What the best way to mount HIGH voltage semiconductors to grounded coolers? - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: November 17, 2018, 01:54:28 pm
content: What the best way to mount HIGH voltage semiconductors to grounded coolers (radiators)?For example TO-247 transistor at 600VDC?

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 17, 2018, 01:59:33 pm
content: Use insulating thermal pads.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2018, 02:18:32 pm
content: There are several things you have to get just right for long term reliability.Burr free and flat heatsink surface.Spring clip across the package so you don't have to worry about creepage distance from the metal pad in the package base to the mounting bolt or sleeve the bolt with an insulator. The clip provides spring pressure on the center of the package that does not vary significantly as its thermally cycled.Mica insulating sheet, which if a good grade of natural Mica is typically more reliable long term than a silpad.Light smear of heatsink compound on both sides of the mica sheet.If the Mica thickness is appropriate, the above is good for over a kilovolt for many years. An appropriately rated glass-fibre reinforced silpad (no heatsink compound needed) is better for automated assembly and will be good for a few years, but I wouldn't bet on its reliability a decade later as silpads can fail by deforming under pressure as they age then pin-holing at or near a package corner.

author: Gyro
author_rank: Super Contributor
timestamp: November 17, 2018, 02:35:10 pm
content: For higher voltages (probably 1kV+) you can get solid Aluminium Oxide washers. They have surprisingly good thermal performance but you need to have some tension control on the screw (crinkle washers etc) to avoid the risk of cracking them.eg.https://uk.rs-online.com/web/c/?sra=oss&r=t&searchTerm=aluminium+oxide+washer

author: kony
author_rank: Regular Contributor
timestamp: November 17, 2018, 03:30:29 pm
content: Alumina is shit compared to aluminium nitride ceramics - that has comparable heat conductivity to the alloy heatsink is extruded from. So if you are running out of the thermal resistance budget, this is a very good fix.

author: 001
author_rank: Super Contributor
timestamp: November 17, 2018, 04:40:17 pm
content: Quote from: kony on November 17, 2018, 03:30:29 pmhow it look? is it expensive? I never seen one

author: kony
author_rank: Regular Contributor
timestamp: November 17, 2018, 06:25:38 pm
content: It depends, I had seen some TO220/247 spacers from chineese vendors, other option is to get them custom made for the purpose from local ceramic hybrids manufacturer. Regular ceramic alike material with ivory to grey color. It is not really a comodity solution, but when you are after utilising most of the power handling capacity of the semiconductor device yet want to keep it with safe galvanic insulation, this considerably outperforms all other options.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 17, 2018, 07:38:25 pm
content: Something else that can be done, if the thermal conductivity of a thermal pad is not good enough, is to mount the transistors or whatever on a block of aluminum or copper, then use a larger thermal pad to couple that to the heatsink.

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 17, 2018, 07:43:52 pm
content: Standard 96 % alumina has a thermal conductivity of around 25 W/mk, which results in an added 0.25 k/w thermal resistance per millimeter of thickness, given a TO-247 metal pad area of 160 mm^2, while the number for aluminium nitride is less than 0.04 k/w in the same scenario. The problem is that it's very easy to add ten times this thermal resistance just from non-optimal thermal compound application. So to fully benefit from AlN insulating washers, it takes a lot of care in surface preparation, thermal compound selection, thermal compound application and optimization of the level and distribution of the clamping force. For cases where you need thicker insulating pads, the advantage of AlN is more dramatic, of course.

author: floobydust
author_rank: Super Contributor
timestamp: November 17, 2018, 08:04:05 pm
content: Surprised to see Wolfspeed using: Kapton Thermal Interface 22mm x 29mm , Fisher Elektronik, KAP 1 P"polyimide-carrier foil with silicone-free phase changing thermal conductive layer completely coated on both sides"0.15 K/W [at 1 inch2; = 6.45 cm2; = TO 3 (KAP 3)]0.45 W/m·K (substrate)material thickness 0.077mm (substrate 0.05mm)dielectric strength 7.8 kV

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 17, 2018, 08:35:40 pm
content: Those kapton foils are nothing but trouble in high voltage applications, especially in cases with high dv/dt. I optimistically used some to isolate some SiC MOSFETs and diodes in a 750 V buck converter, and they failed after a short time in operation. Further testing gave consistent failures of this material. The manufacturer of this material (Aavid in this case) blamed the surface finish on our heatsinks, but they don't actually publish any information on the recommended surface finish for kapton foil insulators. Swapping them with alumina pads solved the problem completely.

