frequent Contributor
timestamp: September 13, 2014, 12:49:15 pm
content: I can see that CPU coolers might not be ideal, but price (at least for me) plays a big part in it. Unless you have any suggestions for a place to get large heatsinks (I'm in the UK, so shipping plays a big part unless it's local) then I'll probably use a CPU cooler or two. It doesn't need to be hugely powerful; 100W would be fine.

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2014, 12:56:30 pm
content: CPU coolers have a big advantage of coming with a built in fan, or if you need more than the 100W typical rating you can use old water cooling units as well, which can go up to 300W or more with the right orientation and care on airflow. If you really want high power a car radiator with electric assist fans and a washing machine drain pump will do multi kilowatt power levels.

author: timb
author_rank: Super Contributor
timestamp: September 13, 2014, 01:03:40 pm
content: I was looking at using IRF'a DirectFET packaging for my load. Tiny metal surface mount packages that are as close as you can get to a bare die.You can place then fairly close together, which eliminates the "small area" problem mentioned above.There's some nice heat pipe based CPU coolers on Amazon that are specced for 250+ Watts of dissipation. Hell, they've got a tiny $15 one that does 100W!My current prototype uses an extruded aluminum case from Hammond. I found an old Thermaltake P4 Heatsink in my fan box; it mounts perfectly onto the top cover of the case. I've got it glued on with Arctic Silver Thermal Epoxy. I then drilled and tapped two mounting holes through the inside top cover of the case (into the heatsink), for attaching two TO-220 MOSFETs.So effectively the entire case becomes part of the heatsink.Works great!Sent from my Smartphone

author: jmole
author_rank: Regular Contributor
timestamp: September 13, 2014, 01:04:01 pm
content: Quote from: SeanB on September 13, 2014, 12:56:30 pmthe newer ones are nice too, you can get a closed loop watercooling system (i.e. low-hassle, runs on 12V) with dual 120mm fans, a radiator, and a water block for pretty cheap. then all you need is some decent thermal epoxy or even thermal tape.

author: poorchava
author_rank: Super Contributor
timestamp: September 13, 2014, 01:11:05 pm
content: I used cpu coolers in my linear 24V/2.5A power supplies. No problem whatsoever even at dead short and maximum current.

author: void_error
author_rank: Frequent Contributor
timestamp: September 13, 2014, 02:21:16 pm
content: I initially wanted to use a CPU cooler for my electronic load but then I remembered I had these in my junkbox (80x80mm fan for reference):A modest CPU cooler can handle up to 100W if you use enough MOSFETs to improve heat transfer.

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2014, 02:35:53 pm
content: Patrick Norton did a water cooler using an old truck radiator as the thermal dissipater. Basically did not need a fan to blow air through it, it was so large compared to the heat input that convection alone worked.I did a machine conversion to cool water using a condensing unit I bought as a spare part for a display counter, the base plate, accumulator and condenser coil on a plate. Just used the condenser and made some fittings to go to 12mm nylon pipe on one end and the other end to dump back into the tank. Has about 2C temperature drop across it water wise and works well.

author: Spikee
author_rank: Frequent Contributor
timestamp: September 13, 2014, 09:34:07 pm
content: I currently have a board under way from china that i designed to be a programmable dc load.It uses IXYS L2 fets that are laying flat (metal tab up). A normal intel775 heatsink bracket is designed to fit.In my design i used the Artic freezer 7 pro rev 2 cpu cooler. It is verry quiet and can handle around 175W using the stock fan. One needs to but an additional fan or a faster one on it to handle more heat.My project is called Megaload. Some details are somewhere on this forum. Will post more details when the board comes in.

author: Balaur
author_rank: Unknown
timestamp: September 13, 2014, 10:12:03 pm
content: TheCBAuses a rather modestly-sized cooler with a fan. Have one, works very well.

author: microbug
author_rank: Frequent Contributor
timestamp: September 13, 2014, 10:19:22 pm
content: Spikee, your project sounds pretty interesting... I just selected the Arctic Cooler 13 (200W) for my load; I'm using 3 D2PAK 100W rated FETs and I'll add a thermal cutoff on the heatsink. How did you design in the bracket? Is it just a set of holes or is there a part you can buy?With the 3 100W FETs, I could get a 300W load with the Arctic Cooler 13 Pro, but I don't really need the extra power. It would be quite easy to upgrade if I did though, just stick in the larger cooler.

author: Spikee
author_rank: Frequent Contributor
timestamp: September 13, 2014, 11:15:22 pm
content: Quote from: microbug on September 13, 2014, 10:19:22 pmIt uses the intel 775 socket mounting holes. Theoretically all heatsinks that support this socket should fit.The heatsink i use now can handle stock 150w and costs around 15euro. The A13pro can handle 300w but costs 35euro.If i needed more power i would just use another board (it is modular isolated i2c control) or put an additional or faster fan on it.

author: rob77
author_rank: Super Contributor
timestamp: September 14, 2014, 06:37:06 am
content: Quote from: microbug on September 13, 2014, 10:19:22 pmsorry, but you simply can'tyou always have to consider the SOA (safe operating area) and also have to count with the thermal de-rating of the mosfet. for my load i paralleled 4 mosfets to get 100W dissipation and keeping the mosfets within SOA - i used IRFZ34N , "68W" each according to datasheet, but considering the derating and SOA, it's only 20-25W each.

author: blackdog
author_rank: Frequent Contributor
timestamp: September 14, 2014, 08:44:04 am
content: Hi,If you want a electronic load that has a good ac respons, look @ my schamatic on this dutch forum.Please use google translate, and read howe to wire is, this is of great importance.There are 2 types of electronic load's, one is the "stupid" type, only DC, and my schematic is a controled AC type.But, the wiring must be done neatly, NO SPAGHETTI.Read the hole Article, there is a lot of useful info in there.Can you use different MOSFET's NO!Can i use more MOSFET's, not in this schematic, the NE5534 cant drive more than 2.Can i switch the BD139-16 for a 2n3904, NO!Can i use a LM324 as a opamp NO,NO,NOIf you build it, folowing my derections, its better than Agilent enz.why would you want to modulate the dummy load, only then watchin you scope you can see the real impedance of youre power supply.And there are other neat tricks to do with a modulated Current Souce :-) (Class A Audio Amp)The schematic,Test setup on a PC heatpipe coolerArticle linkhttp://www.circuitsonline.net/forum/view/121338/1Kind regarts,Blackdog

author: poorchava
author_rank: Super Contributor
timestamp: September 14, 2014, 02:09:53 pm
content: If you don't need this to run at full power you can just bolt the mosfets to some random large heatsink and dump it into a tub of mineral oil. Ice water is even better but then you need to make sure that only fins of the heatsink are immersed. I made quick and dirty 24V/35A load this way using 12 SUP90N03 mosfets.

author: macboy
author_rank: Super Contributor
timestamp: September 15, 2014, 07:15:26 pm
content: A typical higher-end CPU heatsink is asked to dump 125 to 140 W of power into ambient (internal PC temperature) of 35 deg C while keeping the DIE, not heatsink, not case, but DIE temperature of the CPU below 60 deg C. Add to that the fact that it is compact and inexpensive, and it is a winner all around.

author: microbug
author_rank: Frequent Contributor
timestamp: September 15, 2014, 07:29:43 pm
content: Yes- I agree that it seems like an all round good idea.FYI, I'm going with 3 100W rated MOSFETs on a 200W cooler (over voltage/current limits in hardware and temperature sensor on heatsink + emergency temperature cutoff on heatsink). Single channel 16 bit DAC controlling current, dual channel 16 bit ADC measuring voltage through a resistor divider, diode clamp and current limiting resistor and measuring current through an INA196. I chose 16 bit data converters because they are not too expensive (less than £2 each). Precision low offset op-amps are used.Sorry for the brief wording but I'm typing this on a smartphone.

author: eneuro
author_rank: Super Contributor
timestamp: September 15, 2014, 07:40:05 pm
content: Quote from: poorchava on September 14, 2014, 02:09:53 pmIf we connect this tube to such room aluminium heater radiator (<10$ about 0.5m long ) we can probably run at full power without any fansEasy and scalable-if we need more power to disipate simply add another one

author: Spikee
author_rank: Frequent Contributor
timestamp: September 17, 2014, 04:42:56 pm
content: It is alive !It has a few bugs, i placed the 3d model facing the wrong way. So to fix the pinout i needed to mount the heatsink and mosfets on the bottom.Also to rise the mosfets up it is advisable to use some discard able 0805 resistors under it so it rises the mosfet by around 1mm.With some tweaking it is definitely something that is doable.

author: rob77
author_rank: Super Contributor
timestamp: September 17, 2014, 05:34:29 pm
content: Quote from: Spikee on September 17, 2014, 04:42:56 pmwhere are your current sensing resistors ? even if you use 0R1 then it's 2.5W at 5Amps - can't see any resistors capable of 2.5W on your boardand sorry , but i doubt it's alive - half of the parts are missing yet

author: Spikee
author_rank: Frequent Contributor
timestamp: September 17, 2014, 05:46:33 pm
content: Quote from: rob77 on September 17, 2014, 05:34:29 pmThere are two big ones on the bottom =)mouser.com/Search/ProductDetail.aspx?R=WSL36372L000FEA

author: rob77
author_rank: Super Contributor
timestamp: September 17, 2014, 05:55:43 pm
content: Quote from: Spikee on September 17, 2014, 05:46:33 pm2milliOhm ?what op-amp are you using ? 1mV input offset means half an Amp error with a 0R002 current sensing resistor.

author: Spikee
author_rank: Frequent Contributor
timestamp: September 17, 2014, 05:58:10 pm
content: The INA225 monitors each current shunt.but keep in mind that this is still a prototype. Parts might change.

author: kc9qvl
author_rank: Regular Contributor
timestamp: September 18, 2014, 12:15:37 am
content: That looks very nice.Mine is very meager compared. How many watts can it dissipate?

author: Spikee
author_rank: Frequent Contributor
timestamp: September 18, 2014, 12:49:20 am
content: with a better heatsink 300-350w should be possible. With the current smal one only around 150-200w.

author: Nerull
author_rank: Frequent Contributor
timestamp: September 18, 2014, 01:49:30 am
content: Quote from: max_torque on September 13, 2014, 12:32:04 pmWith CPUs out there outputting more than 200W of heat under load and gamers who can't stand to see their CPUs top 40C, I'm sure there are heatsinks out there that can handle that.

author: microbug
author_rank: Frequent Contributor
timestamp: September 18, 2014, 03:13:09 pm
content: Yes, there are PC coolers (e.g., Arctic Cooling 13 Pro) that can do 300W. I imagine there are some that can do more, but CPUs which generate more than 300 watts of heat are generally intended for servers (Intel Xeon). I could switch from the Arctic Cooling 13 to the Arctic Cooling 13 pro - it's only £10 more, but I don't really need the extra power. If I ever do, I can just buy the Arctic Cooling 13 Pro and fit it in place of the old cooler, and change the firmware so it doesn't cut out above 200W.I just realised that a PSoC would be perfect for this application and would cut down on the number of external components I need. I'll order a PSoC 4100/4200 dev board and take a look (I got a 10% off code for Farnell by email)

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 08:58:24 am
content: @spikee - how large is the thermal contact on the Freezer 7 (CPU cooler)? I'm wondering how many TO-220 packages I can get under there. I'll probably use either the Freezer 13 or Freezer A30 (17=200W, A30=320W), which (I think) have similar thermal pad sizes.I'm going to use a different MOSFET to the one I first chose (and stupidly ordered), as the SOA dictates a max of 10V at full current!EDIT: What would be better thermally: D2PAK packages on the bottom side with thermal stitching, and a cooler on the top; or upside-down TO-220 packages directly touching the cooler?

