Dissecting a switch mode power supply / choke and inductor basics - Page 1
Forum section: electronics / beginners

author: GoJuice
author_rank: Contributor
timestamp: March 04, 2020, 06:01:15 pm
content: Hi all. I have a PSU that seemed dead (0v on output). I decided to find out what broke and fix it if possible as a learning exercise. I damaged it when opening it, but I can't see anything wrong so far.I started reverse engineering the circuit but I got stuck at the smaller yellow cube. Is it a common mode choke or what is it? I couldn't read anything with my multimeter which threw me. How should it read on the DMM? I don't recognise it compared to common mode chokes on PSUs when I google for examples. Any info appreciated.Between 2 of the pins of this choke if that's what it is, there is a spark gap (I think). Solder in the shape of triangles ^^^ pointing at each other across a gap. The 2 spark gap pins are each connected to the positive side of the electrolytics. One to the 10uF cap and one to the 33uF cap.Input > 2 ohm resistor in live line > bridge rectifier > 10uF 400v cap > yellow cube > 33uF 400v cap

author: GoJuice
author_rank: Contributor
timestamp: March 04, 2020, 06:20:18 pm
content: The output diode.. I know it's upside down and not connected.. looks silly I know. I was going to replace with hookup wire since I descroyed the PCB where it was soldered.I was measuring the diode like that. It's a dual schottky package. DMM diode test measured around 200 between pins 1 & 2 and pins 3 & 2. I assume the 200 means 200mV? I didn't realise schottky diodes could measure that low.

author: Gyro
author_rank: Super Contributor
timestamp: March 04, 2020, 06:36:48 pm
content: Yes, the smaller yellow cube is a common mode choke, you can just make out the dark line marking the divider in the bobbin under the yellow tape. You should see continuity on the pairs of terminals in line with the divider and not between the sides,Just a reminder that, although it is a small SMPS, it still has mains on much of the board and it will still bite you given the opportunity.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 04, 2020, 06:37:34 pm
content: Quote from: GoJuice on March 04, 2020, 06:01:15 pm

author: GoJuice
author_rank: Contributor
timestamp: March 04, 2020, 07:37:55 pm
content: Ahh, I see it now, Gyro. Thanks. I measure open circuit on my DMM though, between any of the pins. >2M ohms.Quote from: ThickPhilM on March 04, 2020, 06:37:34 pmLike Gyro said, it was the divider I missed. I didn't recognise the two seperated windings.More pics (I marked out live, neutral, and some of the plus and minus rails):

author: AVGresponding
author_rank: Super Contributor
timestamp: March 05, 2020, 05:54:35 am
content: Yep, looks like their budget version of a GDT...

author: GoJuice
author_rank: Contributor
timestamp: March 05, 2020, 05:43:17 pm
content: Who or what is GDT?Anything specifically that's wrong with this design or what is bad about it?

author: AVGresponding
author_rank: Super Contributor
timestamp: March 05, 2020, 05:58:53 pm
content: GDT is a Gas Discharge Tube, basically a voltage surge protection device.The one printed onto the pcb isn't enclosed, so any plasma generated during an overvoltage event isn't going to contained, andmayallow an arc to travel further than is desirable...It's interesting to note there's one with a slot also cut, where the Y1 cap is sitting (that's an interference suppression cap, the blue one) but the gap is huge, I dread to think the kind of fault voltage that would be needed to arc across that gap!

author: GoJuice
author_rank: Contributor
timestamp: March 05, 2020, 06:34:04 pm
content: Ah! I see what you mean now! That's crazy.I've seen the occasional gas dischange tube in multimeter teardowns. Do you ever see them in PSUs like this one? (or in laptop PSUs, phone charger,s etc?)The board has a space for one MOV but it is not populated. If one was installed, would that make the spark gaps (that is the correct term for both those gaps, yeah?) totally redundant? Or maybe one MOV is not enough on it's own?Or the spark gaps, while not the best solution, would work and protect the PSU from being damaged?

author: AVGresponding
author_rank: Super Contributor
timestamp: March 05, 2020, 06:49:54 pm
content: Probably used the spark gap as it's cheaper.The differences between GDTs and MOVs are around clamping speed, energy absorbed, and voltage range.A real belt and braces design would have both.AFAIK GDTs are faster, but clamp higher voltages. Device size pretty much is the determining factor for how much energy they can absorb before the pixies become so angry they spread all over the rest of the device.