author: floobydust
author_rank: Super Contributor
timestamp: November 17, 2018, 11:14:14 pm
content: I would look at what the automotive industry is using.Anyone know what Tesla Motors used here in older Model S? I think it's a modified TO-247 IGBT package. I couldn't find pics of the other side.

author: jbb
author_rank: Super Contributor
timestamp: November 18, 2018, 01:19:01 am
content: +1 for spring clips. Having a hole in you insulator through which you put a metal screw is really bad for creepage & clearance. Also some TO-220 devices very thin metal tabs which deform (causing increased thermal resistance) when the screws are tightened.A Sil-Pad style tends to add about 1 deg C / W of thermal resistance which must be taken into account.Regarding the Tesla inverter; those look like a modified TO-247. By not having the hole you can get more silicon into the same package. IXYS make some standard parts like that, but at Tesla scale they could be custom devices.

author: wraper
author_rank: Unknown
timestamp: November 18, 2018, 01:27:05 am
content: Quote from: Ian.M on November 17, 2018, 02:18:32 pmSilpad should be more reliable because it's thicker and won't crack. However mica + thermal grease have better thermal conductivity.

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2018, 03:53:10 am
content: Mica only cracks if grossly abused, and is very stable under pressure. Its also virtually immune to ageing as its millions of years old to start with! If you start with a good clear piece without any signs of delamination or cracking, and don't mangle it during installation, or have any burrs on the heatsink or device surfaces, its very reliable and fully reusable.Silpads however tend to degrade with ageing and high temperatures, and if they've been in use for any significant length of time should *always* be replaced if disturbed. Their only advantage is convenience on the assembly line for low cost products, as their resilience makes the package base and heatsink flatness, and initial clamping pressure less critical, and if you specify self-adhesive ones, they can make automated assembly much easier.

author: wraper
author_rank: Unknown
timestamp: November 18, 2018, 04:05:16 am
content: Quote from: Ian.M on November 18, 2018, 03:53:10 amIf all of those conditions are met, which often are not. FWIW I've seen plenty of MICA insulators shoot through.

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2018, 05:39:12 am
content: Certainly, it only takes a small surface defect or bit of debris for it to suffer crush damage during assembly. Also there is the problem of obtaining high enough grade mica sheet for HV work - you absolutely cant tolerate significant non-mica inclusions, and I'd be very wary of built-up (composite) mica sheets, so in large enough pieces to provide sufficient creepage distance at the edges, its likely to be expensive compared to other solutions and any attempt made to reduce costs either or materials or by using less skilled labour has a high risk of introducing defects that can cause it to breakdown under electrical or mechanical stress, either rapidly or much later when the heatsink compound has deteriorated enough for it to cease to entirely fill the defect.

author: SeanB
author_rank: Super Contributor
timestamp: November 18, 2018, 09:18:34 am
content: Having taken apart a few power blocks, they all use a large aluminia insulator, with the power devices soldered to the ceramic on silver plated pads, and the bottom side soldered to a large copper heat spreader. Good for 2kV of isolation, and very reliable.You probably will want to get large alumina plates ( at least a lot larger than the devices for creepage clearance), and use a spring clip or a top clamp with Belleville washers to provide the correct spring clamping to the devices. Then appropriate thermal compound and the manufacturer of the devices recommended clamping force per device. Note with the aluminia isolator you do need to have a machined burr free heatsink mounting surface, as extruded is not going to work, it has to be machined to be as close tolerance as the ground aluminia surface to prevent cracking. Remember that the clamp as well needs to be as rigid as the heatsink, so as to apply even pressure, so you will likely either have a bar across the top ( probably with a kapton tape to provide higher isolation) and a clamp assembly both ends, or a longer bar with multiple devices being clamped by it. When doing multiple devices with a single clamp make sure they are all from the same lot number, as they might have slight differences in the thickness of the top epoxy, and tolerances add up.https://en.wikipedia.org/wiki/Belleville_washerBasic info, use with a nylock nut to provide the anti loosen requirement, the washer stack provides the even clamping force with changes in operating temperature.http://www.bellevillesprings.com/belleville-washers.htmlUK company, I have bought from them before, good service even international, and very friendly as well. Good quality items.Worst thing are power SCR pucks, they have a special heatsink assembly that you must use to get the lifetime, and generally no warranty if you do not use it either or use incorrectly. Those things do not let magic smoke out, they instead turn into shrapnel powder with a loud bang, then blow the expensive FF semiconductor fuse.

author: coppercone2
author_rank: Super Contributor
timestamp: November 18, 2018, 04:18:52 pm
content: Quote from: NiHaoMike on November 17, 2018, 07:38:25 pmI really like this idea because the peak pressure on the dielectirc material is much smaller since it has much more surface area to distribute it especially if its something like a TO-220 screw. If its a heat sink clip or a press bar it might not make as big a difference but it should still be better because you can use nice grinded or lapped parts that really make a good fit rather then trying to get equal stress from a very cheap part. I think I will do this from now on.

