[SOLVED] Discreet flyback SMPS subcircuit weird fault - Page 1
Forum section: electronics / repair

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 12, 2021, 08:22:47 am
content: In short: it's a standby power subcircuit of the some old ATX PSU (all solder joints have been reworked); it tries to open the power transistor at the secondary (reversal) stage. This disrupts PWM generation and draws too much current from mains. Schematic and waveforms are attached below.Cause and solution:hereOn the first attempt to start it blew the 5A 250V fuse. Then being powered via 40W incandescent lamp (cold resistance is about 100 ohms) and running at idle, the lamp pulsed from time to time. Hot side waveforms were captured when SMPS was connected to mains via low power isolation transformerAll other circuits were disconnected, so the fault cannot be caused by them. In diagnostic purposes secondary side diodes 1D4, D105 and base pull-down transistor 1Q1 were replaced with no effect. I also experimented with snubbers, but no luck as well. It smells like a design flaw, but it worked somehow many years ago.I know how it’s supposed to work, but I’ve run out of ideas how it does this weird fault and how to fix it. I need a fresh perspective. So, do you have any ideas on it without reworking it to PWM controller?

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 12, 2021, 10:51:24 am
content: It looks like the PS is starting but failed a little bit later. One of the possible causes is problem with the secondary to primary voltage feedback circuit.Please confirm the correct functioning of this feedback circuit. You can do this easily, without the AC mains connection and high voltage. Refer to the following image, you would need a variable regulated power supply capable of providing 4V to 6V adjustable output voltage and a DVM. Connect the external regulated power supply as shown.Set the regulated power supply output to 4V, connect a DVM across the optocoupler secondary to measure resistance. Slowly increase the regulated power supply from 4V to 6V and note for any change in the measured resistance. If the KA431 and its related circuit is functioning correctly, then the resistance would start to fall once the voltage is over 5V. If this test fails, then you can check for bad component until it is working correctly.Please report back your findings.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 12, 2021, 11:39:50 am
content: Quote from: eblc1388 on October 12, 2021, 10:51:24 amHi and thanks for your reply!I tried what you suggested and it looks like the feedback circuit is fine. The phototransistor in the optocoupler (IC4) opens normally when the voltage applied to the output reaches 5.3V.In general, initially I ruled out the feedback loop and didn't tested it, because it drives the pull-down transistor 1Q1, which shuts off the power transistor 1Q2. While the problem is that 1Q2 opens abnormally when it should be closed (at the reverse turn of the field in the transformer (works as an insulated inductor)). I'm starting to suspect a collector-emitter breakdown of the power transistor. 🤔 It's Fairchild KSC5027, by the way, BVceo=800V.

author: amyk
author_rank: Super Contributor
timestamp: October 12, 2021, 01:35:54 pm
content: This design is known as a "ringing choke converter" and they are usually quite reliable, but electrolytic capacitors in them can dry up and cause problems so I would recommend checking and replacing 1C6.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 13, 2021, 04:28:51 am
content: Quote from: amyk on October 12, 2021, 01:35:54 pmThanks for the name of the topology, it could be useful. Unfortunately, the problem is not in the capacitor 1C6: I've already checked it, and just double-checked it: capacity is nominal, ESR is around 4 Ohm @ 10 kHz.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 13, 2021, 04:34:59 am
content: Quote from: Alex Wolf on October 12, 2021, 11:39:50 amOkay, this is really a breakdown, but not the collector-emitter breakdown of the power transistor. 😃 I'll post details later.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 14, 2021, 12:27:13 am
content: Leaky opto?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2021, 03:37:29 am
content: More commonly: blocking oscillator.They're alright, but have weird behavior, so can be hard to control. Like it tends to generate bursts of cycles, rather than varying smoothly from cycle to cycle; or it has operating conditions (particularly at high supply or output voltage) that lead to continuous (often runaway) operation.But that's a matter for design, and presumably it was designed to work correctly.As for those waveforms, it looks fine more or less, until that crunch and stop at the end. I don't know what's doing that, but it's a good bet fixing it will fix the unit. And anything related, either that caused it, or was caused by it (cascading failures are common in electronics!).Tim

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 14, 2021, 08:52:53 am
content: Quote from: shakalnokturn on October 14, 2021, 12:27:13 amNope. 