author: Gyro
author_rank: Super Contributor
timestamp: March 05, 2020, 07:24:46 pm
content: Quote from: GoJuice on March 04, 2020, 07:37:55 pmThat's very strange. It would be pretty much unheard of for both windings of a common mode choke to go open circuit at the same time, even under fault conditions (made me wonder about your meter). Of course either winding being open would stop power getting to the switcher.The main protective element looks to be the component in heatshrink at the end of the board - according to the PCB markings, it should be a 6.3A fuse but that seemsawfullyhigh for an SMPS of that size! You might find a fusible resistor in there instead. I notice that the inrush current limiting NTC thermistor and MOV have been 'value engineered' out.As for the PCB spark gaps. These seem to almost a fashion statement these days (you wouldn't expect to find GDTs in a consumer SMPS, but maybe on a high reliability telecom one - where they would be to protective ground, not secondary). I can't see any situation where breakdown of the primary-secondary one would be an advantage, luckily I can't see it ever happening and at least the PCB is gapped so that it does no harm to isolation. Other than that, the PCB looks to be pretty safely laid out.Once you find out what is actually blown on the primary side, you will need to find out why. Blown switching transistor or something else. You would be better putting the output rectifier back in the original orientation and connecting to the PCB by very short jumpers.One thing I did notice is that the termination of the output lead seems to be a total mess of wire strands on the top side of the board. It would hard to believe that it isn't shorted (or shorts as soon as you move the wires).

author: GoJuice
author_rank: Contributor
timestamp: March 09, 2020, 05:25:13 pm
content: Quote from: Gyro on March 05, 2020, 07:24:46 pmMy multimeter is a £5 job from Rapid. Actually, the bettery has never been changed and I've had the meter over 10 years, if you think that could be making the difference?I'll try powering up the circuit and measuring voltages. See if I can diagnose something. So far, apart from the zero volts output, I couldn't see specifically what's wrong with the PSU.Quote from: Gyro on March 05, 2020, 07:24:46 pmAh! I never spotted that either until now. 6.3A spec'd is bizarreThere is a bare wire jumper where the fuse marking is. Yeah, the component in the heatshrink is a 2 ohm resistor in line with the live input.

author: GoJuice
author_rank: Contributor
timestamp: March 09, 2020, 05:53:09 pm
content: Quote from: Gyro on March 05, 2020, 07:24:46 pmOk, I will turn it round and trim the leads.I'll take a close look at the potentionally shorted output.

author: GoJuice
author_rank: Contributor
timestamp: March 17, 2020, 03:15:47 pm
content: I got a new meter (Fluke 77). The choke is still measuring open circuit.I repaired the PCB. The short on the output wires I'm sure I damaged it like that when opening the case up.The electrolytic capacitor before the choke is charged to ~330v DC. The electrolytic cap after the choke is charged to 0v. Bad quality / faulty choke ? Overloaded / undersized choke for the current draw?

author: GoJuice
author_rank: Contributor
timestamp: March 17, 2020, 03:27:08 pm
content: Does it make sense that both coils in the choke could become faulty and go open circuit at the same time when the PSU went through the mail system?

author: AVGresponding
author_rank: Super Contributor
timestamp: March 17, 2020, 03:29:55 pm
content: Quote from: GoJuice on March 17, 2020, 03:15:47 pmYour best bet for determining that is to remove it from the board and carefully disassemble it.Look for evidence of overheating, like bubbles in the varnish, scorch marks etc if it's been overloaded.It might be as simple as a broken wire going from the pcb solder leg to the coil.

author: GoJuice
author_rank: Contributor
timestamp: March 17, 2020, 09:48:25 pm
content: Thx for quick reply.I had a pretty big shock, forgetting the cap was fully charged, whoops.I got the choke off. On one side it became clear the wires were not connected... but it seems like they were never attached to the pins. I couldn't find the ends under the tapeI don't think my eyesight is that good. Maybe I missed something again.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 17, 2020, 10:28:48 pm
content: The green circled area looks to have a broken conductor to me:It's not unusual for these to break, as I said, either from the unit being dropped, or from desoldering attempts... (been there, done that...)I can't see any obvious signs of overheating.

author: GoJuice
author_rank: Contributor
timestamp: April 02, 2020, 09:09:40 pm
content: You were right..!I spent a long time scraping all the tape off, top and bottom, as I couldn't find the other ends anywhere. I had given up but then I saw the end of the wire where you indicated in new, zoomed in photo.I broke a pin off by accident. I had to unwind the left winding a little bit to find the other end. When I did a small piece fell out.My thinking now is I may have caused the choke to become open circuit after all. Using a screwdriver when trying to pry the case apart.

