Buck converter, ringing issue - Page 1
Forum section: electronics / projects

author: dzseki
author_rank: Frequent Contributor
timestamp: December 14, 2018, 02:44:10 pm
content: I have designed a complex temperature controller system. The output stage (hetar driver) was a result of compromise: the output power had to be rated 10A@30V, but because of other constraits this power supply has to be lowest noise possible. Because of size and practical consern a PWM regulator (Buck converter) topology was chosen, with a rather heavy output filtering.Since this was intended as a somewhat universal controller, various loads are expected on the output. Furthermore by the nature of the system the duty cycle can vary between 0-0.95.It is clear from the above that it is inevitable for the circuit to operate in both continuous and discontinuous modes.The circuit uses a P-FET as switching element driven by an LM5134 that sits on a „tracking ground” that follows Vcc by -5V. The output filtering can be seen on the schematic excerpt, it is tuned for ~200Hz cut off with a damping factor of 1.In discontinuous mode I see some strange behaviour from the circuit. Articles -at least the ones I looked up- did not explained the discontinuous mode in much detail, other than basic equations.In the given setup, with an output load of 4 Ohm and supply voltage of 18V the operation switches over between continuous and discontinuous operation at about 0.4 duty cycle.So the problem is in discontinuous mode before the FET would conduct there is some sort of ringing happening on the drain of Q4.Playing with the PWM repetition frequency: lowering the frequency will make the ringing more severe, higher PWM frequency shifts the switching point towards the lower duty cycles -as expected.Below is an oscillogram taken in discontinuous mode (with duty cycle: 0.2), lower trace is the drain of Q4, upper trace is the output voltage ripple (AC coupled).The output is not affected much by this strange behaviour as I see, but I am also concerned about the radiated EMI. Also I am a little bit confused what is happening here exactly. To my understanding in discontinuous mode neither the FET nor the freewheeling diodes carry currents, therefore nor the coil, but what is oscillating then and why? As I saw adding more capacitance over eg. D9 would definitely detune the frequency of the oscillation, but the energy involved here is clearly larger than it would be practical to dampen with a simple RC snubber, perhaps adding active switch (FET) instead of the freewheeling diode would help to cut the ringing, but I would still like to know why is this happening?

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2018, 04:42:39 pm
content: The inductor forms a series resonate tank circuit with the parasitic capacitance of the power MOSFET and diode. It is completely normal when operating in discontinuous mode.Check out theMAX1722 datasheetfor a little discussion about this. It seems like if you used a synchronous rectifier instead of the diodes, that it would clamp the ringing.

author: Benta
author_rank: Super Contributor
timestamp: December 14, 2018, 05:18:39 pm
content: What you are seeing is 100% normal for a buck converter in discontinuous mode.As no DC current is flowing in the choke at some point (this is where the oscillation starts), both power switch and free wheeling diode are off.This isolates the choke DC-wise.The result is, that the choke is a parallel tank circuit comprising inductance and inter-winding capacitance. There is basically nothing you can do about it. Reducing inter-winding capacitance will simply increase ringing frequency.The upside is, that the ringing has low energy, so it won't radiate a lot. And the ringing is only present at the input side of the choke, the output is kept steady by the output cap.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 14, 2018, 06:14:57 pm
content: C15 / R25-R26 would seem to be the wrong value. Or they are there for high frequency (loop inductance + switch capacitance) damping.You can add another R+C to dampen that, same C, different R of course (R ~= sqrt(L / C)), at the expense of more switching loss of course.I don't usually mind that, because it's lower frequency and not prone to radiating. The energy stored is either dissipated (when the transistor switches on during a low peak) or saved (during a high peak), but it averages out either way.Beats me what the hell the resistors are doing there -- the FET is 5mohm yet there's 80mohm in series after it. And the diodes drop ~0.6V at most load currents. The efficiency optimization seemsreallyoff balance here.So, the combination of an unusually "stiff" (i.e., low impedance, because Z ~ sqrt(L/C) and C is large) ringing on the switch node, and an excessively low Rds(on) transistor, seems to suggest that the component choice is very well past peak efficiency. The diode, resistors (and inductor value, to some extent) imply a much lower operating current than the transistor ("70A"!) does. Or if high current and efficiency are required, then this needs to be a synchronous switcher.Mind that PMOS have 2.5x worse performance than NMOS, so there is further savings in not just using a modestly sized transistor, but using a bootstrap gate driver and NMOS type as well.Tim