😃 It's much worse: literally a breakdown.First of all, I should correct myself. There is some confusion about terminology: under "the transistor is open" I meant "its n-p-n junction is open and passes current" (like a road junction), that is, it is completely turned on. There is also an analogy between a transistor and a switch, where "open" means "open contacts" or "open circuit", which could cause this confusion. In general, I meant that the power transistor turns on and passes current abnormally when it should be turned off at the secondary stage of the reverse polarity in the windings (relative to the polarity of the primary winding).Both according to the schematic and on the real circuit board there are only two paths, which the base current of the power transistor 1Q2 can flow, which would turn on the transistor: the startup resistors 1R1, 1R2; and the auxiliary winding via 1C2, D102, 1R14. All other current paths in the circuit can only turn off the power transistor.I desoldered, doublechecked and replaced all the related components, then desoldered and checked literally every component. All of them were perfectly fine. But the problem wasn't even on the circuit board or in the schematic, damn Murphy. When I decided to run further tests without a hot side heatsink (to get better access to components) and disassembled the transistor from it, I found THIS:However, the hint was on the waveforms from the very beginning: the primary winding's faulty current passes around the current shunt resistor 1R7. It looked as if the transformer/inductor T1 was reversing its polarity on its own (or due to diodes leakage on the secondary side, but I checked and ruled that out right away). How this happened:And this totally makes sense now, everything fell into place. When the PWM generation starts, it creates high frequency high voltage on the collector of the power transistor: the peaks are from around 0V up to around 600V, which sums from the main power rail voltage (around 300V) and the primary winding reverse polarity voltage (up to 300V, didn’t measured, sorry). Since the BJT doesn't have reverse diode in its structure to recuperate reverse energy flow back to the main capacitor, all its pressure lies on the insulation. In this case it breaks down, effectively making a spark gap to the high frequency HV. The first pole of the spark gap is the tab of the power transistor's TO-220 package, which is connected to the collector of the transistor. The second pole is the heatsink, which is connected to the power ground (zero-volt rail). And that's why the fault appeared only after the start of PWM generation: +325VDC is not enough for this breakdown.I had replaced the insulating washer to a spare one from another heatsink, but it broke down again after few seconds of working.Note to yourself: don't trust old insulators if it’s about HV circuit.Since I didn't have spare new and long enough insulators as well as high-voltage BJTs in TO-220FP package at hand, to fix this fault I decided to go the radical way: I wrapped the screw in 3 layers of the UL listed 600V tape and one layer of the 600V heatshrink tube. I also drilled the mounting hole in the transistor and heatsink to 5mm to fit the new diameter of the custom insulator (my compliments to Fairchild: the transistor survived drilling).And this worked: the SMPS feels fine. It just doesn't like low load conditions, but that leads only to a "singing coil", no malfunctions or critical conditions.Case closed. 👌

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 14, 2021, 09:29:53 am
content: Quote from: T3sl4co1l on October 14, 2021, 03:37:29 amThanks! This is exactly the name that was in my mind. However, this is a slightly modified version of this circuit: the auxiliary winding is in phase with the primary winding. It powers and amplifies the on and off switches of the transistor, reducing its linear operation time and switching losses. While in the original blocking oscillator windings are in antiphase: the base current of the transistor flows through the auxiliary winding from the power source.QuoteYeah, this one is absolutely alright, just the plastic insulator broke down from heat and age degradation. By the way it's suprisingly quite high frequency, low switching noise and no audible noise at all. More than I could possibly want to power the TL494, STM32, and ADS1115 to control the main half bridge SMPS. Yep, it's a mod to create an intelligent powerful charger from ATX PSU. 😃 I choosed TL494 based PSU because it works from the cold side and has 2 error amplifiers with reference input outside (very easy to tie with MCU).

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 14, 2021, 10:58:07 am
content: Nice write up, thanks.Lesson learned. If a 5A input fuse blew but no component suffer any damages, it could be pointing to an electrical breakdown somewhere.

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 14, 2021, 11:27:36 am
content: Quote from: eblc1388 on October 14, 2021, 10:58:07 amPrecisely! 👍There can be one particular exception: since in this case the power transistor is able to withstand a pulse current up to 10A, the NTC thermistor limits the starting current, and the startup occurs on the only partially charged mains capacitor, IN THEORY, the transistor can survive, while the ultrafast fuse will blow. According to my experience, in this regard power BJTs are more rugged than MOSFETs (especially counterfeit).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2021, 05:13:02 pm
content: Quote from: Alex Wolf on October 14, 2021, 09:29:53 amWell, power source doesn't matter, it's a dynamical system exhibiting that behavior; how it gets there, isn't so important. Power is required sooner or later, which 1R1+1R2 provide. They're doing it in parallel rather than series, which is more suitable at the high voltage.I think you will find the windings must be phased for positive feedback in all cases -- otherwise it would be an amplifier rather than an oscillator.You may also want to check 1C3, 1R6 and D101, which are intended to clamp peak voltage at turn-off.Hm, schematic is missing a capacitor between 1D4 and L10.Quote from: Alex Wolf on October 14, 2021, 11:27:36 amWell, the transistor isn't sinking any of that current, it's in a side path. It has the effect of igniting the spark, which is then able to maintain itself by direct mains supply; the fault current flows through the transformer, largely shorting out whatever AC voltage might appear on it -- hence the waveform goes flat and the transistor ceases switching. Indeed, even if the transistor could turn on, it would be conducting in the same direction, "helping" the spark.BJTs are generally less rugged than MOSFETs, but this isn't really something that can be concluded from this scenario.Transistors fail under fault conditions, after on the order of 100us; fuses (even fast ones) clear under fault conditions in more like 10ms, a hundred times slower. Even semiconductor fuses (so called because they are fast enough to protect the most robust of semiconductors: diodes, SCRs) take about 1ms to clear.Tim