author: GoJuice
author_rank: Contributor
timestamp: April 05, 2020, 05:04:21 pm
content: I've just found out about the flyback effect with inductors: if you put a moderately high current through an inductor and then disconnect the current while measuring the voltage across the inductor, you can get a voltage spike of hundreds of volts.I was wondering... does the spark gap under the choke have anything to do with protecting against this flyback voltage spike?I don't really understand why sometimes you need a diode like across a relay's coil to protect against this high voltage spike and other times like a SMPS filter (such as this PSU's input common mode choke), a diode is never called for.BTW, I broke the choke in the photos by accident trying to remove the tape at first, thinking it was only plastic and removing more to get to the tape... but I soon realised afterwards that it is ferrite. Is that correct? How significantly has it's performance/inductance been affected? It has had material snapped off the opposite side too in the last photo.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2020, 02:20:59 am
content: It's useless, but don't worry, it was a common mode choke (CMC) -- better called a transformer -- not an inductor.That is, a transformer is intended to transfer power, while an inductor is intended to store energy.More specifically, a CMC is used not so much to transform power -- well, that's kind of the point, it's used to transform power to itself, so that the mains current cancels out. Seems kind of useless, right? Well, in the process, it introduces a ton of impedance, at high frequencies (kohms, typically) -- which means if we have a noisy environment inside the SMPS, we can isolate that noise from the mains wiring. In other words, along with the capacitors, it makes a lowpass filter.The other component wrapped in tape, is a flyback transformer, also known as a coupled inductor. You will probably find that its primary winding is a few mH, and to its secondary, it has a turns ratio of, eh, 10:1 or so, give or take what voltage the SMPS was rated for. (Note that inductance ratio is turns ratio squared, so if it's 10:1 and the primary is 1mH, the secondary will be 1mH / 10^2 = 0.01mH = 10uH.)Tim

author: GoJuice
author_rank: Contributor
timestamp: April 13, 2020, 01:16:50 pm
content: Thanks by the way anyone answering my simple questions.Hmm. I’ve not thought about it as a transformer before. That makes sense I think. I can use that to differentiate a choke from an actual inductor.I’ve heard an inductor is used to store energy before, but I’ve never intuitively understood that or really accepted it. I’ve always thought of it as resisting changes, like inertia resisting changes in speed, an inductor resists change in current flow. Opposite to a capacitor which I think of as accelerating the current draw.I know that a moving magnetic field produces electricity in wires and I watched video on inductor basics. I suppose it's when the magnetic field from an inductor collapses, electrical energy is produced. Is this a correct way of explaining the stored 'energy' of an inductor being released?Since you have to put energy in to charge an inductor, I guess you get that out when it's discharged (conservation of energy and all that).Ahh, I wanted to ask about a low pass filter with the capacitor. I thought the electrolytic caps were just for making DC after the bridge rectifier at first.It seems fairly typical for the input choke to be placed before the bridge rectifier (you could say it's the traditional place for it?). Why not always sandwich the input choke between two electrolytic caps after the bridge rectifier? That will then mean you'd need a smaller choke, right? I assume it will save money and perhaps be easier to implement, or give you better noise suppression. What am I missing? Probably something obvious once I hear the answer.