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2014, 09:53:32 am
content: Upside down TO247 packages are better, lower thermal resistance. Plus you can fit a bigger die inside which has lower resistance to case as well. You probably will only get 9 under the thermal contact, you need a little room to bend the leads and board to attach them to. Use the thickest PCB you can get, otherwise you will need to machine a back brace to get even contact to all 9 dies, otherwise the warping of the board will cause the middle one and the ones not in the corners to not have good thermal contact. A back brace made from thick insulating material with cutouts for the lead stubs will probably be needed in any case, or a thinner one with an aluminum backing plate to give even pressure.

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 10:25:39 am
content: OK, thanks. For a back brace, could I use a block of aluminium (6mm thickness, seethiseBay listing for source) and drill holes in for the leads? I have a drill press. I could get a thicker aluminium block (1cm for example) if that would be better for the back brace.Thanks for your help!

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2014, 11:59:39 am
content: Yes, any plate that is stiff, big enough to act as a load spreader and which you can insulate from the board with something like a blank board with milled holes for the leads. You just need it to press evenly on the board so it presses the transistors evenly onto the heat plate.

author: timb
author_rank: Super Contributor
timestamp: September 20, 2014, 12:59:54 pm
content: Quote from: SeanB on September 20, 2014, 09:53:32 amWait, 9??? Look at the picture of the heatsink portion. (5th Picture)That looks like *maybe* enough room for two TO-220 packages...http://www.amazon.com/gp/aw/d/B006TCSH4Y/Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 01:02:03 pm
content: Yes... I agree. I'll measure it up when I buy the heatsink.EDIT: that link is correct, if anyone is wondering.

author: mrflibble
author_rank: Super Contributor
timestamp: September 20, 2014, 01:08:37 pm
content: Quote from: timb on September 20, 2014, 12:59:54 pmHeh, I was wondering the very same thing.I don't know of any cheapo cpu coolers that would accomodate 9 TO-220 packages. You could do something custom, but that would automatically discard that "cheapo" word in there.Image of the cooler being discussed. Inline because I am lazy today:

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 01:11:45 pm
content: Judging by the details onthispage (4x8mm heat pipes), I'd say that the thermal pad is 32mm by 32mm or thereabouts. That's just enough for two TO-247 packages.EDIT: I think it would just do 6 TO-220 packages, because in one dimension it is slightly more than 32mm. My current design (which needs some significant changes anyway) has an OPA4188 controlling 3 MOSFETs and acting as an amplifier for the DAC. I could stick in another OPA4188 and use 6 op-amps for 6 MOSFETs. I'm sure I can find a use for the two spare op-amps.

author: timb
author_rank: Super Contributor
timestamp: September 20, 2014, 01:16:48 pm
content: What you could do is get a 1cm+ thick square of copper and mount 4 to 6 FETs on it with the heatsink mounted in the center. (So basically you'd have 2 or 3 FETs mounted bent-legged-face-up on the copper just past where those heat pipes curve up.)The copper pad could be thermal epoxied to the board.This way you wouldn't require any reinforcements on the bottom either!Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 01:34:12 pm
content: The way I see it, I'm going to have to have a block of metal around the FETs for reinforcement and good thermal contact. If someone could suggest a source for copper blocks, that would be good; a cursory look on eBay didn't yield anything thicker than 3mm. TBH, the deciding factor (if I can find a source for a block of copper of the right dimensions) is likely to be cost. I don't want this thing, in total, to be more than around £100.Since the PCB will be comparatively large, I'll probably get it manufactured in China and sell the 8 or so spare ones to any willing buyers on this forum, which will help a bit with cost.Estimated cost breakdown:Components: £15MOSFETs: £15 (max, could be substantially less)Cooler: £30PCBs (x10): £20-25Enclosure & mounting hardware: £15 (Aliexpress)EDIT:FYI, here are some rough specs:Max power dissipation: 300WMax voltage: 50V+Max current: 20A (4mm binding posts are generally rated for 20A max)Constant current / voltage in hardware, constant power / resistance in software (I don't know what options I'll have with the PSoC 4, but it might be possible to get an analog control loop for constant power and resistance modes).

author: Marco
author_rank: Super Contributor
timestamp: September 20, 2014, 01:37:01 pm
content: Wouldn't it be easier to just get some two component thermal epoxy and glue them on?

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 01:45:12 pm
content: I doubt that just thermal epoxy would give the required pressure and thermal contact.

author: timb
author_rank: Super Contributor
timestamp: September 20, 2014, 01:57:05 pm
content: You'd be surprised. I've got some arctic silver thermal epoxy here. Once it's dried, shit ain't coming off. Ever.Epoxy is very, very strong stuff.Sent from my Smartphone

author: rob77
author_rank: Super Contributor
timestamp: September 20, 2014, 02:15:14 pm
content: constant voltage in hardware is kind of dangerous... imagine what will happen if you set CV to 4V while connected a 3cell lipo. i would rather implement the CV in software with cutoff current/power.

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2014, 02:19:55 pm
content: My bad, I was thinking of regular CPU heatsinks, that have a flat bottom that is actually large enough to have the 3 rows of 3 TO247 devices on the bottom. You might have the one edge not in contact on the outside, but the surface would do 9 tightly packed. Those copper rods are going to make poor contact, so you probably want the 3mm copper plate, machine it smooth on the faces and use the thermal epoxy to hold it and the devices together. You will not get good contact onto those flattened pipes without thermal epoxy or a good thermpad.

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 02:24:14 pm
content: Can one get pre-machined copper plates? I don't have access to a CNC mill

author: rob77
author_rank: Super Contributor
timestamp: September 20, 2014, 02:29:58 pm
content: Quote from: microbug on September 20, 2014, 02:24:14 pmyou don't need a CNC mill to flatten the surfacepeople were flattening metal surfaces on milling machines before computer control was inventedi bet you have a tons of small businesses or even individuals with a milling machine in your area... for few bucks they'll flatten the block for you.

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2014, 02:31:28 pm
content: Cut to size, use a file on the edges and make flat with a flat surface and sandpaper. Flat surface can be a surface block, a granite kitchen top or a sheet of thick glass.

author: timb
author_rank: Super Contributor
timestamp: September 20, 2014, 02:36:51 pm
content: As luck would have it, I just got a nice mill! I could make some copper pads for you, it would be good practice!Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 02:43:32 pm
content: @timb Sounds great! What's shipping like to the UK?@rob77, there will be OVP/OCP in hardware as well as a thermal cutoff on the heatsink. I'm using an INA196 for current sensing, so I'll use that to drive a comparator switching the output relay.I could get access to a CNC mill at my local makespace... but they don't accept members under 18 (I'm 15). I don't know anyone else with access to a mill, but I could probably find someone on the forum if need be.

author: timb
author_rank: Super Contributor
timestamp: September 20, 2014, 02:48:45 pm
content: Shipping's not bad, I'll cover it. Let me find the raw mats locally and you can just chip in a few bucks towards it if you want?Wow, you're 15? Man I am going to load your package up with some random goodies I've got laying around. I've been waiting for a good opportunity to send them to someone who can use them!Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 03:18:53 pm
content: Thanks! PM sent.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 20, 2014, 03:36:21 pm
content: Intrigued by this topic, I went looking for a way to mount the common TO-220 mosfets (or a bts141) in a low profile way such that pc heatsinks (cpu or northbridge) will fit.I've come up with this:This also leaves space for isolated TIM material, which is very important.

author: microbug
author_rank: Frequent Contributor
timestamp: September 20, 2014, 05:05:44 pm
content: QuoteIf it works, I'm all for it.

author: Christopher
author_rank: Frequent Contributor
timestamp: September 20, 2014, 07:44:39 pm
content: Why not attach the fet to the heatsink, bend the pins up thru the board?Some tall-ish spacers on the heatsink for some thermal insulation?

author: microbug
author_rank: Frequent Contributor
timestamp: September 21, 2014, 08:35:41 am
content: OK, I've found my MOSFETs:IXTH40N50L2. They're designed for linear operation so they're ideal for this. Two of them will be fine for 300W operation, I think I'll have a max voltage limit of 100V. They're TO-247, so they'll fit nicely under the cooler as previously discussed. There were a couple of alternatives from the same company, but that's the cheapest one I can get from Farnell UK which has a minimum order cost of £20 rather than £50 like everyone else.On the subject of cooling, as SeanB said, the copper heat pipes won't make great contact without a copper plate underneath them (thermal epoxy again). What about the back side of the MOSFETs (between them and the board)? Options as I see them:- Cutout on the board, glue the MOSFETs to the copper plate;- No cutout on the board, glue upside-down MOSFETs to the copper plate;- No cutout on the board, MOSFETs sandwiched between back brace made from insulating material (e.g., FR4) and copper plate.I'm currently investigating hardware over-voltage / over-current protection (watching Dave's video on multimeter input protection seems like a start).

author: Spikee
author_rank: Frequent Contributor
timestamp: September 21, 2014, 08:56:54 am
content: Quote from: microbug on September 21, 2014, 08:35:41 amget the 75v version and contact IXYS. They like these kind of projects and should have no problem sampling 2-4 mosfets for free.I have used the arctic silver adhesive before and it works reasonably. To get it off again just soak it in ipa and you can twist the mosfets off the heatsink.