author: wraper
author_rank: Unknown
timestamp: November 18, 2018, 04:51:59 pm
content: Quote from: NiHaoMike on November 17, 2018, 07:38:25 pmExample of this in old Soviet amplifier.

author: coppercone2
author_rank: Super Contributor
timestamp: November 18, 2018, 05:13:43 pm
content: I am pretty sure though that you want to use a tensioning bar or one of those special heat sink clips to tension the package rather then a screw for optimal heat conduction

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 18, 2018, 07:55:25 pm
content: Quote from: floobydust on November 17, 2018, 11:14:14 pmThat's a Roadster inverter phase leg. This inverter used some polymer based insulation between the IGBTs and the water cooling blocks, and these are failing in large numbers:https://teslamotorsclub.com/tmc/threads/roadster-pem-failure.88028/. The Model S uses 0.5 mm alumina pads and I haven't heard of them failing. I'm not aware of any pictures of them as the whole phase leg assembly is potted in silicone gel and removing the transistors is a pretty destructive affair.

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2018, 08:18:10 pm
content: Quote from: NiHaoMike on November 17, 2018, 07:38:25 pmYes, that works very well. In the past I soldered an LM317 to a piece of copper bar which I then fixed to the case with an electrical insulator sandwiched in between.I was still learning when I built this, so don't give me too much grief about the messy construction.I repeated the project but made it dual rail, tracking and fixed the LM317 and LM327 with screws to copper blocks, again mounted to the case with electrical insulators. It's a little neater, than the previous project, but could've been better.Obviously for 600V, the above examples are inadequate. The insulators need to be larger than the copper blocks to ensure adequate creepage distances.

author: coppercone2
author_rank: Super Contributor
timestamp: November 18, 2018, 08:52:29 pm
content: I don't think solder is permitted for earth ground connections officially, because of mating cycles and flex of the ground pin if the cord is fucked up. The solder lug on the chassis is a bit more acceptable because that point is not being flexed, but you obviously still need excellent strain relief so the motion does not transfer through the ground wire from the plug (which is less if you use multi strand wire).and the earth solder joint in the lower picture looks down right suspicious. I think you should at least make a half hook through the hole to solder it with good clearance to the pin if you insist on soldering it, it looks like its bearly attached. Did you clean and flux the joint (perhaps with a brass brush?). The wetting on your solder joints looks horrid but I am not sure if its a camera thing.Also, where are the fuses at?cant hurt to put the levitating rectifier on a bracket either.Also would recommend a softer dielectirc between the torroid and the tensioning washer.also are those earth ground lugs star washered?This one is unnecessary but there is a kind of cheap trick to wire-pcb junctions, where you drill a hole away from any conductors and zip-tie the wire to the PCB through the holes or between the hole and the edge of the PCB. It works very well I had a hobby project that got yanked very hard and it held up fine without damage to the PCB.

author: Gyro
author_rank: Super Contributor
timestamp: November 18, 2018, 09:13:49 pm
content: Quote from: Hero999 on November 18, 2018, 08:18:10 pm"I was still learning when I built this, so don't give me too much grief about the messy construction."Wouldn't dream of it, but I think you owe it to the forum to post the picture in the 'ugly-but-flawless-project' thread for posterity - if only for the 'securely bolt the lightest components and glue the heavy ones' methodology. Sorry, I really couldn't resisthttps://www.eevblog.com/forum/chat/show-us-your-ugly-but-flawless-project/

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: November 18, 2018, 09:42:12 pm
content: Quote from: Wolfram on November 17, 2018, 08:35:40 pminteresting.can you elaborate on this? i.e., what was the isolation thickness, what switching frequency were you using, what slew rate on VDS did you observe, what was the heatsink temperature, and how long was the operational time of your converter before the isolation broke down?apologies for being nosy here, but i find your information extremely valuable and i'd like to understand the actual circumstances of the isolation break down.

author: Zero999
author_rank: Super Contributor
timestamp: November 18, 2018, 10:06:50 pm
content: Quote from: coppercone2 on November 18, 2018, 08:52:29 pmI agree: the earth should be crimped, but there's no flex. It's connected to the mains via an IEC connector.I can't remember whether I wetted the joints, as this was well over ten years ago.QuoteThey're in the black holders which aren't very clear on the pictures.QuoteI agree, it might come off, if dropped.QuoteIf I remember rightly there is a rubber or foam washer between both the large tensioning washer and the other side of the toroid but they're not visible on the photograph.QuoteI doubt it.QuoteI don't know what you mean. I'm not familiar with that.

author: coppercone2
author_rank: Super Contributor
timestamp: November 18, 2018, 10:28:37 pm
content: you drill a hole next to the edge of the PCB so you can put a zip tie into the hole and around the PCB to clamp down on the wire and you can drill another hole to loop the wire through also.

