Question about grounding techniques for ATX SMPS - Page 1
Forum section: electronics / projects

author: Giordano Lanzola
author_rank: Newbie
timestamp: December 26, 2021, 12:40:03 pm
content: I recently fixed some ATX SMPSs (bad caps, burned mosfets or PWM ICs) and before working I always took some time to identify the "hot" section of the circuit just to stay safe :-)I saw that the hot section is always properly set apart from the low voltage section through transformers and optocouplers.Moreover the separation is also nicely emphasized through physical grooves digged on the PCB for faster recognition.In so doing I noticed a recurring design technique about grounding the hot section of the circuit with two capacitors that I highlighted with a red oval in the attached image (original fromhttp://www.pavouk.org/hw/en_atxps.html).Additional schematics are available here (http://danyk.cz/s_atx_en.html) all showing the same grounding technique.Based on the following arguments I cannot really figure out the function of those caps:-) Given the complete decoupling of the SMPS hot section from the rest, would not it be better to leave the whole hot section floating ?Just connecting the negative of the low voltage section to the chassis and grounding it would be adequate. Moreover it would still protect from accidental leakages in the hot section.In case the two caps are part of the EMI filter they could be replaced with a single one (value halved) linking the paths from the AC line and avoiding connecting the ground.-) Companies supply AC in terms of a "live" (240 V rms) + "neutral" (0 V).When the socket has no ground the node where the two caps are joined, not being tied to zero, exposes 120 V rms.I just checked the PC in my bedroom where sockets are NOT grounded and to my great surprise I discovered that the voltmeter read 110 V between the PC frame and the radiator of the heating system connected to metal pipes going to the basement. Given the value of the capacitors in the schematics (4n7 F) the current flowing to ground (50 Hz) would be close to 0.4 mA (on my PC I read 0.8 mA). This is unable to harm but may be nevertheless noticeable if someone touches the chassis with a sensitive skin area, especially if wet.-) When the ground wire is available, the whole chassis is tied to 0 V and so is the node where the two caps join. However just the cap sitting between the "AC live" circuit path and "ground" will be working, while the one between "AC neutral" path and "ground" will not. This means that 0.4 mA will flow from the "live" to "ground" unbalancing the mains circuit. The current delta tripping RCCB/RCBOs is 30 mA max (mine trips at 22 mA) which is much higher. However ground leaks produced by multiple SMPS definitely sum up. Since almost every appliance nowadays has a SMPS, I guess that quite a few of them will end up tripping RCCB/RCBOs.In summary I see only drawbacks for this design technique that essentially introduces some sort of leakage "by design".However since it is so in widespread use I'm sure that I'm missing something very important.So the question is what is the purpose of those caps and grounding the whole SMPS hot section ?I will appreciate any suggestion and explanation and thank everybody in advance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 26, 2021, 04:21:31 pm
content: Here, let me strip away a bit of superfluous information, and also show a physical reality that doesn't come through on the schematic normally:If the output isn't grounded (they used an earth symbol throughout this schematic, but I don't think it was intended to mean chassis ground), then the capacitance between windings drives switching noise between mains and output. A double whammy. Well, the load is usually enclosed in a metal box too, so maybe it doesn't go far that way, but the part sent up the mains wiring is important. And if it is grounded, then that output-side noise is shorted out, putting the full magnitude onto the mains side.And what is a capacitor? Two metal plates with some space between 'em, right? A transformer has, well they're not plates, but wires, but they still cover a lot of facing area -- they have capacitance alright! So the source of interference as inter-winding capacitance is a major one. Also capacitance from switching node to heatsink to chassis, via transistor tab, is a notable one -- maybe not so much in these units, where the heatsink is usually a board component, but it's also hit and miss whether the heatsink is grounded (say to primary DC +/-, or the center tap between them), which is a prime way to deal with that capacitance, shunting its current to ground through a local loop rather than allowing it to couple onto the chassis. (The capacitance from heatsink to chassis is fairly small, 10s of pF I would guess, but still, that's more than nothing.)So, the transformer has some ~100s pF between windings, and assuming the output is grounded, a line-to-ground capacitor in exactly the location shown, will help to shunt the noise from inter-winding capacitance, back towards its source. A common mode choke then increases the impedance, and another cap (usually) serves to filter it some more. Sometimes the latter isn't required; the CMC working against mains impedance might be enough by itself. It depends.We can reduce the circuit even further, to an equivalent something like this:Note that the mains wiring is modeled as two 50 ohm resistors to ground. This is a convenient fiction, compared to the real world where it could be anything (transmission line and antenna effects galore!), but it's also not far from this on average. (That is, the impedance may have peaks well above or below this magnitude, but it still centers around about this figure.) C1, L1 and C2 form the usual CLC mains filter; note that L1 has leakage inductance which acts to provide differential-mode filtering with C1 and C2. The leakage is much smaller than the total value, so much larger capacitors are needed (typically 0.047 to 1uF). R2 represents the AC load of the converter circuit itself. V1 and C3 represent the inverter (RF) source, and any offending coupling like inter-winding or heatsink capacitance. C7 is another, more direct option for 'Y' capacitance, shunting across the source directly (primary to secondary). Note that C3/C7 represent the isolation barrier. Finally, the secondary may have its own 'Y' cap to earth if it's intended to float, otherwise when hard-grounded we can take it as ground as well.All three positions of 'Y' capacitor are shown: C4, the mains-inlet cap, not used on the above ATX supply; C5 is equivalent to the ATX's C2+C3 (note that C2 in the above diagram makes the lines act in parallel for RF purposes, so using a pair of 'Y' caps is somewhat redundant; either way can be used, though without an 'X' (line-to-line) cap at the FWB, they didn't have that option). And finally, since the output is ultimately earthed, we recognize the secondary-grounded condition, so C6 is N/A, and C7 is equivalent to C5.Also, note that R2 effectively produces RF as well, hence the need for differential mode (line-to-line) filtering. This is smaller anyway -- it's shunted by big huge (electrolytic) caps for one -- so we don't usually mind that L1 is somewhat improvised as far as its DM filtering value. (Or perhaps not. Evidently they needed quite a bit more in the ATX supply, hence adding 'T' in series with the leakage (on the other side of the FWB; for RF purposes, the diodes are always conducting so we can ignore them), and an extra stage of DM-only filtering between C1-T1-C4. Perhaps their electrolytics had unusually high ESR. Which, compared to modern formulations, eh, yeah they probably did.)Tim