author: microbug
author_rank: Frequent Contributor
timestamp: September 21, 2014, 09:06:38 am
content: I assume you mean IXTP80N075L2 (TO220)?EDIT: Yes, they look suitable. They have a significantly lower Rds(on).Given the extended SOA, two would be fine for 300W. Should I get two in the TO247 package or 3/4 in TO220?EDIT 2: So far for input protection, I'm thinking of a fuse for each MOSFET, reverse protection diode, MOV(s) and a thermal cutoff on the heatsink. There will be some protection in software too (shut off output relay in overload condition).

author: timb
author_rank: Super Contributor
timestamp: September 21, 2014, 10:25:48 am
content: I ordered some 5mm copper today. You're using the A30, right microbug? I'll grab one off Amazon and make something very slick for mounting. I've got a really good idea. (I'll draw it up tonight and post it here.)Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 21, 2014, 10:40:20 am
content: Yes, it's the A30. I'll probably have two TO-247 packages under it (the linear-optimised MOSFETs from the previous post are available in TO-247 and I think two TO-247s will be better than 3 TO-220s. Thanks timb!

author: Spikee
author_rank: Frequent Contributor
timestamp: September 21, 2014, 11:27:59 am
content: I have soldered the initial prototype but i soon found out there is a short somewhere on the board.I was unable to locate the problem. I have started populating a second board and assembling and testing between each step to find the culprit. The power segment seems to function without any issues.Edit:This one IXTH75N10L2http://ixdev.ixys.com/DataSheet/DS100200(IXTH-TT75N10L2).pdf

author: timb
author_rank: Super Contributor
timestamp: September 21, 2014, 11:30:18 am
content: Micro, what sort of driver are you looking at?Edit: I mean OpAmps to drive the MOSFETs. Sorry, have been working on a SMPS tonight and have MOSFETs drivers on the brain!Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 21, 2014, 04:15:14 pm
content: Tim, it's an OPA4188 in a typical constant current configuration (100mOhm sense resistor). The current implementation is based onthisthread, but I will probably change it to a more 'standard' configuration (input to the non-inverting terminal, shunt voltage to inverting) to make it easier to do a hardware constant voltage mode. I'm working on the schematic at the moment and I'll post a screenshot of the power stage soon.Thanks for pointing out the right FET, Spikee. That one looks like itcould(in theory) do 300W at the right current on its own, but I'll use two of them in the design (separate op-amp for each) to account for all the graphs other than the SOA in the datasheet that I didn't bother to read. I'll send an email to IXYS to ask for 3 (in case I blow one) and post the result.EDIT: Spikee, that FET looks 100V rated to meEDIT 2: Uh Oh! I just looked up stock of IXTH75N10L2 on Octopart having just sent a grovelling email to the IXYS sales rep office in the UK, and it turns out neither Mouser nor Digikey have the part in stock (it's a 'factory direct' part)! I hope that IXYS has some internal stock or something... I suppose it wouldn't be too hard to find another suitable MOSFET.

author: Spikee
author_rank: Frequent Contributor
timestamp: September 21, 2014, 07:10:18 pm
content: IXYS has it and will send you a few free because it is not in stock at their distributors.

author: microbug
author_rank: Frequent Contributor
timestamp: September 21, 2014, 07:11:07 pm
content: Oh, good!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 21, 2014, 08:02:19 pm
content: You will get about max 150W from 2 of those if your Heatsink has a Rth of about .25 K/W (allowing NFet junction temp to reach ~130 C) .150W is plenty anyway to test most things you will come across ( a typical bench supply 30V@5 A =150W) .Why do you need 300W ? thats alot of power .p.s I have made a 150 Watt one and if I ever needed to extend the power range then it's easy enough to make a power range extender .(Say 4 * 50W 1 ohm resistors fixed on to the side of a can you can fill with water ) .You could then put these resisitors in series with your eload and the DUT so that they dissipate 50% of the total power and your eload drops the other half .(i.e the resitors drop ~ half the total voltage ,( thats why youd have say 4* 1 ohm resistors permanetly fixed fixed to your water can so then you could wire them to have the exact required resistance you needed to drop 50% of the total dut voltage . (i.e with 4 *1 ohm resistors you can have 1/4 ,1/2,1 ,2,4 ohm values by wiring them in various ser/par combinations.)

author: microbug
author_rank: Frequent Contributor
timestamp: September 21, 2014, 08:35:45 pm
content: The heatsink is rated at 320W power dissipation. Unfortunately, it doesn't specify efficiency in K/W, but I'd expect better performance than that! The SOA covers 300W operation easily; I'd expect some thermal derating, but that's why there will be two. I chose a 300W limit because the extra 20W of heatsink capacity gives a little headroom should things go wrong.Anyone else care to weigh in?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 21, 2014, 09:36:40 pm
content: Well I dont know about the heatsink you have chosen (does it's spec give a max temp at 320W dissipation ? if so then to get it's Rth is just (temp@320W-25)/320.I used 2 standard amd K6 sinks+fans on mine they each had a Rth of .45 K/W each when I measured them .Forget about these soa graphs they dont contain much usefull info for dc operation(they simply reflect the Rth j-c if you look),manufacturers use them to produce nice banner spec headlines on there product's like '300W dissipation ' .They produce them by emersing the whole transistors in a cooling fluid to hold the case temp at 25 and then measure junction temp rise wiht power dissipation .You can't do that of course you have to cool your transitors in a conventional fasion .So you have to take into account the extra thermal impedance of the Case to Sink(Rth c-s) an also + any insulating material you have between case tab and sink (which can add another ~ .2 C/W to thermal resistance ).So looking at your data sheet for to264 case style we haveRth j-c .28 C/W (notice if we only had this one Rth we could get the SOA graph)Rth c-s .2 C/W + another .2 C/W if we use tab insulation .total Rth j-s = ~ .7 C/Wnow your thermal calculation is easy .lets say you have 150W power into that ,so that would be150 * .7 = 105 C temp rise at fet junction above whatever your heatsink temp is at 150W dissipation .since I dont know what the sink you using is rated at , lets guess and say it's rated at ~ .25 C/Wso at 150W this will reach a temp of = 150*.25 =37 + 25 ambient temp =62 CSo with a single fet at 150W your junction temp will reach 62+ 105 = 167 C (to HOT)with two fets the j-s temp rise would be 105/2 = 52 Cso now your junction temp at 150W is 62+52 =114 C (so at least it's now it'sunder your 150 C absolute max for junction temp) .DONT take to much notice of manufacturers headline grabbing specs and soa graphs done under liquid cooling conditions , Just gather your various Rth's and do the simple calculations yourself.

author: Spikee
author_rank: Frequent Contributor
timestamp: September 21, 2014, 11:06:24 pm
content: I tested two ixys75L2 mosfets on a arctic freezer 7 pro (150w max rated). and i was able to get the full wattage. The A30 heatsink should be twice as good. Around 0,01 - 0,005deg c/w should be possible.

author: timb
author_rank: Super Contributor
timestamp: September 22, 2014, 01:16:03 am
content: @Kevin.D These MOSFETs are specifically designed for linear operation in the SOA, so the graphs in the datasheet are pretty accurate for the conditions we’ll be using them in.

author: timb
author_rank: Super Contributor
timestamp: September 22, 2014, 02:50:11 am
content: @microbugSo here’s one idea I had:The idea here is your FETs would mount to this copper block (50x75x5  in this example). The hatched area would be where the A30’s heat pipes press against. It would be wise to use thermal epoxy or thermal compound here. The area on the board the copper slab will mount should be a solid copper pour and have holes drilled so the FETs and slab can be bolted down. Obviously you’d still need to use the heatsink mount itself to keep that stabilized on the board.Copper has such a low thermal resistance, it would be very close to mounting them straight on the heatsink itself. The advantage here is ease of mounting.I can also design one to work with only two FETs (one on each side).

author: mrflibble
author_rank: Super Contributor
timestamp: September 22, 2014, 03:10:17 am
content: I'll just accidentally drop these links here, and let the coolness factor do the rest.http://www.mouser.com/new/panasonic/panasonicthermalgraphite/http://www.digikey.com/product-highlights/us/en/panasonic-pgs/3953

author: timb
author_rank: Super Contributor
timestamp: September 22, 2014, 03:57:09 am
content: Yeah I saw those the other day... I was toying with the idea of placing one on top of the copper block. The FETs and heatsink would rest on it.Cool stuff. Pun intended.Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 22, 2014, 06:57:24 am
content: QuotePhew!Tim, your idea looks great! I wonder if thermal graphite is conductive, because if not, it could insulate the drains of the FETs. I probably will use two of the previously mentioned MOSFETs, they look like they should cope with the heat dissipation; the heatsink needs space for two TO-247 packages. Spikee's experience is encouraging.Thanks!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 22, 2014, 09:08:04 am
content: Quote from: timb on September 22, 2014, 01:16:03 amNo it isn't, the conditons you will be using them aren't the same conditions that those soa graphs are plotted .IF you look at the data sheet you see those conditons that the sao are taken in is at a fixed case temp .(you will see T.C =25 ,or t.c =75 or something) .For a normal use you can't hold the case at some fixed temp unless your using a nice cryogenic cooling fluid like they do when they do these soa plots .You have to transfer heat from a transistor to a heatsink via a tab .which has an extrathermal impedance Rth c-s (case to sink thermal resistance ,also any extra thermal resisitance of sil pad's if you use them) .IF you'd had looked at the soa plots on the graphs and noted the various power's being dissipated at the junction temp of 150 then you would have noticed thisvalue (150 - case temp) = Rth j-c * power dissipated ( i.e these soa at dc only plot rth j-c * power .they dont include any other thermal resistance).Being able to use your Rth's to calculate junction temp @ a certain power dissipation (because junction temp is the one that really matters )is really simple to do . And if you can't do that first basic bit then you aren't yet ready to design a multiple mosfet electronic load.Regards

author: timb
author_rank: Super Contributor
timestamp: September 22, 2014, 09:27:52 am
content: Actually, IXYS has graphs for Tj @ 25, 75 and 150c...Anyway, I'm not sure what your point is? Obviously you need to know the Tj at a certain power dissipation. Calculating it is (as you pointed out) simple.My point is that IXYS also provides graphs for a range of junction temps for these MOSFETs, as they're designed to run in the SOA.Sent from my Smartphone

author: mrflibble
author_rank: Super Contributor
timestamp: September 22, 2014, 11:47:31 am
content: Quote from: microbug on September 22, 2014, 06:57:24 amBy itself PGS is a good electrical conductor, but Panasonic also manufactures sheets with an insulating film.

