Safety considerations when mounting high voltage MOSFET on Heatsink - Page 1
Forum section: electronics / beginners

author: rstor22
author_rank: Regular Contributor
timestamp: June 18, 2022, 08:24:09 pm
content: I am attempting to repair a Wharfedale SPC 12 subwoofer (for someone else) that does not power on. The unit has an IEC power connector however there is no grounding pin on the IEC connector, only live and neutral. On the SMPS the IR2153 was physically blown and several capacitors nearby had high ESR / capacitance out of spec. I also found that the two IRFP460A mosfets (Q1 and Q2) on the high voltage side are testing bad. Attached is a picture of the board (with some components removed).I have ordered replacement parts to attempt the repair. The mosfets were originally mounted on a heatsink with what appears to be silicone interface caps. There is a metal plate that is used to push the mosfets against the heatsink when the interface caps are over the mosfets (see attached)The heatsink on which the mosfets are mounted to is connected to a larger heatsink that has its fins sticking out the back of the subwoofer and is exposed to the end user. Therefore there is a direct connection between the mosfets heatsink and the outside fins (end user).For some reason the original silpad(?) cap had thermal paste applied to it on the outside as well as on the inside. I decided it would be a good idea to replace the cap. The order for replacement parts was through Mouser and I could not locate a rectangular silpad cap however I was able to get the following silpad tube:https://www.mouser.ca/ProductDetail/Bergquist-Company/SPT400-12-135-25?qs=xZ%2FP%252Ba9zWqbcUwc9gNXWQQ%3D%3DThe datasheet on the silpad tube indicates a dieelectric breakdown voltage of 5000 Vac. The thickness of the silpad tube looks to be thinner when compared to the original rectangular cap.Should this be a concern?I am thinking to use this silpad tube instead of the original cap and not use any thermal paste.Would this silpad tube provide sufficient electrical insulation for the end user?Is this type of SMPS design common where a high voltage device is mounted to a heatsink via an electrically insulating material and the heatsink can also be in contact with the end user?

author: Gyro
author_rank: Super Contributor
timestamp: June 18, 2022, 09:55:23 pm
content: Repairing mains powered stuff for other people is a bit of a minefield, insurance and 'public' liability wise, these days. You should maybe get a signed piece of paper saying that they take responsibility for putting it into service after your repair.Having covered that, the Silpad tube's 5kV rating corresponds with the required test voltage rating of a Y2 capacitor, commonly used between line and output or metal case on a Class II double insulated consumer product, those carry agency approvals though. This does constitute a change from the originally specified parts and I think it still counts as single insulation, even with the high voltage rating.Personally, I would carefully inspect the existing silicone 'caps' for punctures or other damage [EDIT: eg. thinned by previous clamping] and consider re-using them, cleaning off and replacing the heasink compound (it's very unusual for it to be used with compliant pads). It is possible that they used thicker material as a reinforced insulation measure - we don't know the specification of these items. Whichever you use, you need to be careful not to overtighten the clamping screw and pinch the insulation too hard.You say that the IEC connector ground pin is unconnected. A belt and braces approach would be to run a suitable thickness grounding wire from the IEC ground pin and a screwed solder tag on the exposed metal heatsink (ensuring thatbothends are mechanically secure and heatshrinked). You would first need to check that the heatsink is electrically isolated from signal ground so that you don't introduce a hum loop.Yes, adding a heatsink grounding wire would constitute a design change, but I can't immediately think of a reason why adding a Class I grounding scheme for exposed metal, to something that already has Class II grade insulation, would make it less safe.... As long as it is properly and securely done.You are right to be concerned. I've designed quite a lot of metal cased consumer Class II products, but I've never had to design anything with a single layer of insulation between a live (primary side) part and exposed metalwork - it kind of goes against the definition of double insulated.

author: rstor22
author_rank: Regular Contributor
timestamp: June 18, 2022, 10:51:41 pm
content: Thank you Chris for your reply. This information is very helpful!Quote from: Gyro on June 18, 2022, 09:55:23 pmEarlier I had spent some time looking at the original silicone cap to see if I could re-use it as you had suggested. I noticed on the side that was mounted against the heatsink, it had a slight impression of the "circle" from the TO-247 package. That area seems to be a little bit compressed but maybe okay to use. This side also has thermal paste on the outside and on the inside. I was also checking to see if using the opposite side of the silicone cap could be possible. This side has some indentations from the metal clip. The thickness in this area appears more compressed. These factors along with imagining how I would clean the silpad (perhaps distilled water as IPA might degrade the silicone...) So that had me wanting to replace the sil-pad with a new one. Perhaps the original can be used together with another layer of something inbetween...Is there any suggestions / ideas to double insulate this area with minimal modification (lets say we accept the higher thermal loss / lifespan of the mosfet -- they did use thermal paste with the silcap which probably wasn't very thermally efficient to begin with). Would putting Kapton tape on the heatsink + old silpad cap provide greater safety? What about if I try to fit the thinner silpad tubing and then place the silpad cap over that? Alternatively, I could use the original silpad cap (after cleaning) and I cut up the new sil pad tubing and place that between the cap and the heatsink?Quote from: Gyro on June 18, 2022, 09:55:23 pmThe IEC connector physically doesn't have an actual ground pin for the ground on the power cord to mate with. I have never seen such an IEC connector without a ground pin before. I believe Wharfedale is located in the United Kingdom though I think I saw somewhere that it was made in China.