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 15, 2021, 02:26:54 am
content: Quote from: T3sl4co1l on October 14, 2021, 05:13:02 pmYou're right. I just meant that in the most famous example of the blocking oscillator topology – the Joule thief circuit, the switching is made from the secondary side. This circuit is more of the boost converter topology than the flyback. But yes, they share a common principle, and in all cases the auxiliary winding of the transistor base is antiphased to the secondary winding, which powers a load. And you're right again: to make an oscillator, the feedback loop must be positive relative to the switching to make it unstable and oscillate.QuoteNah, they are fine, I've already checked them. The snubber is just not designed to dump so much energy. It can dissipate into heat only a small ringing energy of the stray inductance, which is much higher frequency than the switching. If it was a direct short of the primary winding in reverse polarity (like a single diode snubber), this would practically reduce the efficiency of the converter nearly to zero, because at the reverse polarity energy must be transferred to the secondary winding. There is no workaround to reduce the reverse high voltage of the primary, other than the heavy loading the secondary. This is a typical drawback of flyback converters.QuoteYeah, that's a bit how ya goin. 😁 There are some mistakes as well as on the real circuit board, but they are not critical, like a resistor leading to nowhere because of an omitted capacitor or a jumper link under an IC. What I like the most about this schematic is the shorted optocoupler's LED.QuoteYes, I've already described the fault current path above. 🙂 The ignition coil and spark plug were accidentally created by the insulator fault.But the quote you are responding to has nothing to do with this. This was just an abstract example of how a transistor can survive in such conditions.QuoteIn specs: yes, MOSFETs usually can pass very large amount of current due to their junction low resistance, faster switching and low heat generation as a result. In practice (according to my experience): no, because their internal structure has a very unreliable gate insulator, which very often fails short (source-gate-drain) for a million reasons, especially in nearly-critical operating modes. A comparable in current limit and heat dissipation BJT lives much longer, again according to my experience.QuoteDid I ever claim something like that? It is rude to ascribe to someone words that he/she never wrote or said, and then argue with these non-existent statements... 😕 But okay, let's raise the stakes: I bet the 3N80 MOSFET won't survive this fault scenario with the spark gap right at its tab. Are you in? 😃QuoteWhere did that "100 us" come from? It should be an exponential curve, not a constant.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2021, 08:11:30 am
content: Quote from: Alex Wolf on October 15, 2021, 02:26:54 amWell, if you let it experience high gate voltages, that'll happen, yeah. So, as says the doctor in the classic joke, "stop doing that".For same switching ratings, the BJT will likely have a smaller die, and be more prone to 2nd breakdown, thus failing in a shorter time frame when subject to thermal stress (which I should've noted was the mechanism I was concentrating on, and true, there are other mechanisms). The difference is even more extreme in IGBTs, which have still higher power density; they may fail in 20 or even 10 microseconds.QuoteAh, OK. It wasn't clear if you were extrapolating from the subject, or stating more generally.But I don't have any particular misgivings about such waveforms around a less modern MOSFET, given its ratings are respected (which, if you mean this exact environment and an RF/signal MOSFET, clearly that's not the case).QuoteHm, exponential with respect to what? I'm not sure what you have in mind, but am curious.I can explain my figure:Switching into a fault condition, typically applies say 50-80% of rated voltage, and draws as much current as the device can muster, getting very hot, very quickly. In the present situation, it would be if the transistor got stuck on somehow, for much longer than the transformer takes to saturate, so it just looks like DCR; basically, playing the role of the spark, instead.Broadly speaking, switching capacity (VA) is proportional to die area, as is fault power. Same power density is same heating rate. So, the time taken to reach failure is relatively independent. Again, it's a ballpark figure; you'll have to consult the thermal data of a given device to get the exact figure. The most important factor is drive voltage, as this is in the linear range (Vds > Id * Rds(on)) and there may be transconductance to spare.Though some devices actually "saturate" in that regard: SuperJunction types seem to be more-or-less fully "on" for Vgs > 7 or 8V, and draw about the same short-circuit (fault) current whether at Vgs(on) = 10 or 15V, actually providing a built-in limit.Somewhat similarly, BJTs run out of hFE under high injection; they simply can't be made to turn on any harder than Ic(max), more or less.The same may not be true of MOSFETs! It seems it may be true for SJ (high voltage) types. But I've seen low voltage, very-low-Rds(on) parts where the curves appear to extend beyond Id(max pulsed). These usually have extraordinary peak current ratings to begin with; high enough that, even if they don't cause silicon self-destruction, they can literally blow their own bondwires!Anyway, actual failure time could be as long as a few ms, especially at lower Vgs(on), or with control circuitry; or it could be as short as ~10us like as mentioned for IGBTs. Even less still for GaN, their active die area is extremely tiny.Oh, there's also pulse avalanche breakdown, which can literally burn a hole through a BJT in tens of nanoseconds; but this is easily avoided by obeying the RBSOA, not a big deal in practical switching circuits. MOSFETs can exhibit the same failure mode, actually, but it requires not just high voltages but high rates as well (above the dV/dt limit), again easily avoided by design.Tim