author: microbug
author_rank: Frequent Contributor
timestamp: September 22, 2014, 06:00:46 pm
content: What should I do for reverse input protection? The max sink current is 20A - I'm planning to have a high-current diode in a TO-247 package on the heatsink pad (heads up @timb!).Also, what about the binding posts? I could use 4mm banana jacks which are often rated for 20A, but what about those hex-screw ones on the BK precision loads - would they offer a significant advantage?I'm currently redesigning the power stage.EDIT: the DAC I'm using is the DAC8411 from TI. I read that most DACs have buffered output stages, and so can't get lower than 10 or so mV to ground. Is the right approach to add, say 20mV negative offset in hardware and to trim in software (the negative supply voltage for the op-amps is -2V)?

author: timb
author_rank: Super Contributor
timestamp: September 22, 2014, 09:32:54 pm
content: By the by, made this up quickly in Pixelmator:The dimensions are loose, based on an image from Arctic’s site that showed the width and height at both angles.@microbug Using a positive offset is easier sometimes. TI even makes dual output voltage references now that have Vref and Vref/2 outputs; specifically designed for single supply bi-polar applications. For your application you’d use it like this: Say you want a 0 to 2.048 output range. Get a TI REF2040 and use the 4.096 (Vref) output as the reference for your DAC; then use the 2.048 (Vref/2) output to bias your OpAmp.Now your control range will be 2.048V to 4.096V which is the same as 0V to 2.048V! The other benefit is it will reduce overall noise between the DAC and OpAmp since they’re being powered from the same source.Anyway, it’s just one approach, but one that I like. =)

author: microbug
author_rank: Frequent Contributor
timestamp: September 23, 2014, 03:10:35 pm
content: @timb, I can't find a part by the name of 'REF2040'! I have already got a DAC8411 and a 16 bit ADC (I forget the part number - will add it later), both of which have built in references. I can replace them if I have to, but I'd prefer not.Typical zero-code error @ room temperature (according to the datasheet) for the DAC8411 is 200uV - I won't need that much trimming, if any.

author: mrflibble
author_rank: Super Contributor
timestamp: September 23, 2014, 03:16:03 pm
content: Maybe he meant the 2041?http://www.ti.com/product/ref2041

author: microbug
author_rank: Frequent Contributor
timestamp: September 23, 2014, 03:20:20 pm
content: @mrflibble (I like your username!) Yes, that's likely.EDIT: Of course, it's unlikely that I'll be using the load below 10mA, however; it would be nice to have accuracy right down to 1mA.FYI, I'm using two INA194s with .01 ohm resistors for current sensing; the output feeds into the op-amp(one IC and resistor per driver). I can put both current sense voltages through their own resistor (to get the average current sense voltage), and feed it into the ADC - I'll double it in software. A voltage divider will set the full scale current sense range.I'm going to include a constant-voltage setting in hardware, as I can get the PSoC to act as an analog mux so all that is required is a few resistors (my plan is to divide the input voltage down and compare it to the set voltage using the op-amps). I did look into constant resistance / power modes in hardware briefly, but I don't know whether it would be worth the effort. I won't be limited for op-amps, though; the PSoC has two so I might be able to make use of them.I put two MOVs which go at 200V or so (haven't chosen the parts yet) across the input for protection. I don't know what to do for protection against voltage above 100V but below 200V though... suggestions?EDIT 2: For hardware constant-power, an AD633 multiplying load voltage and current would be fine. Constant resistance, which would require comparing either using a digipot (not much resolution) or doing V/I in hardware (difficult - I can't see how it could be done), is harder. I think constant-power would be more useful, anyway.

author: timb
author_rank: Super Contributor
timestamp: September 24, 2014, 01:24:28 am
content: Measure the current with your INAs and the input voltage with a resistor divider using the PSoC's ADC. Now use your MCU to control the current via the DAC. This way you can do CC, CV, CR and CP through software.BTW, doesn't the PSoC have like a 16-bit DAC and ADC? Why use external parts?Sent from my Smartphone

author: microbug
author_rank: Frequent Contributor
timestamp: September 24, 2014, 06:18:24 am
content: Won't the transient response be much worse? I through that was what C2000s (DSPs) were for.The PSoC 4200 has a 12 bit ADC and a 12 bit IDAC (current DAC), which is not very useful. I chose 16 bit resolution with external parts because they aren't inhibitingly expensive so I might as well; it will let me get closer to my 1mA resolution ideal.

author: Joenuh
author_rank: Newbie
timestamp: September 24, 2014, 03:19:11 pm
content: I'm planning on building something similar, also with the 1mA resolution. But thinking about that, wouldn't it be easier to use a 1 ohm shunt resistor instead of 0.1 ohm?A 1mV error on a 0.1 ohm shunt would already be 10mA if I'm not wrong. A 0.1 ohm shunt would dissipate less power but it still seems like a 1 ohm resistor would make more sense, or is that just me?

author: rob77
author_rank: Super Contributor
timestamp: September 24, 2014, 04:22:51 pm
content: Quote from: Joenuh on September 24, 2014, 03:19:11 pmi built one with a 1 Ohm shunt (50W 1R resistor) and even that is not good enough for 1mA with a jellybean opamp (LM324). so i decided to go for a 10mA resolution for my high power load and in addition going to build a new low power load with high resolution.btw.. frankly speaking - i don't care if the load is 4.0 Amp or 4.003 Amp - for high currents the 1 mA resolution is useless.

author: Joenuh
author_rank: Newbie
timestamp: September 24, 2014, 04:32:51 pm
content: Well as you mentioned your opamp, a high precision opamp would probably fix that problem. And I agree that for high currents the 1mA resolution is useless. But if it doesn't get to expensive, I would like to build an all-in-one solution that accurately can go from small currents, as 1mA, to higher currents like 5-10 Amps.Edit: You also mentioned a 50W 1ohm resistor, I hope that is realised by putting multiple resistors in parallel. If not, that also may be part of your accuracy problem. As power resistors are not really accurate/low tolerance.

author: microbug
author_rank: Frequent Contributor
timestamp: September 24, 2014, 04:35:09 pm
content: The op-amp I am using is the OPA4188 which has a 6uV (25uV max) offset. The INA194 which is measuring current has a max 2mV offset voltage and typical +/- 500uV. I'm thinking about removing it though.EDIT: Welcome to the forum, Joenuh!

author: rob77
author_rank: Super Contributor
timestamp: September 24, 2014, 05:10:43 pm
content: Quote from: Joenuh on September 24, 2014, 04:32:51 pmi compensate for inaccuracy of the resistor in hardware - both set voltage coming from a DAC (well... PWM with low-pass) and the readout for the ADC are tunable (voltage divider followed by a opamp buffer with tunable gain), so the exact resistance of the shunt is not relevant. and yes, it's a single big-ass 50W heat-sink mounted resistorthe smaller brother of the DC load will have a spec of 0-250mA and will be built with a 4R7 (or 10R) shunt and OP07 op-amps.

author: Joenuh
author_rank: Newbie
timestamp: September 24, 2014, 05:43:50 pm
content: Quote from: rob77 on September 24, 2014, 05:10:43 pmYes ofcourse that would fix that problemMaybe it's my own stupidity but why are people using ADCs in these projects? I know, for measurements.. But if you can accurately set the current you don't need to measure it right? Or are you using it to measure the input voltage in some way? To be able to limit in power?

author: microbug
author_rank: Frequent Contributor
timestamp: September 24, 2014, 05:45:12 pm
content: ADCs are useful for software constant resistance / power / voltage modes. For all of these modes, you'll want to easily see the current and voltage flow.EDIT: Yes, over power limits come into it as well - software controlling a relay, for example.

author: timb
author_rank: Super Contributor
timestamp: September 24, 2014, 08:22:06 pm
content: Quote from: microbug on September 24, 2014, 06:18:24 amNah, your transient response will be just fine. Personally, I’d use the built in ADC and comparator to control an external DAC. Hook your INA up to a single to differential amplifier and use the built-in 12bit differential ADC.That PSoC is 48MHz with single cycle multiply, so it should be more than fast enough for your needs. You should be able to get mA resolution with a 12-bit ADC and software calibration.

author: microbug
author_rank: Frequent Contributor
timestamp: September 24, 2014, 09:21:38 pm
content: 20A/2^12 = 4.88mALike rob77, at higher loads I don't care much about milliamp resolution, so are you suggesting a PGA on the DAC (for multiple ranges)? Unless there's a reason not to, I want to use a 16 bit ADC/DAC as I already have the parts, and if I do there's no need for a PGA or anything. I was thinking of doing away with the INA194s altogether and running the sense voltages from the shunts through a couple of resistors, then scaling them for the ADC with a resistor divider.Schematic / example to come.EDIT: The other reason that I'd prefer not to do it in software is that the PSoC will be driving a graphic LCD and sensing a rotary encoder / keypad. This will definitely worsen the transient response!

author: Joenuh
author_rank: Newbie
timestamp: September 25, 2014, 07:38:25 am
content: Quote from: microbug on September 24, 2014, 09:21:38 pmIs that the right way of calculating it? Maybe it depends on how your output is. But the way I see it is that which voltage you set on the output of your DAC is the current that will flow, if you use a 1R resistor ofcourse. So wouldn't that be the Vref of the DAC divided by 2^12? So a Vref of 4.096V with a 12 DAC should give you 1mA steps. The problem is now that you can ''only'' go to 4.096A.That's a problem that I'm having. I'm thinking of having multiple gain options on the opamp so that a switch can switch between different feedback resistors of the opamp. That switch could be something like a ADG884.