author: GoJuice
author_rank: Contributor
timestamp: April 13, 2020, 01:21:35 pm
content: Quote from: T3sl4co1l on April 06, 2020, 02:20:59 amBTW, I'll try and remember that. I didn't know that. Cheers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2020, 11:43:34 pm
content: Quote from: GoJuice on April 13, 2020, 01:16:50 pmSame thing, just a change of variables. Consider the equations of both:Capacitor: I = C dV/dtInductor: V = L dI/dtThe d's may not mean much to you if you haven't taken calculus, but the swap of V and I is plain to anyone.(What happens if you put two, er, swaps, together? They oscillate of course. That's what an LC tank is doing, the energy just "sloshes" back and forth between one and other container, between voltage and current.)QuoteI never liked either phrasing. "Magnetic lines of force" works fine for relative motion, but it leads to terrifically bad intuition with relative rotation -- consider this for example:https://mysite.du.edu/~jcalvert/tech/faraday.htmAnd it completely breaks down in a transformer. When the transformer is deenergized, where do the field lines go? There are fewer of them, or they are... less intense, or something, but then how do we go from an enclosed area containing some, to none?The reasonable explanation would have to be that there's an infinite supply of "lines of force" sitting out at infinity. When we apply flux to the transformer, some of those lines are pulled inside, into the core. In the process, they "cut" both the primary and secondary, explaining the induced voltages in both.But the idea of objects at infinity isn't a very beginner subject; the study of infinity, poles and zeroes, real and complex analysis, is college level, to varying depths depending on major.As for "collapse" -- consider a mechanical analogy. You hold a rock in your hand. You drop the rock. It falls through the air, and clunks on the ground.Did its gravitational potential "collapse"?What if you continue holding it in your hand, and just lower your hand? Did its energy "collapse"?This phrasing leads to an incorrect intuition about the voltage and current to expect from an inductor. For example, it's a common sight to see huge power diodes on signal relays -- even though the relay coil is switched by a piddly 2N3904 or the like. If we think about "resisting change of current", well, if that relay coil is only carrying say 100mA while it's on, then itcan only ever possibly carry100mA through that catch diode. And the voltage is perfectly constrained by the diode's voltage drop. Nothing is poorly defined here. No need to use a 1N4001, or 1.5KE24A, or something; a 1N4148 will do.Whereas if you didn't have that diode, the turn-off voltage would merrily rise up to, well, whatever the 2N3904 can handle (about 60-100V), which may toast it in short order (again, the peak current isn't any higher, but the peak power is -- 100V * 100mA = 10W, quite a bit more than a 2N3904 can handle, at least for very long).In the ideal case, that is with an ideal switch that instantly goes to infinite resistance and has no breakdown voltage -- the flyback voltage can be very high indeed. (Infinite, for an ideal inductance. But also for zero duration... so, ehh?) But such a case can't happen in reality. The closest we can get is perhaps, say, undesired arcing of a real switch, in which case we might ask if we can constrain the voltage to prevent that arcing, or the momentary shock hazard, or for various other reasons.In an SMPS, the voltage is always perfectly constrained, or left to decay in which case it pretty quickly drops to zero. When the driving/input switch is on, Vin (or Vin-Vout, or some such permutation) is applied to the inductor; or the switch is off and the load switch (usually a diode) is on, and Vout (or such) is applied to the inductor; or neither is on, and the inductor's voltage freewheels down to zero, at currents much lower than when driven, and the remaining energy being dissipated into circuit resistances (losses).Well, in the same way that we can pick up and put down rocks, we can "pick up" and "put down" the current in an inductor, and at all times the voltage (force) is well defined and controlled.Or likewise, the voltage in a capacitor. A capacitor only "collapses" when you drop a screwdriver across it. That was clearly your fault for putting too low a resistance across it; likewise the same for the inductor, iftoo higha resistance is put across it.QuoteYup. Minus some that leaks away over time, because real components aren't ideal at energy storage. That's, fairly directly, the flyback supply: an inductor is charged with one polarity of voltage (up to some peak current), then discharged with the opposite polarity of voltage (at the same initial peak current, and by "same" I mean same direction as well). It doesn't tell you anything about the in-circuit dynamics (what the voltages and currents are doing over time), but that's the thermodynamic level view, yup.QuoteDepends. Putting it at the AC line does give you one bonus: the rectifier itself produces some noise (basic harmonics if nothing else, but more importantly, reverse recovery can generate quite high harmonics), and it will be left unfiltered otherwise.On closer inspection, I see the present example actually puts the CMC after the FWB, not before. Might be it didn't make enough difference for their design (...or they just didn't bother meeting actual regulatory standards...) and the layout was just better this way. Or something.Ah, and yeah, it's between the electrolytics at that.There are almost always relatively large caps in parallel, on either side of the CMC: on the AC line, these are X1/X2 rated film caps, maybe 47nF to 1uF, sometimes more. Or in this case, just some uF of electrolytic on the DC side. This works with the leakage inductance of the CMC, forming a (differential mode) CLC filter. The motivation is reducing lower frequency ripple (near the switching frequency).You might also see a single (two pin) choke between electrolytics caps, which is a cheap way to reduce low frequency ripple, without also using a CMC, or in combination with one to reduce it even further.Devices with very little common-mode coupling (small size, no output wires, etc.?), may not need a CMC at all. CFL and LED bulbs are a good example of this. They may use a single choke and two caps, nothing more.Tim