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 15, 2021, 09:38:48 am
content: Quote from: T3sl4co1l on October 15, 2021, 08:11:30 amSometimes it's easier said than done. 🙂 Effectively we have two capacitors between gate-source and gate-drain. This is a rather serious challenge to eliminate all possible extreme overvoltages. If it was that simple, MOSFETs would live forever. In the harsh reality, any engineer can easily collect a full bucket of burnt MOSFETs. And they also have SOA.QuoteThese waveforms were captured at 16 MSa/s, there could be a lot of interesting things remained behind the scenes. 😉 Especially voltage spikes on the collector (or drain), which is one pole of the SG1 and from which waveforms were not captured at all. One day when I have more free time, I will definitely conduct this experiment, it sounds interesting to me.QuoteWith respect to proximity to critical operation modes, any parameter: current, voltage, power, temperature, etc. In most cases, this is exactly an exponential curve: the closer to the critical value, the faster the component fails. This is why it is considered good practice to take components with a decent margin at their parameters (with respect to real operating conditions).QuoteEverything that you have pointed is indeed true. But in real circuits the amount of energy needed to break something down is always a time-dependent function. And taking into account that the energy (J) is the result of the expression I^2*R*t or U^2/R*t - this is literally an exponent connected through a factor. It may be smoothed or sharpened depending on the values of all associated multipliers, but it always looks exponential. I just cannot agree that there is some kind of failure time constant, sorry.Everything else is just a typical Internet "holy war" "which is better?". Of course, I will use MOSFETs instead of BJTs for many applications, especially switching applications, because their specs are just much better and they don't need constant current to stay turned on, but I will not stop considering them less reliable. 😃

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2021, 08:39:07 pm
content: Quote from: Alex Wolf on October 15, 2021, 09:38:48 amAh, but D-G coupling acts to turn on the transistor, which slows the rise of voltage. And significant Cdg is only present in old parts (e.g. your classic HEXFETs); modern parts have significant C at low voltages, then it falls off a cliff.I find it strange that some engineers seem to celebrate buckets of their conquests; in my whole career I would be able to hold all my destroyed devices on two hands [if I saved them], and that's just because two of them were modules that need a hand each.The most valuable function I've used is desat protection. This reduces failures due to short circuits. I've had one MOSFET cook off (fail shorted) and its half-bridge partner survived to live a long life. It's typically not worth employing in smaller converters but saves a lot of development time and cost with industrial modules.QuotePerhaps; arcs can do nanosecond-scale stuff. At the same time, it looks to be pretty stable, I would guess the noisiness arises from the arc moving around, evaporating bits and pieces of debris in the gap. But arcs are strange beasts, hard to say.QuoteAh, in terms of lifetime at parameter value, sure.Acute failure modes tend to have a... very aggressive lifetime curve, perhaps hyperbolic rather than exponential. Heh, maybe it is still exponential, I don't know. But something fundamentally changes when gate oxide punches through, or the plastic case separates from the die, ripping bondwires, etc.At least, I think that's part of the thermal failure mode? 150 to 175°C is well below destruction for silicon as such, but in the right ballpark for package GTT and thus, marked expansion or whatever. Whatever the case, I've had transistors quite reliably fail, quite quickly, up in that range. (It is most likely a variable duration until failure, not crossing a sharp threshold, for which an exponent would fit.)As I recall, Tj(max) is typically selected to be somewhere down on the curve, where lifetime is reasonable, like 10k or 100k's of hours. (I wish that semiconductors standards were more accessible, to cite that..) Evidently the curve falls off quite quickly beyond that (which could just be an exponent with a rather sharp curve, like how avalanche I(V) seems like a threshold, but is really an exponent).QuoteStill not quite sure what you're getting at; 2 is an exponent, but it's not a variable, so it's not an exponential. Considering Tj(final) vs. pulse energy is also not exponential. The only thing that should be exponential, then, should simply be as above, lifetime reduction vs. Tj?So yes, there'll be some curvature to it, but for practical purposes, when are you going to rely on surviving that 95%-of-failure pulse, while disallowing the 105%-of-failure pulse? (Or for how many times at 95%, or 90% or whatever, as a wear mechanism. Also, thermal cycling stress or whatever.)And since, under fault conditions, I and U are set by device ratings, and transient RthJC is proportional to those ratings, time is really the only free variable. And Tj(final) ~ t, so when t nudges over that exponential cutoff, failure soon follows.But again, if I'm reading this right, let me know?Tim

author: Alex Wolf
author_rank: Regular Contributor
timestamp: October 16, 2021, 03:03:02 am