author: metacollin
author_rank: Contributor
timestamp: September 25, 2014, 08:28:46 am
content: I came to this thread kind of late, but I've been reusing CPU heatpipe/sink assemblies with and without fans for years with FETs and transistors with all sorts of packages. Doesn't matter if they're surface mount or have a screw hole, I use both. Use whatever package has the lowest junction to case thermal resistance.This is what I've learned:1. Do not use thermal interface compound (TEC). It's terrible and using it often ends in ritual suicide. TECs do little beyond fill in imperfections between two flat surfaces with something that isn't air. But otherwise, it's basically about as bad at conducting heat as a solid, non-air filled material/over priced goop can be. Toothpaste performs nearly as well as thermal grease while smelling significantly better (spearmint is my favorite). There is less than a 10% performance difference between quality thermal paste and shitty toothpaste. And TEC dries out or otherwise deteriorates, though much more slowly than toothpaste (years vs. hours).2. Do not drill holes in the heat spreader, it's a huge dickchafe and you'll probably breach a heat pipe and effectively ruin the entire assembly's ability to shed those phat watts.3. Brackets? Clamps? Maybe even bears, oh my? Don't bother, and it will be a huge pain in the dick for inferior thermal performance.4. Hot air heats heatsinks as effectively as cool air cools them. Heatguns aren't just for heatshrink and bacon.5. The important bit -Just solder the damn FETs directly to the copper heat spreader.They're *made* to be used that way. No holes, no brackets, no bullshit, just bitchin' awesome thermal performance and a bond as strong as the heatspreader is bonded to the rest of the assembly and heatpipes. And you won't have to commit Seppuku to restore honor to your family. (The last one was a joke, of course. Probably.)Use low temperature Bismuth-Tin solder. Sometimes you can find it in paste form in small quantities, but its quicker and easier to just make your own. Find that old bag of bismuth ingots and that bar of grade A tin you have stashed somewhere in your garage, melt them in an aluminum container on the stove, mixed 42% Tin, 58% Bismuth, Sn42/Bi58. This is the same thing bonding the heatpipes to the cooling fins and heatspreader. It melts around 140°C, give or take. You can tweak the temperature by moving the ratio tinward. Remember, heatpipes work in reverse, just blow a heatgun through the part a fan would blow air through and the heat pipes will eagerly shit all of those watts into the copper heat spreader where you want to solder your FET, and you'll have things up to temp and wetting nicely in tens of seconds. Just, uh, make sure the whole thing doesn't fall apart due to gravity or something. Also don't attempt this with normal solder, you'll vent one of the heatpipes and it WILL shoot a jet of methanol directly at your genitals.Those wattage and SOA curves in the datasheets are all done with those mofos soldered DIRECTLY to some big ass hunk of copper. If you solder a FET to copper, it adds negligible thermal resistance and, depending on the total thermal resistance of the junction to heatsink/heatpipe assembly to ambient, you will actually achieve the specs in the datasheet. Now, often the wattage is assuming you cast level 99 Heatsink of the Infinite and have a heatsink with 0 thermal resistance cooled by unicorns breath, but if you run the numbers for your non-magic non-unicorn breath cooled heatsink, you'll get exactly what all the thermals predict. The heatsink is the limiting factor if its for a CPU, it probably won't handle much more than 100W shed to ambient with a fan, but a TO-247 will dump 100W into those sweet sweet heat pipes without even breaking a sweat, no problem. Single package. With a bigger heat pipe assembly, 200W out of a single package is totally doable, I've done 240W out of a TO-264 soldered to a big ass aluminum heatsink (solder wets aluminum wonderfully, just sand it in an aquarium or similar and fill it with CO2 using dry ice or whatever is handy, this will remove the surface layer of aluminum oxide and prevent a new one from forming instantly. The aluminum oxide surface layer is what makes aluminum seemingly unsolderable, but actual unoxidized aluminum wets nicely). Not even copper or heatpipes, just shitty old aluminum.Some quick examples attached. In order: 1. A big ass fet soldered directly to a big ass cooler assembly salvaged from an MSI graphics card. 2. One of those shitty chinese 100W LEDs, soldered directly to the spreader. 3. For a modest reduction in wattage, I quite like laptop coolers. This one cost me $2.50 USD on ebay. 4. My ghetto home made Solder Slab™ of low temp Sn42Bi58 solder, with little nibbles from my iron evident at the edge. 5. Finally, a CPU heatsink actually being used as such, AMD processor soldered directly to it. Yes, it still works. It's running FreeBSD as I type this.

author: Spikee
author_rank: Frequent Contributor
timestamp: September 25, 2014, 10:36:53 am
content: I tried the soldering method on my arctic freezer 7 pro rev 2 but it was a no go.I have a top of the line soldering iron with around 240W of capacity (JBC) but the heatpipes of the cpu cooler are to good. After a fuckload of time the temperature only achieves around 60-80 deg C.I have tested the Arctic silver thermal adhesive up to 600W dummy load using 4 mosfets and it works fine. The only hassle scraping it off when you don't need it anymore. Just normal thermal compound and a press fit heatsink is the best solution.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 25, 2014, 11:08:49 am
content: Quote from: Spikee on September 25, 2014, 10:36:53 amAs described above, you must use the heatpipes in reverse by heating up the fins using hot air. All energy will channel and concentrate back up to the heatspreader plate. Making it incredibly hot very fast.It's quite interesting to test if soldering is a lot better. Intel recently changed from solder to glue between the heatspreader and the chip die. This seems to have decreased the performance.

author: microbug
author_rank: Frequent Contributor
timestamp: September 25, 2014, 03:23:43 pm
content: @Jeroen3 Yes, I heard the same story about solder being better.I can see that solder would be better once it's there. How would the thermal connection between the cpu cooler and the heat spreader be when soldered together? Also, would this be doable with the Atten 858D hot air gun and Hakko FX-888D soldering station (my soldering equipment)?@Joenuh, multiple gains on the op-amp is one way of doing it. It would make sense as you are unlikely to need high resolution at high power. The reason, however, that I am not using 1 ohm shunts - apart from the heat, which has to go somewhere - is that in constant power mode with a current source of 20A (the max current) connected to the input, they would limit the minimum power to 200W (I2R = 400*0.5 = 200). On a 300W load, that's almost useless.EDIT: No reply yet from IXYS...

author: metacollin
author_rank: Contributor
timestamp: September 25, 2014, 05:01:28 pm
content: Thermal paste is often still adequate, but will up your over all thermal resistance and you'll have less headroom. It just seems like once you've gone through all the trouble of mounting something with sufficient force, drilling a hole, cleaning the surfaces, blah blah, you could have had a much easier time just using the soldering method. I am really not sure why its not used more (in prototyping/personal project contexts, I am clueless about how well it would scale to some sort of volume production setting, probably not well though). I started doing it simply because it was *much* easier and quicker.Yes, it's totally impossible to get it to temp using an iron. I use a $20 1500W heatgun from amazon. Hot Air is terrific for heating any kind of surface-area heavy thing like a heat sink. The radiator fins suck up the heat if you blow hot air between them like crazy. All I'm saying is anyone interested should give it a try, and especially if you get ahold of or make some low temp solder, and you'll never go back. I used to drill and tap stuff and mount it the classic way, but haven't done that in ages, simply because I'm, ahem, a convenience enthusiast and the soldering method got the most done for the least amount of effort.But seriously, I've yet to encounter a heat sink large enough that a cheapie heat gun couldn't bring up to temp with relative ease. Also, it's a lot easier to hit 140° than 180° or higher for lead-free SAC alloys.

author: microbug
author_rank: Frequent Contributor
timestamp: September 25, 2014, 05:05:08 pm
content: @metacollin, OK. I'd probably have to buy a better heat gun if I connect the heat spreader to the cooler with solder, since the Atten 858D 'only' does 700W.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 25, 2014, 05:09:13 pm
content: Quote from: Joenuh on September 25, 2014, 07:38:25 amTheres a couple of ways you can have a multiple range load without any switching .I did one a year or so ago it's got 4 decade ranges 12A ,1.2 A ,120mA ,and 12mAon the lowest range you get 10uA adjustment precision . I designed it to be used as a current sink/source as well as use as a eload hence the four lower decade ranges (and it has true reverse polarity protection which is a must to protect the DUT) .It's also fully protable so I can take it to the garage outside where I use it on large lead acid batt capacity tests.Search forum for 'eload' and you should find it

author: microbug
author_rank: Frequent Contributor
timestamp: September 25, 2014, 05:33:58 pm
content: I had some trouble finding it, so here's the link:https://www.eevblog.com/forum/projects/fully-portable-150w12a-e-loadcurrent-reg-with-multiple-ranges/EDIT: I think multiple ranges are a good idea. The end project will be digitally controlled, so I'll use analog switches (such as AD884) to control the range. I have dropped the INA194; the input offset voltage was too high. The 6uV of the OPA4188 should be much better.EDIT 2: I have been looking into the DAC some more. The DAC8411 uses the supply as the reference, and as it only uses 160uA (max), I'll power it from a 4.096V reference.EDIT 3: This is better because I can make the current ranges 409.6mA, 4.096A and 40.96A (that one will be software and fuse-limited to 20A) and have adjustment with a resolution of exactly 6.25uA / 62.5uA / 0.625mA (instead of a long decimal number). Hardware constant-voltage ranges will probably be double the values of the current ranges (819.2mV, 8.192V, 81.92V, maybe 819.2V (software limited to 100V, of course!)).EDIT 4: I'm going to use 100mOhm shunt resistors. With 21mOhm MOSFETs, that's a 61.5mOhms impedance altogether (MOSFETs and shunts are in parallel). If I assume the actual impedance will be 70mOhms (accounting for connections, traces etc), then the minimum power dissipation (I^2*R) is around 30W at max current. Graph attached - x is current, y is minimum power dissipation.

author: microbug
author_rank: Frequent Contributor
timestamp: September 26, 2014, 07:48:57 pm
content: Bump (I often edit my messages rather than replying, so see the last message I posted).What should I do to generate a small (250-300uV) but stable negative offset for the current/voltage set line (do I have to use a negative voltage reference or can it be something cheaper)?I'll make the constant current ranges 400mA, 4A and 40(20)A so I have some headroom to trim the DAC in software.The minimum constant current will still be 1mA (or maybe 500uA), even on the 400mA range, because of the offset.

author: rob77
author_rank: Super Contributor
timestamp: September 26, 2014, 08:12:35 pm
content: i strongly suggest a negative rail for your op-amps in the design and strongly suggest to use op-amps with offset compensation pins (all old-school precision op-amps have those available - pins 1+8 if it's a 8 pin package) and trim down the offset to zero.otherwise forget the accuracy you're trying to achievebtw... i'm always nulling the offset before i solder the op-amp in - setup a non-invert amp with gain of at least 5000 (non invert input grounded) and tweak the offset to zero. then solder the op-amp into the final board and either solder the 20turn trimpot with it or replace the trimpot with resistors for the final board.

author: microbug
author_rank: Frequent Contributor
timestamp: September 26, 2014, 09:12:28 pm
content: There will be a negative rail for the op-amps, generated by an inverting switching controller. I don't know if an 'old-school' trimmable op-amp will be better overall than an OPA4188 - it has a very low offset anyway and the drift over temperature is extremely low.EDIT: Do you have any specific suggestions for a better op-amp?