author: TheMG
author_rank: Frequent Contributor
timestamp: December 26, 2021, 04:49:50 pm
content: As mentioned, they are indeed for EMC compliance.Note that on ATX power supplies the output common is directly tied to chassis ground (not floating).I've never run into issues with the leakage from multiple power supplies tripping GFI/RCD but those are much less common in Canada (typically only used for outdoor receptacles and those in "wet" locations such as kitchens and bathrooms). In any case, you would need to have quite a large number of them and there are not typically that many ATX PCs in a single home. Are the RCDs in your country also used in office buildings?Smaller SMPS like phone chargers are often floating, and also have smaller values of RFI capacitors so don't really contribute any ground leakage.

author: Giordano Lanzola
author_rank: Newbie
timestamp: December 26, 2021, 08:27:40 pm
content: In Italy you are required to have GFI/RCD also in office buildings. Actually due to safety regulations they are also strictly checked at every workplace (offices, universities, but also restaurants, pubs and the like) more that at home (which is considered as a private property). In my office, for example, we have open spaces with even more than 10 PCs controlled by a single circuit breaker.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 26, 2021, 10:01:54 pm
content: Simply, ground current is within limits for the equipment. If there are too many such loads on the circuit, that's the installation or user's problem.The limit is lower for medical devices, which may necessitate other design choices: shielding around the capacitances shown, smaller filter caps, larger or more filter chokes.Tim