content: Quote from: T3sl4co1l on October 15, 2021, 08:39:07 pmAnd I find it strange that some engineers don't. 🙂 Could you explain the underlined: how is it possible? Don't you run stress-tests on your prototypes to find their real limits, but just calculate them theoretically? Or do you use pricy components with a huge margin of parameters (like >100%)? Or do you specialize exclusively in low energy devices? It all sounds really "strange" (unusual) to me: like an exception to the rule, but not the rule. And the rule sounds something like this: the more times you find a way to kill your prototype, the less chances you left for this to the end user of the end product. In my opinion, this is the only way to make the created device truly reliable and most importantly safe. But probably this is just a point of view, one of the approaches. But yes, theory and practice always diverge, even the parameters values from a datasheet and the reality almost always diverge for the worse (or there is a note in small print about completely unrealistic testing conditions under which the value of this parameter was obtained for documentation). As they say, only for reference. 🙂QuoteExactly, nanosecond-scale stuff or very high frequency stuff. That's why I am wary of an insulator of a capacitively coupled MOSFET gate, since even a very small capacitor for this frequency is effectively a short circuit. In addition, a small capacitor is also formed between the heatsink (which sits on the power ground) and the MOSFET tab (drain). And if that's not enough, these spikes also tend to spread all over nearby conductors through the electromagnetic field. Will there be enough energy to damage the gate insulator? Who knows... As you absolutely rightly noted, the arc discharge is a rather unpredictable thing. In essence, it's a plasma-formed resistor and a capacitor at the same time. The parameters of this "hybrid" are extremely difficult to calculate, since they are based on poorly controlled and rather chaotic variables. And this is one of the very good reasons to conduct tests (including those that burn components). At the moment, I cannot say for sure; I can only theoretically assume that the arc damage scenario is not excluded, but not 100%-probable.QuoteI think they are the same lifetime curve, that's the point. Let's take LEDs for lighting as a simplest example and build a curve of their MTBF vs current. As a result, we will get exactly an exponential curve from just picoseconds to almost infinity (virtually immeasurable decades, without taking into account the degradation of the phosphor). The curvature of this characteristic depends on the specific expression, private variables used and their values. It is quite possible that the curve will be hyperbolic, I haven't dug that deep, I don't need it (since I don't manufacture semiconductors). But I can definitely say that a hyperbolic function is a special case of an exponential function, just as I can say with precision that the amount of energy per time is exactly an exponential function that strictly depends on squared current or squared voltage. In turn, all other parameters are strictly dependent on the amount of energy. And the Tj mentioned by you, and even Rth do not make any sense without energy, and in their essence, they are only either a variable in this exponential expression or its result. As for the mentioned Tj(max), while we are not talking about the physical destruction of materials, this is a very approximate value, roughly calculated from this expression at the given test conditions (I, Ta, etc.). In practice for specifically well-defined conditions, we can deduce a conditional MTBF or "failure time" constant or some limit for some variable - this will be perfectly correct in my opinion. But I prefer to think that this is a special case of solving this exponential expression as it is better to keep in mind than to overlook.And I think that it's not only physics, it is also a rather philosophical question. Well, you know: the harder you push, the less chance of dying of old age. 😁QuoteYeah, I wish that too... But this is business, including the programmed failure in design. Sometimes this is really justified by physics: you can't jump above your head. Sometimes this is a deliberate degradation of parameters: it's not profitable to make components or modules/devices that live forever... So, such questionable information is unlikely to be ever published.QuoteNice, you got me! 😃 Well, it’s not an exponent, it’s a power function f(x)=x^2, and it is non-linear as well. My math terminology was always a little lame, especially in a non-native language, duh… 🤦🏻‍♂️QuoteI think this is exactly what leads you to the conclusion that is incorrect in my opinion. Some pre-calculated limits are not constants in a real circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2021, 08:01:58 am
content: Quote from: Alex Wolf on October 16, 2021, 03:03:02 amWell, when I left the one place, we were doing burn-in tests, and over a hundred units or so, never saw anything that couldn't be blamed on something obvious, like a loose or missing bolt, etc.In particular, I worked on inverter modules of 5kW (up to 400kHz) and 150kW (up to 50kHz) capacity. These were stacked to get induction power supplies up to 50kW, and 600kW or more, respectively. 