author: rob77
author_rank: Super Contributor
timestamp: September 26, 2014, 11:23:21 pm
content: actually those "zero-drift" opamps from TI look pretty good, the thermal stability is impressive. but i'm afraid the 25uV offset is too much for your 16bit DAC. datasheet states 6uV typical , but you know you have to consider the max value which is 25uVbut anyways - you can compensate for that even if there are no offset trim pins - but it's less elegant than a trimpotbtw... if you'll use a 4.096V reference , then why not to use a 12bit dac ? 16bit is a overkill - you can't build the circuit stable enough anyways... your opamp will be zero-drift ,but the resistors notsomeone will sneeze in the room and the whole thing will drift away - rendering the extra 4 DAC bits useless

author: microbug
author_rank: Frequent Contributor
timestamp: September 27, 2014, 07:14:32 am
content: @rob77, I guess you're right. Besides, the current/voltage set resolution will still be perfectly good - 100uA on 409.6mA; 1mA on 4.096A and 10mA on 40.96A. If I go with a 12 bit DAC, I can use the built in 12 bit ADC on the PSoC 4, both saving components and improving transient response for software constant-power/constant-resistance.

author: microbug
author_rank: Frequent Contributor
timestamp: September 27, 2014, 09:27:58 am
content: I found a good DAC:DAC7613. This will let me use the REF2041 as timb suggested!

author: microbug
author_rank: Frequent Contributor
timestamp: September 27, 2014, 04:28:00 pm
content: @timb: how should I negate the offset voltage (nominal +/- 4LSB) of the DAC buffer? You mentioned that it's easier to use a 'positive offset voltage' - what do you mean?EDIT: Unless there's a better way, I'll use a resistor divider between 2.048V and ground to generate 2.045V (6 LSB down), and trim in software.EDIT 2: I attached my attempt at a negative offset voltage for the DAC output. Will it work?EDIT 3: ... or would I be better off having the same voltage divider on the VREFL and VREFH inputs?EDIT 4: Since the range of the DAC is now effectively 0-2.048V, it makes more sense to make the current ranges in multiples of 10 of 2.048V. Therefore, I will have current ranges of 204.8mA, 2.048A and 20.48A.

author: eneuro
author_rank: Super Contributor
timestamp: September 27, 2014, 06:09:02 pm
content: Quote from: Spikee on September 25, 2014, 10:36:53 amThat is why I'd like to try spot weld mosfets TO220 tap in 3-4 spots to copper heatsink or flat heatpipe.This should prevent cooling spot area, but of course high energy short pulses needed, so there will be no time to disipate heat.Just wondering if 30A on spot welder trafo primary 230VAC, so close to 7kW will do the trick, while copper has quite low resistance, but cross section area of those spots will be much lower than transformer a few turns secondary, so maybe?Anyone tried this?

author: macboy
author_rank: Super Contributor
timestamp: September 29, 2014, 02:20:05 pm
content: Quote from: eneuro on September 27, 2014, 06:09:02 pmSpot welding will give you a good (and irreversible) mechanical connection to the heatsink, but what about the thermal connection? You will still have air gaps in the imperfections between the device tab and the heatsink. A metal-air-metal thermal interface is very poor. The soldering technique provides a metal-metal-metal (tab-solder-heatsink) thermal interface, which is as close to ideal as you can practically get.

author: eneuro
author_rank: Super Contributor
timestamp: September 29, 2014, 09:15:19 pm
content: Quote from: macboy on September 29, 2014, 02:20:05 pmFinally found tips how to spot weld those tabsHow to Spot Weld CopperQuoteExtracted recipe from this link below:QuoteThis is what I was thinking about and it looks like that Silver Solder Paste could be used for brazing copper like in this video using classic high power heaterI'd like to solder heatsink to tap using powerfull spot welder, while I do not like to put mosfets in high temperature stress when heater is used during assembly, so it is time to try those tricks above and see what happends if high energy short pulses will be apllied to TO220 mosfets tapsNote:Just checked SSQ-6 Silver Solder specs herehttp://muggyweld.com/ssq6-silver-solder-pasteand it looks like itQuotewhich means1050ºF =565ºCMaybe classic solder pase used in SMD assembling will be better for brazing copper to copper , but it is interesting to test spot weld copper to aluminium heatsink using this SSQ-6.

author: Joenuh
author_rank: Newbie
timestamp: September 30, 2014, 09:26:24 pm
content: @Microbug: Did you get a reply from ixys? Because I sent an email with a sample request yesterday and I already got a reply.Also, are those ranges correct? I assume from the calculations that you are using a 4.096V Vref and a 16bit DAC? Wouldn't that give a lowest range of 62.5uA instead of 6.25uA? Or are you using a gain of 0.1, which would make no sense to me?That resolution would be really hard to get I think, considering input offset voltage and stuff like that. I'm even in doubt about getting my (accurate) 1mA resolution with a shunt of 0.1 Ohm.16bit DAC would be the way to go I think or would it be overkill and am I thinking to hard about this?

author: microbug
author_rank: Frequent Contributor
timestamp: October 01, 2014, 06:51:54 am
content: I didn't get a reply from IXYS, I think the email might not have sent. I'll send another!I'm now using a 12 bit DAC (you can have an external reference, and it's more accurate) with the REF2041, as suggested by timb. There is a mux which controls the range by connecting the appropriate resistor divider. Moe details to come later!

author: metacollin
author_rank: Contributor
timestamp: October 02, 2014, 07:39:10 am
content: @microbug: I use a 1300W heat gun. But anyway, you've got things under control and in the way you want thermally, sorry to keep talking about much earlier parts of the project. Can't wait to see more of this project though!@eneuro and the other guy talking about welding/brazing:I would discourage anyone from spot welding copper for thermal purposes, and even more so actual IC packages to copper. CPU coolers have heat pipes soldered to a thin copper plate which is intended to have heat from some watt nugget conducted into it. It then spreads the heat and distributes it into the heat pipes. Then, very thin copper sheets will be soldered to the heat pipes, and sometimes additionally the copper sheets/fins are skived together to make the outer edges of the fins a rigid structure that is more diffidult to bend. That is all that is holding together the CPU cooler in your computer, laptop, GPU, etc, and its more than enough.But its soldered using Sn42Bi58, which is used extensively in thermoelectric applications, mainframes (for its tin whisker retardant properties), and niche industrial applications (sometimes used for large BGA packages, fro example). It is very strong mechanically compared to electrical solders and has excellent wetting, is eutectic, very low thermal expansion, and fairly importantly in the context of thermal performance, it expands slightly as it solidifies, filling in any gaps and making a very secure thermal and mechanical bond between two surfaces.You are using heat sink/heat pipe assemblies inappropriately if you require them to be stronger than what the thermoelectric soldering used in every commercial passive or forced air CPU and GPU cooler on the market can provide. Heat conduction is a function of cross section, spot welding is, uh, not known for the cross section of the welds, and any weld will only penetrate so far. You could not weld a semiconductor package such that the entire back surface was bonded without destroying the device. And beyond that...why? What benefit does welding or brazing heatsinks to each other offer? Why use silver filler? Surely not for its thermal conductivity. If you want heat to move from one place to another you don't wait for it to heat up a bunch of mass in between, watching it spread on timescales measured in _minutes_ through solid material. If you want to move heat out of and quickly away from something, ore move it from one spot to another spot, you use a heat pipe. Heat pipes transfer almost all heat energy to the 'cold' end in a couple seconds.The thermal conductivity of heat pipes used for CPU cooling is in excess of 100 times that of copper.Anyway, I'm done rambling about heat. and using a CPU (or GPU for even phatter watts) for a dummy load is a great idea, and the heat pipes they use will keep the actual MOSFET junctions at much lower temperatures as long as your airflow is removing heat as fast as they make it. And, surplus server coolers, for xeons and the like, which usually need to shed 125W+, are both abundant and cheap on ebay. I'm eager to see how things develop with the digital control and DAC.

author: macboy
author_rank: Super Contributor
timestamp: October 02, 2014, 03:35:23 pm
content: Quote from: metacollin on October 02, 2014, 07:39:10 amI agree and tried to make this point earlier. Spot welding a device to heatsink is a non-starter idea for many reasons, especially the one above.

author: microbug
author_rank: Frequent Contributor
timestamp: October 02, 2014, 07:40:09 pm
content: @metacollin, I'm glad you're interested!I'm using an ADG715 for the analog mux rather than the PSoC 4; the PSoC 4's 400 ohm resistance makes it unsuitable (the ADG715 has a resistance of 2.5 ohms).EDIT: Any ideas for removing the op-amp / DAC offset? I'm using the REF2041 to bias the op-amp at 2.048V, and the DAC outputs from 2.048V to 4.096V.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 02, 2014, 08:18:05 pm
content: Socket 775 is relatively small compared to the newer ones. If you get some old Pentium 4 heatsinks for servers with heatpipes you'll also have somewhat compact model.Like these: 66h080000-046.The 3 heatpipes can all serve 1 TO-220, with a total of 3, or 2 and an LM35 in the middle, not sure if there is need for copper filling plates. Awaiting dimension spec from oem.Does anyone know of temperature sensor ic's in TO220 that do not have the tab connected to anything?

author: microbug
author_rank: Frequent Contributor
timestamp: October 04, 2014, 09:23:56 am
content: @Jeroen3, unless I'm wrongly estimating the size, that heatsink won't get close to 300W.SeanB said this:QuoteI think he probably knows what he's talking about!EDIT: For the MCU, I'm going to use a PSoC 4 chip (not the PSoC 4 dev board). I can use the dev board to program the PSoC 4 with a bootloader that will allow firmware updates over USB (there's going to be a USB to UART chip too).

author: Jeroen3
author_rank: Super Contributor
timestamp: October 04, 2014, 10:31:39 am
content: Quote from: microbug on October 04, 2014, 09:23:56 amCorrect, 300 Watt is an enormous amount of energy. I'm not designing for that kind of currents or voltages.For typical use, a 35-40 Volt OR (not and) 10 Amps should be enough. If more is required, I'd need to buy a better heatsink. Which will be heavier, bigger and more expensive.The specification received from the OEM say: 0.25 °C /W @ 3000 RPM.

author: microbug
author_rank: Frequent Contributor
timestamp: October 04, 2014, 06:01:45 pm
content: I finally got round to resending the sample request email to IXYS. I think the last one didn't get through, as I sent it from my website's domain so it may have been spam filtered.@Jeroen3, I think I'll stick with the Freezer A30 as it's more widely available if it fails. It's not too expensive and will easily fit two TO-247s.

author: microbug
author_rank: Frequent Contributor
timestamp: October 04, 2014, 06:04:04 pm
content: @timb Aargh! I just realised my PM to you didn't send! I've sent another; let me know if it gets to you.