author: Gyro
author_rank: Super Contributor
timestamp: June 19, 2022, 10:50:05 am
content: You're welcome.The principle of Class II (double insulation) is that you either have reinforced insulation, eg. doubling up the creepage and clearance distances on PCBs from 4mm to 8mm, or you use two physical insulation barriers (and occasionally both, eg. putting an insulating sheet between a PSU PCB and the chassis in case a standoff fails). A good example is on captive mains leads, where the inner cores are covered with heatshrink from the point where the outer jacket is stripped back. The principle is that the item will withstand physical breakdow, nick or whatever, of one layer of insulation but still be protected by the second layer.It does look as if Wharfedale have been flying close to the wind here - unless there is something in the spec of the insulating caps that makes them count as reinforced insulation, but I'm not aware of a clause that would allow this. Not having any way to access the spec of the original insulating caps is a pain (we can't tell if they are UL recognized for this). It does sound as if they have deformed a little though (as you would probably expect). The indentation on the back from the clamp is less of an issue (as long as it isn't punctured) because the epoxy package will count as one layer of insulation. You may be able to remove the heatsink compound residue with some water and dish soap if you don't want to risk IPA.Given the situation, my best advice would probably be to use two thicknesses of Silpad tube between the Mosfets and the heatsink (hopefully you have a couple of spares). Obviously the same diameter won't fit one inside the other, but you could slit the outer one on the non-heatsink side. The epoxy package on the clamp side counts as one layer of insulation, so you only need one thickness there. Hopefully this won't introduce too much thermal resistance (you mentioned that the material is thinner). As I mentioned, it isn't normal to use heatsink compound with silpads. You mentioned Kapton tape, this is another possibility in place of the second thickness. Kapton is sometimes used for insulating pads, so, as long as you use genuine 3M tape with specified breakdown voltage, then this ought to be workable too.You haven't included a photo of the heatsink and PCB assembled together, but I suspect that they may have taken some liberties with PCB trace to heatsink clearance too. Kapton tape on the heatsink might be advisable here too.It's annoying that they have omitted the ground pin from the IEC connector - that's a new one for me too! I don't know how feasible it would be to replace it (to make it Class I).Yes, Wharfedale is a British brand of many decades (90 years) standing. Unfortunately all of their stuff, including speaker cabinets and drivers, is made in China these days. I think I heard that the company is Chinese owned now too, although they do still have UK premises and probably design.P.S. Sorry to mention this, but have you established the underlying cause for the Mosfets failing in the first place.

author: rstor22
author_rank: Regular Contributor
timestamp: June 19, 2022, 02:17:55 pm
content: Quote from: Gyro on June 19, 2022, 10:50:05 amI will attempt to clean the cap with distilled water and perhaps some mild soap.Quote from: Gyro on June 19, 2022, 10:50:05 amI only have two new siltubes. I will see if I can fit the mosfet-siltube within the original silcap and if that doesn't work I will try modifying the silpads. I have 3M 5419 Low Static Polyimide Film Tape and the dielectric strength according to the datasheet is 7000V. The width of my tape is 6.35mm. I might try to triple insulate and use both the sil-tube and sil-cap along with kapton tape on the heatsink. Unfortunately I don't have a picture with the heatsink and PCB assembled. The heatsink is attached to a platform on which the PCB is mounted via standoffs (attached).Quote from: Gyro on June 19, 2022, 10:50:05 amI found someone with a similar issue as posted here:https://www.diyaudio.com/community/threads/tricky-smps-fault-in-wharfedale-spc-12.348101/(post #17). Their IR2153 was cracked (mine was blown) and one of their IRFP460A mosfets appeared dead. One of the suggestions on that forum for the cause was"Also replace all the electrolytics around that high/low driver, my bet is on some external capacitor going high impedance". When I checked the capacitors around the IR2153 I found:C14 (10uF 50V) to be 6.75uF and ESR was showing open.C18 (100uF 25V) to be 90uF and 18R ESR (out of spec).C19 (2.2uF 50V) to be 1.8uF and 31R ESR (appears to be in spec however will replace)C10 (100uF 35V) to be 0.7uF and ESR was showing open.I am thinking this had contributed to the components failing. I ordered replacements capacitors to replace these and also C6 which appears to be in spec but will replace anyways.

author: tooki
author_rank: Super Contributor
timestamp: June 19, 2022, 04:55:51 pm
content: Quote from: rstor22 on June 18, 2022, 10:51:41 pmYou mean an IEC C18? It’s part of the same standard that defines the standard C14 IEC connector.

author: tooki
author_rank: Super Contributor
timestamp: June 19, 2022, 04:57:25 pm
content: Quote from: Gyro on June 19, 2022, 10:50:05 amYeah, I immediately noticed the CapXon brand electrolytics in the first photo.

author: Gyro
author_rank: Super Contributor
timestamp: June 19, 2022, 06:06:52 pm
content: Yes, it's possible that excessive capacitor ESRs could degrade the Mosfet drive to the point that they fail (sorry, I missed some of your comments in your op). Of course it's impossible to say for sure without a schematic, but at least you have a documented precedent to go on.Looking at your ltest image of the devices mounted on the heatsink and the height of the PCB standoffs, the clearences look ok. Those silicone caps do enclose the Mosfets very nicely and also guarantee the isolation of the top of the mosfets from the (rather close) fold in the Aluminium.It's a shame you don't have some wider Kapton tape, yes, its breakdown rating is very good. An uninterupted width under each silicone cap would maintain good isolation on re-use, even if they have compressed down a bit.

author: rstor22
author_rank: Regular Contributor
timestamp: June 21, 2022, 07:15:52 pm
content: A diode, zener, and transistor also tested bad. I have removed my reply to this post and created a new thread requesting help identifying replacement components:https://www.eevblog.com/forum/beginners/wharfedale-spc-12-smps-repair-help-with-identifying-replacement-components/