480V 3ph mostly, with 240V an option for the smaller units.To your point -- this was lowish quantity, kinda specialized, industrial sort of stuff. Not like motor drives, or uh, solar farms or something; we didn'thaveto be especially picky. Whereas in the qty 100k's, you're likely to start seeing spooky failures that can possibly be blamed on semiconductor ratings. Also, conditions slightly outside of normal operation, like unusual swells and surges, in patterns outside of what you'd normally test for.We did pick up a guy from Rockwell with drives experience, though I don't recall picking his brain on semiconductor reliability. (I do remember one thing he had an opinion on, mask screened heatsink paste -- practical and effective. Though I don't remember anymore if he got the service techs to use masks instead of fingers, when replacing modules. Hm, what a strange sentence, out of context.)QuoteWell... an alternative explanation would be: mediocrity is the rule, not the exception.I don't know your experience, you haven't mentioned much yet, so I don't want to go and assume. You do sound experienced. Not having found this particular observation suggests either an early career -- or an extremely lucky one! (Jealous!)I've seen more than enough things, that are hardly worth an eye-roll anymore. To the best of my knowledge, the 80-20 rule applies in all fields. Like, take the range of experience you see on this forum -- it isn't much different in professional spaces, or academics, or etc. It's recursive, too: you can take the top 20% of, say, electrical engineers, and they'll be reasonably knowledgeable in their respective subsets; 20% of them will be expertly knowledgeable in those subjects in turn. And 20% again, and so on until you're in such a rarefied set that you've found the dozen subject matter experts in, well, some very obscure corner of the field, but damnit do they ever know it well!Conversely, the 80% might not realize which population they're in, partly explainable by a modest misunderstanding of how one should characterize that set (most people think they are reasonably knowledgeable about the set of things that, well, theyknowthey know about; and chances are, few will share exactly that set of facts, and I would admit it's a fairly easy error to make regarding the definition of ones' set, assuming something more personal of onesself or less personal about another, y'know?), but also a large part of that is simply Dunning-Kruger at work.And not to be outdone, the 80-20 rule applies almost uniformly across intersections of sets. Take that superlative dozen from before; they might have as much knowledge of, say, sociology, as any rando off the street. You need to search quite far indeed to find someone truly in-depth in many subjects, and truly broad in general knowledge.For my part, I would ~guess~ I'm maybe two levels deep (three might be pushing it) regarding general EE and science knowledge; but let me also be the first to admit I'm likely in the base 80% on most general-knowledge, business, cultural, etc. subjects. (I do have my own business, but it's merely self employment, and I surely make much less than I would in a normal salaried position... some might suspect I must be an utter moron! And I'm not saying they're necessarily wrong...)Anyway, that might be very non-sequitur, but if nothing else, anyone reading can gain whatever benefit this lecture entails, if it should prove redundant or too off-topic for your taste. At least, that's what I like to tell myself when I get into these long posts... I digress...QuoteAnyway, it's certainlyonemethod of testing. Sometimes it's the only way. But it's anextremely low information method.I've read many posts/comments from those beginning with SMPS and such, and until they understand the ratings, dynamics and so on, it's just a lot of feeling around in the dark. And that experience can take years (I know, I've been there!). But they're still expected to producesomething. The project clock is ticking. Budget is spending. We need to getsomethingout the door forThe Show!And, perhaps as much as a mental self-defense as a cultural phenomenon, those buckets of de-magic-smoked transistors become a peculiar badge of pride.I far prefer high-information methods. I powered something up and blew it; well, that doesn't work, great, butWHY?So try it at lower voltage, or current, or frequency, see what the dynamics look like, see if something changes at extremes, see if something is getting too hot or too much voltage or current or something. Anything that directly suggests a course of action, rather than throwing more spaghetti.I will admit that that first 5kW inverter got up to, revision E I think it was. First big project I did out of college. The first two or three were built with the best information I could find -- postings, application notes, a book or two. Theyfucking SUCKED.The key factor that was missing, I learned, is that traditional advice is faulty, and in particular,whatis missing from that advice. It is an approximation, and like any other, incurs certain assumptions. Which, sadly, always end up going missing from rules of so many thumb. So when it fucks up, you're clueless why; there's no information to suggest what's right or wrong, or by how much.And, in that moment, you realize true knowledge is not a statement of fact, but a relationship between them. A relationship that suggests direction and action. But if all you have are statements, and those statements fail -- what do you do?The particular rule I disproved, is: design your switching loop with "minimal inductance".The assumptions missing from this statement are:1. You will be able to achieve a low enough inductance in your layout.2. "Low enough" is defined by the switching speed of the devices used.That last point actually says something. It suggests a relation: for a device with some rise/fall time \$t_r\$, the switching loop, which includes stray loop inductanceand device capacitance,must have a time constant \$\tau = \pi \sqrt{L C} / 2 < t_r\$.I suspect this particular statement is very old, perhaps dating to the 70s or 80s, when all we had were bipolar devices, or maybe into the 90s too when MOSFETs became mainstream. Most applications wouldn't have any way to violate the time constant of the switching loop: a whopping 50nH loop (which might be typical of single-side layouts like in ye olde ATX power supply) with ~1nF devices is some 11ns (that's the 1/4 cycle time, by the way), while the fastest power BJTs might turn off in a whopping 100ns. And why would you push a MOSFET harder, going above 150kHz attracts more attention from the FCC, why worry about it?But statements get repeated, as they are wont to do -- just another meme, they absolutely exist in professional