author: CaptDon
author_rank: Super Contributor
timestamp: December 27, 2021, 03:33:57 am
content: It is common in 240vac land to see 110vac from ungrounded chassis to earth ground!! Of course in 120vac land 60vac from ungrounded chassis to earth ground is common. In some cases the leakage current is significant to cause sparking and in 240 land you WILL feel the 120 leakage path!! In 120 volt U.S. with 60vac leakage you don't really feel it as much but you will know it's there!!! I have seen ungrounded units with the 'Y' caps suffer damage because even though you are working on some gadget that has an internal step down transformer to power let's say 12vdc circuits, if the low side of that +12vdc power supply also returns to the chassis then those internal devices on the circuit board also appear to be floating at 60vac above earth and thus any earth grounded test equipment you use for trouble shooting can take advantage of that 60vac (or 120vac in most of Europe) between its ground lead and your circuit under test to blow up stuff like CMOS and FET's. Seen it happen more than once!!! Ground those floating chassis!!! I once had a Heathkit signal generator weld itself fast to a B&K tube tester. They both had 'big blade / little blade' polarized plugs and each had one side of line directly tied to chassis!!!! Obviously one was wired wrong and could have easily killed someone under the right conditions!!!! Here's the kicker, the Heathkit unit was factory assembled and not bought as a kit and it was the bad actor!!!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2021, 06:54:14 am
content: Funny thing about the old like Heathkit, Eico, etc. stuff... they'd put the 'Y' caps in there but use bog standard waxed-paper types, of massive values. I mean, who knows if the Eico 377 I have was actually stock or not, but it had like 0.22uF's inside it I think... yikes!Neat thing about chassis voltage, you can feel it on your skin; there's a sort of piezo/kinetic effect that makes the surface feel like it's just... really rubbery, you get kind of a stick-slip motion on it, but even for very light touches. This takes over 50V or so, I think, and fractional mA. So, you might feel it on your smartphone (on the charger, with the line-to-output Y cap), lots of kinds of stuff that's ungrounded, and, I mean you shouldn't be handling live wires for obvious reasons, but you'll feel it there too if it's not zapping you through a ground return path of course.Tim

author: Giordano Lanzola
author_rank: Newbie
timestamp: December 27, 2021, 11:04:39 am
content: Thanks a lot for your interesting explanations and tips pointing me to the problem.Everything is clearer and I'm already searching documentation to better grasp this subject.Thus far I strongly believed that GND was only meant to be a safety feature to prevent hazards just in case of malfunctioning.Essentially I considered it as a kind of life jacket not meant to take part to the device regular operations.I see instead that GND now plays an active role in circuit operations, at least for reducing CM interference through the use of "Y" caps.Thus if you disconnect GND, not only you spoil that functionality but immediately cause a "controlled" hazard due to such a spoiling. This is really a huge paradigm shift that was beyond my imagination when I first posted my question.I'm still not fully convinced of its need considering that some laptops power cords lack the GND wire altogether (e.g. C7/C8)https://en.wikipedia.org/wiki/IEC_60320. Why should a desktop be causing more CM interference than a laptop ?Moreover I understand that SMPS may send some DM interference up to the mains that needs to be stopped by "X" caps.But why should a desktop be causing CM interference at all if it is almost always used as a terminal device ?Maybe this is another example where regulatory issues take over technical ones.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2021, 04:55:35 pm
content: The key fact is where grounds are connected, around the power supply, not so much that ground itself is connected on the power cord. If you take the simplified diagram and remove ground, notice it reduces to 'Y' caps from primary to secondary -- which you will indeed find inside such (2-prong) power supplies.The important part about emissions is, anywhere you have a box (like a power supply on cords), you can only have differential mode emission (between pairs of wires in a given cable), or common mode emission (between pairs of cables). That is, maybe the RF pushes H up and N down, and nothing much happens on the DC output (or, it has its own ripple that behaves the same way), that's differential. Common mode, maybe the RF pushes mains up and output down, and each pair acts together so it's not producing much differential in this case, but there is an end-to-end difference across the box.Tim