author: Mr.B
author_rank: Unknown
timestamp: October 08, 2014, 03:32:39 am
content: I am also designing a DC Load.Seems to be project of the year here.I am considering using a Corsair H60 watercooler. A bit more expensive than a Freezer A30, but should work well.The copper plate is removable from the waterblock/pump assembly.This way I should be able to directly reflow solder the FETs to the copper heat exchanger face.I have emailed Corsair Tech Support asking for more detalis:What is the thermal spec for the unit in deg C per W?What is the maximum power dissipation of the unit?Can the 'O' ring on the copper plate be easily removed and put back after soldering?I eagerly await their response... I hope their Tech Support is quicker than Zelman Coolers.One disadvantage of using the watercooler is that I don't know the electrical conductivity of the coolant.This means that the fan forced radiator needs to be kept isolated from everything including the chassis.Any thoughts as to whether this is a good or bad idea...?

author: SeanB
author_rank: Super Contributor
timestamp: October 08, 2014, 04:11:15 am
content: The coolant will be water based, and likely quite conductive.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 08, 2014, 04:26:36 am
content: Those packaged water cooling kits don't perform that much better than good heatsinks, certainly not enough to offset their cost. They're really designed for those who want water cooling for the "cool" factor and compact machines that don't have the space for a big heatsink.Something that really works in our favor is that power MOSFETs withstand higher temperatures than CPUs, and power resistors and SiC FETs even higher. Just beware that for the heatpipe coolers, beyond a certain temperature (somewhere above 100C, well above what most CPUs are rated for), the vapor pressure will get very high and could cause the tubing to burst.

author: Mr.B
author_rank: Unknown
timestamp: October 08, 2014, 06:07:33 am
content: @SeanB - Yip, aware of that. Thats why I would maintain electrical isolation for the radiator.@NiHaoMike - Thanks and yes, I would not want the waterblock exceeding about 90 degrees C.The main reason for using a watercooler is to reduce the size of the enclosure that is required as you mentioned. The A30 is huge.It would be nice if TiN noticed this thread. He appears to have loads of experience with cooling... being a (somewhat) professional overclocker.(I am not rude enough to PM him)

author: Jeroen3
author_rank: Super Contributor
timestamp: October 08, 2014, 12:26:24 pm
content: Those water coolers don't perform much better. Instead, they excel in a transporting the generated heat over a distance.Where air heatsinks transport the energy a few centimeters away before convection to air. Watercoolers can do tens of centimeters.Very useful when you're putting your project in a desktop case.Unfortunately electrical isolation pads directly on the fets severely reduce thermal performance. Especially short peak performance or impulse load changes.

author: Mr.B
author_rank: Unknown
timestamp: October 08, 2014, 06:25:43 pm
content: Yes, i have seen the performance specs on various 'review' sites.I am not expecting the watercoolers to be much better than a good quality standard fan cooled unit.My DC Load will be in a desktop case and the water cooler is slightly more compact than something like an A30.The thermal efficiency between FET case and cooler should be better as I intend to solder the FETS directly to the cooler. (As I said above)The cooler face is flat copper. Fairly easy reflow job.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 08, 2014, 06:42:52 pm
content: If you're looking for great cooling in a small heatsink, look at rack server heatsinks.

author: Mr.B
author_rank: Unknown
timestamp: October 08, 2014, 06:54:29 pm
content: Thanks, I will investigate them as an option.I was after something with a solid copper face so that I can solder the FETs directly to it.The removable copper face on the water cooler is attractive because it will have lower thermal mass while soldering.

author: eneuro
author_rank: Super Contributor
timestamp: October 08, 2014, 07:31:25 pm
content: Quote from: metacollin on October 02, 2014, 07:39:10 amHeating those IC units by classic soldering or putting them to ovens is thermaly better?Haven't got a chance to test spot welding IC taps with brazing yet, probably next week while my buffy spot welder should be ready and tested this week after upgrade to 25A SCRs in pararell on its primary and just developing its software to add support for brazing mode

author: mrflibble
author_rank: Super Contributor
timestamp: October 09, 2014, 03:21:22 am
content: Quote from: eneuro on October 08, 2014, 07:31:25 pmThe issue is the cross section of the resulting thermal interface. Spot weld ==> smaller cross section (compared to soldering) ==> poorer thermal conductivity across that thermal interface. Anyways, it has already been explained pretty well earlier on in the thread.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 09, 2014, 07:14:05 am
content: Quote from: NiHaoMike on October 08, 2014, 06:42:52 pmThere are plenty for sale on eBay of recycled servers.

author: eneuro
author_rank: Super Contributor
timestamp: October 09, 2014, 06:33:48 pm
content: Quote from: mrflibble on October 09, 2014, 03:21:22 amThis is not in the case when brazing method is implemented.Spot welder is only used to create huge short pulses to join copper areas by brazing, so it is simply source of high energy released in short times and those two surfaces should get soldered, but not by heating everything around including semiconductors chips.Withput brazing it could be simply not possible join together copper metals without huge currents while it has very low resistance and very good thermal conductivity.I hope it will work, then will tell more after experiments are made, but I do not dismiss it until tests are done.Without brazing it couldn't have any sense to play and melt even with 1mm copper TO220 taps.Tips for Soldering and Brazing Copper Tubing

author: mrflibble
author_rank: Super Contributor
timestamp: October 10, 2014, 03:42:10 am
content: Quote from: eneuro on October 09, 2014, 06:33:48 pmWell, your remark (complete with that bit you bolded for emphasis) and my reply to your remark were about spot welding. No remarks from me about brazing, since brazing is just soldering with an attitude. And as you point out, with brazing you don't get a poor cross section as with spot welding. What with it being the same kind of filler action as with boring old lower temperature soldering.

author: rob77
author_rank: Super Contributor
timestamp: October 11, 2014, 05:34:05 am
content: Quote from: mrflibble on October 10, 2014, 03:42:10 amand actually brazing would kill the mosfets easily - the max soldering temperature for most of the components is around 300 degree celsius while brazing happens well above 400 degree celsius. so brazing is a big big NO-GO for mosfets.

author: eneuro
author_rank: Super Contributor
timestamp: October 11, 2014, 06:51:39 am
content: Quote from: rob77 on October 11, 2014, 05:34:05 amDid you tested it with your own spot welder?By brazing using spot welder and short high power pulses I mean something different than that

author: mrflibble
author_rank: Super Contributor
timestamp: October 11, 2014, 07:14:22 am
content: @eneuro:Just to avoid confusion, whatdoyou mean when you say "brazing"? Essentially the same thing as soldering, but with higher temperature melting point for the filler material? Or something different that I am not aware of?Because you mention"brazing using spot welder"? Wut? How does that work?At any rate, as far as I am concerned for a direct join of the metal tab of say a mosfet to a heatsink:- spot weld ==> noooope, crap cross section ==> poor conductivity- brazing ==> noooope, high work temperature ==> high risk of killing semiconductor- soldering ==> yupThat is not to say that I would pick soldering over boring old clamping, but it is to say that I would pick soldering as the prefered method out of the above three choices.Unless I am missing something, in which case please explain.

author: eneuro
author_rank: Super Contributor
timestamp: October 11, 2014, 06:08:47 pm
content: Quote from: mrflibble on October 11, 2014, 07:14:22 amI wrote about those instructions in post #118I found thereHow to Spot Weld Copperthose instructions, and applying brazing in 1st step is a key to do it.QuoteDidn't tested it yet, but makes sense.Brazing pase between two much better conductors should behave like this steel ball, but it will be diffrent thanks to point #3 of those instructions above and I have much more power full spot welder I think.Paste should be melted quickly and since datasheet of one of mosfets I used says about 300*C for 10 seconds it is a lot of time to glue two pieces of copper.Probably at the end of next week I will make some experiments-using spot welder with brazing pase to join together good conductors like copper and aluminium.

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2014, 06:14:35 pm
content: 300C for 10 seconds on the LEADS, not the case. While you probably could get away with reflowing them at 250C in an IR oven to reflow it to a copper clad board you would be severely stressing the package attachment. Brazing at 300C will probably cook the case off.

author: mrflibble
author_rank: Super Contributor
timestamp: October 12, 2014, 09:23:15 am
content: Quote from: eneuro on October 11, 2014, 06:08:47 pmI thought you might have meant that, but better to be sure. Reason I asked is because even that article you linked to refers to the joining action as spot welding, as opposed to brazing. Anyways, that's semantics, now I know what surface joining action you mean.I suspect many molten mosfets lay in your path to enlightenment, but we'll await your experiment results.

author: eneuro
author_rank: Super Contributor
timestamp: October 12, 2014, 01:52:24 pm
content: Quote from: SeanB on October 11, 2014, 06:14:35 pm300*C on TO220 tap was taken only as comparision to its leads solder temperature 1.6mm from case, since I'm not sure if there are tap soldering temperatures and time in datasheet

author: Jeroen3
author_rank: Super Contributor
timestamp: October 13, 2014, 06:46:02 pm
content: Earlier in this topic there was talk about an IXYS mosfet. Which one if I may ask?IXYS seems be manufacturing quite fancy stuff. But the seemingly most effective is an depletion mode, which equals to a short without a control supply.Those TO-247's and 264's are great for mounting underneath cpu coolers. Don't fool around with TO-220.

author: microbug
author_rank: Frequent Contributor
timestamp: October 13, 2014, 06:50:28 pm
content: I just sent an email to an IXYS sales representative in the UK asking for two IXTH75N10L2 N-channel enhancement-mode MOSFETs. The first sales rep (h-rep) turned me down, and I can't find the MOSFETs online in minimum quantities less than 30. I might go for a different one if I continue to have no luck.

author: Mr.B
author_rank: Unknown
timestamp: October 13, 2014, 06:55:35 pm
content: I am going to use IXTH30N50L2.They are a couple of dollars dearer than IXTH75N10L2, but at least they are in stock at Mouser.

author: microbug
author_rank: Frequent Contributor
timestamp: October 13, 2014, 07:25:21 pm
content: Yes, those would be OK. I'll have to do some searching to see if there are some available online which have similarly good specs to the IXTH75N10L2s (low RDS(on)).

author: Mr.B
author_rank: Unknown
timestamp: October 13, 2014, 07:34:27 pm
content: The IXTH75N10L2 has about 20mOhms.The IXTH30N50L2 has about 215mOhms.In the bigger scheme of things, is it going to make a lot of difference?

author: microbug
author_rank: Frequent Contributor
timestamp: October 13, 2014, 07:35:18 pm
content: Not really, but if they're around the same price I might as well.