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 19, 2018, 11:45:15 am
content: Quote from: Le_Bassiste on November 18, 2018, 09:42:12 pmThis particular test was done with 25 µm material (4200 V AC breakdown rating), the switching frequency was 75 kHz, the slew rate was around 30 V/ns, temperature was around 60 degrees on the heatsink, and it failed after around two hours. There were actually two failures, the first one was between the heatsink and the diode. This one went unnoticed. Then the second failure, between a transistor and the heatsink, tripped the converter protection. The failure left some nice craters in the backs of the transistor and diode, but the converter was still working afterwards.

author: coppercone2
author_rank: Super Contributor
timestamp: November 19, 2018, 01:38:21 pm
content: the surface finish they should give should be specified in Ra (roughness) and flatness inch/mm (if its not flat enough you get uneven pressure distribution).They seriously could not provide it when asked? So long you gently stone the surface before hand and give it a wipe down there should be no penetration.I wanna know lol

author: Wolfram
author_rank: Frequent Contributor
timestamp: November 19, 2018, 05:44:00 pm
content: They didn't specify it. This is a company that also does not provide any mechanical data on their mechanical products, and no thermal data on their thermal products so that's to be expected. Needless to say, we don't deal with them any more. The reduced capacitance from alumina with similar thermal performance reduced switching losses and EMI challenges, so there's no great motivation to make a kapton solution work in this application.

author: 001
author_rank: Super Contributor
timestamp: November 19, 2018, 06:23:06 pm
content: Hey HeyHo to do it without rocket since?

author: stj
author_rank: Super Contributor
timestamp: November 20, 2018, 03:14:35 am
content: did i miss it, or was the working voltage not mentioned??i'm thinking berilium-oxide spacers - old-school tech.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: November 20, 2018, 11:57:12 am
content: Quote from: 001 on November 19, 2018, 06:23:06 pmif i get it correctly from your opening post, the important keywords are "600V" and "grounded heatsink".hence i assume that your application is mains-connected and requires double or reinforced isolation, so you have to consult the applicable safety standards for your application wrt voltage categories, etc.have a look at silicone tubing for power transistors,https://thermal.henkel-adhesives.com/en/eimea/thermal-materials-products/product-detail/sil-pad-400.htmlmounting clips (already mentioned by others in this thread) and heatsinks that are made to accomodate those clips.https://www.digikey.de/en/product-highlight/a/aavid-thermalloy/max-clip-heat-sinksagain, keep in mind that you will have to use double or reinforced isolation for your application.

author: N2IXK
author_rank: Frequent Contributor
timestamp: November 20, 2018, 04:19:46 pm
content: Beryllium Oxide "thermal links" were used on conduction cooled vacuum tubes, where the anode may be operating at thousands of volts above ground.

author: Zero999
author_rank: Super Contributor
timestamp: November 21, 2018, 10:55:59 am
content: Quote from: N2IXK on November 20, 2018, 04:19:46 pmAnd back then, electrical safety standards weren't so strict.QuoteIf the heatsink is grounded then reinforced/double insulation is unnecessary. Only basic insulation is required and whilst it does have to fulfil certain regulatory requirements, above functional insulation, they're nowhere near as strict as double/reinforced insulation which would only be required if the heatsink isn't earthed and the user may come into electrical contact with it.Reinforced insulation typically needs to be double that of basic insulation. Here are some links about creepage and clearances which is relevant to heatsinking and PCBs.https://www.infineon.com/dgdl/Infineon-ApplicationNote_MOSFET_CoolMOS_Electrical_safety_and_Isolation-AN-v01_00-EN.pdf?fileId=db3a30433d1d0bbe013d20e0cbf017fehttp://www.reo.co.uk/files/safety_7_-_low_voltage_directive.pdfhttp://blog.optimumdesign.com/clearance-and-creepage-rules-for-pcb-assemblyhttps://www.frontdoor.biz/HowToPCB/HowToPCB-extra/CreepageandClearance.pdfhttps://www.silabs.com/documents/public/application-notes/AN583.pdfQuoteAt a glance it doesn't appear that those solutions will be applicable for basic insulation at 600V, but it depends on the materials used. Read the data sheets in great detail.