spaces as well as casual -- again, nothing is REALLY different, we're all humans here -- and people encounter, uh, Learning Experiences, like my above story.And so, applying this knowledge, I was able to take stock of where I was, and what I could do.Transistors were something like this,https://cdn.ozdisan.com/ETicaret_Dosya/587044_5214130.PDFThe design was this: half bridge, two transistors in parallel each side, 320VDC in, 70A RMS out. 4-layer board, so the power, switch and ground nodes are all interleaved as closely as possible. I calculated that the total loop inductance was something like 17nH. It literally can't possibly be any lower. The SOT-227 devices are, well they clearly have a loop area, but they're close ~enough~ to the board I guess, and a whack of TO-247s isn't going to do any better for example. So, truly, I've done my due diligence, inductance is as low as it physically can be, while still using off-the-shelf parts. (And nevermind MOSFET modules -- the best I could find at the time was, as best I could tell, something like 30-40nH to the terminals. Preposterously useless!)The measurement. When one of the transistors turns on, in hard switching, it becomes a short circuit over 10ns or so, and yanks the full supply voltage across its opponent. This suddenly charges its drain capacitance, through the loop inductance. The capacitance is about 2-4nF ballpark (it's nonlinear, so whether you count the fat or thin end of the curve is a matter of concern!). So the loop time constant is 9-13ns let's say. And we're switching fast enough that substantial energy is going to be put into that LC circuit.How much energy? Well, the measurement was80% overshootwith a resonant frequency around 60MHz. Oh shit!Fuck me, I've never made anything that fast and hot before, and I certainly didn't mean to...(Aha, on-topic content! There was no evidence of parasitic turn-on, despite the magnitude of rising edge (~25kV/us). Like I said, the feedback capacitance is minuscule, above much Vds. Also good that the gate drive impedance was low. Arguably too low, given how much trouble that risetime is causing, but I couldn't afford a whole lot of switching loss, either...)So the first thing I tried was just brute-force clamping the voltage spike, because that was blockingprogress. I can't run at full power at all with these derated transistors; I have to make this thing work with 600V transistors, to deliver the nameplate spec.So I ass-wired some TO-220 diodes across some nearby pads. 600V 8A parts should do it, right? It's like 80A peak, sure, but the average is a pittance, like, <1% duty cycle, that should be fine...Nope, they blew up right after hitting the switch. Huh. Also, that's one SOT-227 transistor to the pile. (Not all four, because desat protection from the start, remember?) Or maybe there was another few before this, I don't remember; I might've tested this at 100V and noticed the overshoot problem before testing at scale. I was SMRT like that, even back then...Okay well, try 12A diodes... runs a few seconds? They.. don't even get hot? That's weird...Okay, fucking 30A diodes... alright, now it runs steady state.I think what was happening here is, under such high peak currents, the diodes were suffering from electromigration, or maybe some weird super-high-injection thing with the semiconductor itself, I don't know. They don't heat up, it's not a thermal thing, at least not a bulk thermal thing. They just die. (Wish I'd had an SEM to inspect the die and see how it failed, but yeh, this was 2010, not like today when every other science YouTuber has one.)Anyway, with the diodes in, the spikes were clamped to a modest 20% -- that's right, still a whopping ~60V applied to the poor diodes -- and even accounting for lead inductance (about 7nH for TO-220), I'm pretty certain that's largely dropped across the die itself. Forward recovery is absolutely a thing, and you get to see it in full swing at these kind of time scales.Skip forward a few board revs. I analyzed the circuit, did some simulations, and figured out a solution.I made two major changes:1. The module is now H-bridge. Four transistors per board, but they're complementary. This solves problems with wiring (in a half bridge, the load current returns through the power lines, such a mess!), and reduces load current (it's only 35A now).2. I added a slot in the copper, which DC+/- looped around -- thus adding about 100nH to the supply loop, at each half of the H-bridge. On the input side there's a row of film caps for bypass, then this series inductance, then the transistors in the middle of the board. At the transistors, there's a peak clamp snubber, a diode into something like 0.1uF || 20R, across the supply. The inner switching loop is still fairly tight, maybe 20 or 30nH between transistors and the snubber -- but the extra 100nH to supply allows some room for the transistors to bounce against each other. In return, the capacitor clamps, and the resistor dissipates, that extra switching energy. The overshoot was something like 20%, so that 600V transistors were feasible at 320V DC bus, and 900V at 650V bus (480VAC input).I think the figure was around 100W switching loss per pair of transistors, at low load current and full frequency (hard switching at 400kHz). Switching loss decreases under load (as ZVS is achieved), then goes back up (as, what (at turn-off) used to be load current through one or the other inductor, has to be dumped into the snubber). I designed it so that the snubber dissipated 100W in the two extreme conditions, with a minimum inbetween.So that was a, not wholly off-topic story, but it illustrates the thought process I had, in a very real example.To be clear -- sources of information are best matched by frequency, or data rate or density. By that I mean, consider error correcting methods for example: when a comm channel has very low bit error rate (BER), it's feasible to run with no error correction at all, and simply retransmit