Primary side FET heatsink in offline flyback connected to earth or primary GND? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: January 20, 2021, 10:58:21 am
content: In an offline Flyback SMPS, using a plastic case TO220 FET such as FSCQ1565, do you agree, its better to connect the FET heatsink to primary ground rather than earth? (Wouldn’t common mode EMC be worse if it was connected to Earth?)Also: Same question as above but for the secondary TO220 diode?FSCQ1565 datasheethttps://www.onsemi.com/pub/Collateral/FSCQ1565RT-D.pdf

author: Gyro
author_rank: Super Contributor
timestamp: January 20, 2021, 11:05:31 am
content: That's an unanswerable question from the information you provided. The FET part number and datasheet are irrelevant.

author: ocset
author_rank: Super Contributor
timestamp: January 20, 2021, 07:22:43 pm
content: Surely common mode noise would be worse if the flyback primary fet is connected to an earthed heatsink (even via a thin insulating pad).As you know, Common mode noise is noise that goes off into earth ground, so having an earthed heatsink so close to the switching node is surely bad?Better to put a quiet node that is not earth, eg primary gnd , and connect that to the heatsink?I gave the FSCQ1565 part number to demonstrate that its not a metal tabbed to220...but is plastic....but even with it being plastic, the switching node is in that fet, so its surely nice to keep it far from an earthed heatsink?I once had a 240VAC 500mW LNK304 buck on a pcb on an earthed heatsink and had to shield its switching node from the earthed heatsink with bottom layer quiet node copper (PRIM GND), and some common mode inductance in the mains input..........the original emc (before the said fix) was a massive fail from 150k to 6meg.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 20, 2021, 07:57:32 pm
content: Depends on primary side filtering network, secondary side filtering, whether it's grounded at all, etc.; in short, unanswerable given the above.Tim

author: ocset
author_rank: Super Contributor
timestamp: January 21, 2021, 06:36:12 am
content: Pri side filtering is usual diff mode and common mode filter, with Y caps to ground from AC inputs and Y cap across flyback transformer.

author: ocset
author_rank: Super Contributor
timestamp: January 21, 2021, 06:47:41 am
content: Thanks, in the following design note, Power integrations state that from an emi point of view, its best to connect the primary heatsink to primary ground rather than earth.......(they say so on their forum)https://ac-dc.powerint.cn/design-support/reference-designs/design-examples/der-547-275-w-pfc-front-end/Would you agree that if this is the case, then it must also apply for any offline PFC stage? In other words, earth should be kept as far away from switching nodes as possible...as coupling to earth is the dreaded causation of common mode noise?I cannot think of a situation in an offline SMPS where you would want to actively choose earth as the net to connect to your fet heatsinks? Can you?

author: trobbins
author_rank: Frequent Contributor
timestamp: January 21, 2021, 07:10:57 am
content: Think of it from the perspective of the transient current that will flow through the capacitance from FET drain to heatsink. Initially the aim is always to make that current loop have as small a loop area as possible (ie. directly back to the FET or V+ related filter cap gnd node - and not by some more circuitous path where it can couple in to control or emi reduction related traces/paths). Even better is to use a special silpad with internal screen.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2021, 03:37:15 pm
content: Quote from: trobbins on January 21, 2021, 07:10:57 amThat would be cool, but do they actually make those?I think you can get hard-anodize plates with a pin though.Tim

author: ocset
author_rank: Super Contributor
timestamp: January 21, 2021, 06:59:10 pm
content: QuoteThankyou, and i believe you have answered in confirmatory nature.....because the switching node coupling in to earth would give a very large loop of current, and worsen EMC...so from what you have said, i decree that there is no offline SMPS in the world, where you would actively seek to heatsink switching-node-containing FETs to an earthed heatsink....you would choose any quiet node in the vicinity of the fet...preferably primary ground....but never wouLD you choose earth...would you agree?There are of course, FETs which are heatsinked to the metal case of the power supply (if it happens to have a metal case), which has to be earthed for safety...and in that case, you just have to put up with the earthing of the "heatsink"...and use a better common mode filter to pass EMC...but i "get" the nature of the above answers...basically, i have asked an "Ohms Law" type question here and thus i don't even deserve an answer, its so obvious.The final page "Conclusion" of the following "bastion of EMC" document decrees that "thou shalt not connect your power switching transistors' heatsink to earth".http://www.elmac.co.uk/Heatsink_emissions.pdf

author: trobbins
author_rank: Frequent Contributor
timestamp: January 21, 2021, 11:03:04 pm
content: Tim it was the Bergquist Sil-Pad Shield - I think it is just an option nowadays. I did get a sample decades ago but can't locate the original datasheet - but a datasheet is on-line:http://disti-assets.s3.amazonaws.com/quistelectronics/files/datasheets/6363.pdfOf course a rough and ready diy sandwich could also be practical.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2021, 12:57:22 am
content: Hmm, looks pretty special-order at that.Yeah, a foil shield between two will do, assuming you can afford the resistance of two in series. Still kinda need something to clear the screw, or use whole sheets with a spring/clamp to secure the device.Incidentally, pyrolytic graphite is conductive (of course?); it's mainly useful for heat spreading (lateral conductivity several times better than copper(!)), but has shielding value as well. Of course, that's the hazard as well, you can't use it just anywhere, and the insulator or adhesive used with it, costs thermal resistance.Or in the complete opposite direction, there are even HIP alumina heatsinks available -- the conductivity isn't as great as AlN or Al, but for something convection cooled, that hardly matters, and it's only a few points off under forced air. In return, the capacitance is minimal, great for high frequency operation.Tim

author: floobydust
author_rank: Super Contributor
timestamp: January 22, 2021, 05:53:43 am
content: OP, you've cross posted the same question at multiple forums:power integrations technical forumselectro-tech-online discussionsedaboard Power ElectronicsYou asked related questionin 2017.Stop it already. We can't answer your vague question, you'll leave the thread necro in few days.Those forums have a no cross-posting policy for a reason.You're just wasting our energy.edit: the no cross-posting rules seem to be within a forum.

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2021, 04:27:39 pm
content: Quote from: trobbins on January 21, 2021, 07:10:57 amThat sort of idea may have been behind Tektronix including an extra winding on some of their early off-line switching power supply designs which was tied through a low value capacitor between the power ground at the negative side of the input bridge rectifier and chassis ground. I have read that they did this to create a counter current which cancelled EMI.I have seen screens used between the transistor case and heatsink before in exceptional applications.

author: temperance
author_rank: Super Contributor
timestamp: January 28, 2021, 01:59:52 am
content: You wrote long ago about a switching power supply course written by you. From that point of view this is a very strange question and one you should be able to solve on your own.Under how many names do you appear in here coffeefet?

author: jonpaul
author_rank: Super Contributor
timestamp: January 30, 2021, 04:23:50 pm
content: Hello:Designed SMPS to 12 kW and 55 kV in epoch 1970s...1990s.Never use earth ground for anything but chassis safety earth and EMI filter/Y caps.Use only circuit return for interception of CM noise via the HS capacitance.Kind Regards,Jon

author: ocset
author_rank: Super Contributor
timestamp: January 31, 2021, 06:44:50 pm
content: QuoteThanks, I would agree.Though as we know, some Metal cased SMPS's actually use the earthed metal case as a heatsink for the FETs and diodes. The Sil Pads kindly recomended by TRobbins would be useful here. I wonder if anyone's got much experience of offline SMPS in this case that don't use the special sil-pads kindly recomended by TRobbins? It must be pretty tough to get through conducted & radiated emissions with those? (ie fets/diodes screwed to earthed case without the special sil-pads.)