author: Mr.B
author_rank: Unknown
timestamp: October 13, 2014, 08:57:35 pm
content: For me they are about NZD 3 each more expensive.I ordered 16 from Mouser late last week.Arrived this morning.BIG DC load.

author: rob77
author_rank: Super Contributor
timestamp: October 13, 2014, 10:04:20 pm
content: Quote from: Mr.B on October 13, 2014, 07:34:27 pmRdson is unimportant unless you're planning to build a dummy load capable of tens of amperes at 1 V or less

author: Mr.B
author_rank: Unknown
timestamp: October 13, 2014, 10:11:50 pm
content: Exactly rob77.In my design I am not wanting to do anything under about 3v3 anyway.

author: Mr.B
author_rank: Unknown
timestamp: October 14, 2014, 01:04:57 am
content: @AcHmed99 - Yes, they have exceptional thermal characteristics.However, I have already spent NZD 250 on the IXTH30N50L2.On the subject of thermal...The IXTH30N50L2 has a Tjmax of 150 deg C.Datasheet attached.Expert experience question: How hard can you safely push it?140 deg C?6.66% de-rated?

author: rob77
author_rank: Super Contributor
timestamp: October 14, 2014, 06:36:02 am
content: Quote from: Mr.B on October 14, 2014, 01:04:57 amcheck the datasheet for thermal de-rating factor - it might be a surprisingly high number (a 100W mosfet can easilly become 40W max at high temperatures)

author: Jeroen3
author_rank: Super Contributor
timestamp: October 14, 2014, 10:09:17 am
content: I was looking at the linearity of the Id/Vgs curve. It's not ideal if the fet goes from 0 to Imax in a window less than 1 Volts.The IXTP or IXTH15N50L2 seems nice for this purpose, you have less risk of instability when you're running milliamps.And it's affordable on digikey and an ebay seller.I'll look into the ones named above tonight.(EU time)

author: microbug
author_rank: Frequent Contributor
timestamp: October 14, 2014, 02:36:46 pm
content: I got a reply from Delta Components in the UK. They have a sister company, Neutron LLP, that will provide me with two IXTH75N10L2 samples free of charge

author: Mr.B
author_rank: Unknown
timestamp: October 14, 2014, 07:28:31 pm
content: Quote from: rob77 on October 14, 2014, 06:36:02 amChecking the datasheet, I cannot see anything directly called thermal de-rating factor - unless I am overlooking something.The IXTH30N50L2 is quoted as a 400W device (when Tc = 25 deg C).If I take the quoted SOA: 200W @ Tc = 75 deg C.RthJC is 0.31 deg C/W.200 * 0.31 = 62 deg.Therefore: 75 + 62 = 137 deg C for the junction temperature when the case is 75 deg C and I am driving it at 200W.I plan to drive the device at a maximum of 150W.Assuming 137 deg C is within the SOA and working backwards...150 * 0.31 = 46.5 deg C.137 - 46.5 = 90.5 deg C.To remain within SOA, I can allow the case temperature to be as high as 90.5 deg C @ 150W.Now add to this the RthCS of 0.25 deg C/W --> 150 * 0.25 = 37.5 deg.90.5 deg (case) - 37.5 deg = 53 deg.If my math above is correct...I can allow my heatsink face to get to 53 deg C if I am driving the FET at 150W.Have I understood this correctly?

author: Jeroen3
author_rank: Super Contributor
timestamp: October 14, 2014, 07:44:26 pm
content: IXYS is very honest and they specify their SOA at Tc 75.They do not specify a dereting factor. But this fet is designed for linear use.If you stay inside the soa drawn in Fig. 14 you should be fine.

author: Mr.B
author_rank: Unknown
timestamp: October 14, 2014, 07:46:29 pm
content: Quote from: Jeroen3 on October 14, 2014, 10:09:17 amSorry Jeroen3, I dont quite follow you.The graph for the IXTH30N50L2 looks to me to be about 3.5v for the 10A Ids range I want to run mine in.Am I missing something here?

author: Mr.B
author_rank: Unknown
timestamp: October 14, 2014, 07:50:41 pm
content: Quote from: Jeroen3 on October 14, 2014, 07:44:26 pmThanks, will do.

author: Mr.B
author_rank: Unknown
timestamp: October 14, 2014, 07:52:23 pm
content: Apologies @microbug.I am not wanting it to appear that I am hijacking your thread.It is just that we are both designing a DC load and using similar or the same components.I figured a shared knowledge thread was better than starting another one called "yet another DC load".

author: Joenuh
author_rank: Newbie
timestamp: October 15, 2014, 04:05:54 pm
content: Just to let you guys know, I just got a reply that I will be receiving 5 samples of the IXTH75N10L2 tomorrow. I only asked once..

author: microbug
author_rank: Frequent Contributor
timestamp: October 15, 2014, 04:19:17 pm
content: @Mr.B, no worries! That makes sense. I haven't been working on my design for the past week or two; I need to get on with the schematic.@Joenuh I don't know what country you live in, but here in the UK the official IXYS office appears to be unreachable. The only way I could get samples (which arrived today, BTW) was through a distributor (Neutron LTD). I've only got two of them, so I'll be using cheaper ones until I have to test the power dissipation.EDIT: Offset voltage for the AD633 (jellybean multiplier) sucks, plus it's really expensive, so I'm going to do constant-power and constant-resistance in software.

author: macboy
author_rank: Super Contributor
timestamp: October 15, 2014, 05:03:42 pm
content: Quote from: microbug on October 15, 2014, 04:19:17 pmConstant resistance operation does not require a multiply operation. Set the current to be proportional to terminal voltage, rather than setting it to a constant value. Constant power operation does require a multiply.

author: microbug
author_rank: Frequent Contributor
timestamp: October 15, 2014, 05:31:32 pm
content: OK... so I could switch the external reference on the DAC to a divided-down version of the input voltage, and compare the DAC output with that same divided down voltage.

author: Joenuh
author_rank: Newbie
timestamp: October 15, 2014, 06:46:24 pm
content: @Microbug I live in The Netherlands. I used the tech support (emailadress), they were happy to forward me to the Europe section of IXYS. Today I got an email from a IXYS dealer.. Anyway you still got a couple

author: Jeroen3
author_rank: Super Contributor
timestamp: October 16, 2014, 09:27:00 am
content: Quote from: Mr.B on October 14, 2014, 07:46:29 pmPrecisely. With a "normal" switching optimized fet this isn't 3.5 Volts.

author: microbug
author_rank: Frequent Contributor
timestamp: October 16, 2014, 04:03:27 pm
content: I've just ordered the Arctic Cooling Freezer i30 CPU cooler for this project. It's the same as the previously planned A30, but it's for an Intel socket and is slightly cheaper.Would it be best to solder the two TO-247s directly to the copper heat pipes (using low-temperature bismuth-tin solder)? Solder appears to be better than thermal epoxy, and if I had the two packages opposite ways up then the heat transfer would be about the same for each heat pipe.

author: mrflibble
author_rank: Super Contributor
timestamp: October 16, 2014, 04:11:01 pm
content: Quote from: microbug on October 16, 2014, 04:03:27 pmNo. What would be best is tonotsolder it. Best would be to use a thermal paste of your choosing. Then take measurements of thermal performance. Then remove paste, clean it up, and solder it.Then measure thermal performance again, and compare findings.

author: microbug
author_rank: Frequent Contributor
timestamp: October 16, 2014, 04:14:17 pm
content: OK. I'll get on with the rest of the project, and do the thermal stuff last. That way, I can put exactly 300W (or whatever) into the cooler and measure its temperature.

author: Mr.B
author_rank: Unknown
timestamp: October 21, 2014, 09:20:52 am
content: I am using similar components to @microbug.Sharing the knowledge on this thread...Looking for a good closed loop fan controller for my DC load...As you may have noticed in previous posts, I intend using a water cooler for my DC load - One pump, one fan.I want a supervisory device that fits the following profile:Sits on the SPI bus.Has one or two PWM outputs for fan control.Has two tacho inputs for fan speed measurement.Has one or more temperature measurements.Has closed loop temperature control.A good example, but over the top for my application, is the Analog Devices ADT7470.http://www.analog.com/static/imported-files/data_sheets/ADT7470.pdfIn fact, Analog Devices have a lot of cool devices that almost fit the bill... but not quite...Anyone had any experience with this issue?Any recommendations?Much appreciated.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 21, 2014, 10:07:33 am
content: Why don't you mount a 4 wire fan with a ~25 KHz pwm timer and a timer capture input.All should be available in your mcu, excluding the level shifting and protection.After all, you want to be sensing using an RTD in the TO's mount hole anyway. Which is probably not an SMbus device.

author: Mr.B
author_rank: Unknown
timestamp: October 21, 2014, 07:16:34 pm
content: Thanks Jeroen3.Yes, the MCU will be more than capable of doing that, however it is not really the way I want to do it.My design consists of one "Display and MCU" module, with up to three "Power" modules.The idea was to build the Display and MCU module with one Power module, then further down the track I could add more Power modules.By having everything on one serial bus it means the potential three Power modules can be simply daisy chained.The chip selects are managed by simple implementation of a 595 serial to parallel shift register.By using a closed loop controller it is just one less thing to have to worry about in the MCU.

author: Joenuh
author_rank: Newbie
timestamp: November 11, 2014, 08:16:02 pm
content: Any updates on this project?

author: microbug
author_rank: Frequent Contributor
timestamp: November 16, 2014, 10:52:51 am
content: I've been extremely busy of late so this project is currently on hold. Updates as follows:- I got a Freezer i30 CPU cooler.- The heat pipes on the CPU cooler are flattened and milled smooth at the point where they make contact with the load (a 'patented system' by ARCTIC). This will mean less effort and better thermal connection when connecting the FETs.- The cooler is smaller than I expected: I can see that it would do 300W, but it's impressive.- The enclosure will probably be plastic with a hole cut in the top for the heatsink.- I ordered and received one of the $4 PSoC 4 dev boards.- I'm planning to use the PSoC dev board in the final design since it has a USB>serial converter on it and the included PSoC is pre-programmed with a boot loader.- The load will have USB connectivity so that it can be reprogrammed easily. This will mean I can get the hardware over with before the software.I've been designing with daily builds of KiCad. I find them fine in stability, and if a build is unstable it will usually be fixed tomorrow! When I have more time (Christmas holidays coming up) I'll do some more work and upload the R1 design to hackaday.io .

author: kalj
author_rank: Contributor
timestamp: July 06, 2020, 07:06:00 am
content: Quote from: microbug on November 16, 2014, 10:52:51 am@microbug : What happened to the project? Did you finish it?