when a fault has been detected. This is a huge PITA, it clogs the channel when it happens, it breaks all sorts of guarantees you might've otherwise had about timing say -- but your datawill get there, intact.Examples like TCP/IP. And this is a perfectly reasonable compromise to make, when you need to prioritize correctness, don't need perfect timing, and when the BER is low, these faults happen rarely, so the average bitrate is still very good.How does this apply? Trying to do early development, when you're still trying to figure things out at all, requires a lot of information. Learning it from single bits at a time is no way to work. That's mine, and everyone else's, beginner story. But the converse is perfectly true: when things happen very rarely, it can be a big PITA to figure out, but as long as it's rare enough, it can be worth dedicating the time to figure it out. (And that's not as if to say there's less value in the tools that help you figure things out; the classic software ticket dismissal "cannot reproduce" is absolutely justified. If you can't slap a debugger on it, what can you really do? The same is true of our meters and scopes, when tracking down strange, rare failures in power supplies or whatever.)QuoteActually, that kind of helps. The lead inductance is some nH, as mentioned -- or for short enough time scales, we can just as well look at it as some crude length of transmission line, which a lead through air is going to be ballpark like 100-150 ohms. Anyway, that provides a "squishiness", where the die tends to act as one (it's a huge capacitance, remember!), and the wave energy couples out, basically through all three leads in parallel. And maybe the gate drive isn't very strong, there's a 10 ohm resistor to it or something, so not much goes that way anyway, and Vgs basically just stays put while these waves wash around it.QuoteI want to add a quick note on this -- they're obviously not the same function, having different words for one, but also, like, analytically speaking. Practically speaking, they may be close enough not to care, and in what we're talking about here (that neither of us really know about for sure), that's as good as any, so I'm not calling you out here. Just that, when there is statistical evidence and theoretical basis for it, the difference is absolutely stark: an exponential grows (or falls) without bound, for an increasing argument. A hyperbolic growsinfinitely to a single point. The real fun is if you reverse the equation. Normally, when you try and solve for points beyond a hyperbolic asymptote, you get some bullshit complex number -- thereliterally does not exist,a solution to the equation here.* Whereas the exponential will tell you something obviously useless, like a Planck scale lifetime, but it still "works", mathematically speaking.*Unless we want to make some stretches about the quantity being oscillatory out there, or in some hand-waving sort of way, geometrically perpendicular? Which you can sometimes get away with, even. But in general, yeah nah, who knows.But yeah, for our purposes, we have neither data nor theory, all we know is it's some stupid sharp thing. Alas!QuoteWell, it's not so pessimistic for semiconductors, as far as I know. Some things really do seem to inevitably wear; electrolytics for example. They can be made for long lives, but it's always finite; there's no -- aha, I have an excuse for my pedantry -- asymptote in the other direction.I'll explain this property with another example. Some things do appear to have such a claim: some alloys, such as steel and titanium, have a fatigue limit, meaning, the cycle life of a spring for example, tends towards infinity for some (nonzero) displacement or less; or in general, some bulk loaded up to the corresponding maximum stress/strain. This isn't trivial -- aluminum alloys don't have such a limit, so the cycle lifetime increases merely inversely proportional to loading (or with whatever power law it has).Stuff like that, I suppose makes one wonder about things like the millenium clock or whatever it is -- metals, under certain load conditions, can potentially survive "forever", so it would seem. Good frickin' luck with wear, even under rolling contact, but yeah, at least cyclic loading isn't a deal breaker, huh?In a similar way, semiconductors seem to have, at least very good lifetime, if not hyperbolically so, near room temperature. One of the bigger troubles is electromigration, as the conductors in ICs have to be very fine to do their job, thus current densities are high, and lifetime depends on temperature and current flow or dimension. Go figure, it's not the semiconductor, it's the damn wire on it!Which also means you could have random samples of chips that fail at various lifetimes, due to variation in width/thickness of interconnects for example.Electromigration specifically, I forget if it's a hyperbolic cutoff, or a continuous (power law or exponential) relation. So there might be that.I mean, there's a lot we can derive just from the sheer existence of modern technology as we know it -- again a bit of a low-information claim, but appreciate that there are, well and truly, BILLIONS of transistors in, probably whatever you're looking at right now; or somewhere nearby. And they all work together at a, probably <1ppb BER, and most of those errors are soft (induced by noise, radiation..), and total failures (e.g. a transistor stuck hard on/off, broken wire, etc.) are years inbetween. This argument is something of a null hypothesis, setting merely a lower bound on the real figure. But also, you don't see cellphones bricking themselves every day, and you might know -- whatever, dozens, hundreds, thousands of people from whom you're likely hear about it happening.QuoteAh -- fair enough. We're talking fairly precise things here (well, on the couple occasions when we have, heh), so it pays to get it right. I hope my (long winded) explanations have at least been illuminating..?Cheers!Tim