author: dzseki
author_rank: Frequent Contributor
timestamp: December 14, 2018, 08:07:57 pm
content: Quote from: T3sl4co1l on December 14, 2018, 06:14:57 pmThis circuit is the second generation of its kind. R25-6 and C15 is for high frequency damping, their value was determined with the help of a Richtek application note, and I am satisfied with those results.The original circuit was based on the LT1158 synchronous NFET driver, which had other (more serious) issues, and after all I was only able to use it reliably when I gave up with the synchronous mode, so I was thinking to try something new with the second generation.The resistors are there to form a critically damped filter on the output, without them the filter network would have a Q of 4.Again the first generation of this circuit used even heavier output filtering: 100uH coil with 100 000uF capacitance, with that setup the cutoff frequency was 50Hz, and much lower resistance was needed for critical dampening, but a 100uH inductor that is rated for 10A and the 10pcs 10000uF capacitors took some PCB space, it was overkill anyway.Efficiency was not in the first place but low noise level was, it is clear that these power levels are not suiting for linear regulators (even when combined with buck preregulator). The circuit might not be super efficient, but I think it is still decent for what it is.In my philosophy the FET in the circuit is for switching an not for its series resistance, If I want something to drop voltage on I put there a resistor, so I don't have to care about the cooling of the FEt to maintain performance, while the resistor have no problem with heat

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 14, 2018, 08:57:19 pm
content: Quote from: dzseki on December 14, 2018, 08:07:57 pmYeah but why? It's in the loop.Wait, don't tell me, it's open loop PWM? YIKES!QuoteYeah, brute force doesn't get you low noise, patient work does.Per microfarad, a filter is infinitely* more efficient with multiple sections, than one lumped sum. Go for a high order filter.And that's assuming ideal components, which an electrolytic that size certainly won't be (more because of trace length; its ESR at least will be low enough..).*Obviously not infinite, but it can be over 100dB improvement, which is "infinite" for practical purposes.QuoteRight, the FET is there for switching, so you don't want it overencumbered by being too large.Tim

author: ogden
author_rank: Super Contributor
timestamp: December 14, 2018, 08:57:43 pm
content: Quote from: dzseki on December 14, 2018, 08:07:57 pmI am afraid that you think that inductor of buck converter is part of some filter, but it is not. Buck converter is not PWM followed by RLC filter. Also 33uH "coil" have too low inductance for 33KHz. You need to read some papers about buck converters and their design. Copy some reference design (application note) and do not try to invent The Wheel.QuoteYou need knowledge, not philosophyYou don't want to put any voltage drop in form of resistor between inductor and switches. It does not do *any* good. Check application notes of similar converters. Good place to start:TI webbench. Note that most high current 10A converters are synchronous (and no series resistors for god's sake).

author: dzseki
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:29:00 pm
content: OK, then this is not a buck converter...It is working quasi open loop ( as a temperature controller, the duty cycle is governed by the PI control loop where the temperature is the goal and heater power is the control signal).So this is a PWM driver that has an RLC filter on its output to smoothen the switching signal as much as possibleAlso I don' see clearly what would I actualy gain with a smaller FET

author: ogden
author_rank: Super Contributor
timestamp: December 14, 2018, 10:54:10 pm
content: Quote from: dzseki on December 14, 2018, 09:29:00 pmYou need properly built buck converter rather than improperly built and renamed to PWM for some strange reason. Could your show PCB of your converter? - That ringing could be layout problem like way too long or improperly routed driver->gate track/wire.No, don't resize FET's. Better remove those (switch-inductor) resistors and increase inductor to 100uH, switching freq to ~50 KHz. If you want low ripple on the output - filter it after bulk capacitors by adding another inductor and capacitor. That 33uH you currently use as buck inductor may be used as output filter.

author: dzseki
author_rank: Frequent Contributor
timestamp: December 14, 2018, 11:31:57 pm
content: Quote from: ogden on December 14, 2018, 10:54:10 pmSo what is the difference then? This design was actualy started as a filtered PWM design, which "by coincidence" resembles to a buck converter...The shown ringing is only present in disciontinuous mode -if we can call that way now- in continuous mode there is no ringing what so ever. As it was confirmed by others this seems to be a normal behaviour for this circuit.Also, you suggest to use 4th order filtering, with two inductors, did you have 10A 100uH inductor in your hands? It is big... Now with my "ugly" 2nd order filter I may have 8W dissipated on the filter, so what? For 300W of output power the total loss in the converter will be still less than 15W, it isn't particularly bad.With this filter the attenuation of the fundamental switching signal will be greater than -90dB (theoretically), as Tim says you can't go below 100dB anyway, so what to gain?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2018, 01:07:02 am
content: Massive chokes!I'd have done it with, probably:10uH, then 4.7uH, and maybe one more if needed. Whatever capacitors that then requires, given the impedance and cutoff frequency; in the 10s uF, probably a mix of polymer and ceramic.Sync inverter, current mode control, 200kHz say, maybe two phase as well (in which case LM25119 maybe, otherwise a single phase version).Have done it before at ~90% efficiency and higher frequency (with low noise measured). Pushing >95% should be no problem at a somewhat lower frequency, and with somewhat bigger transistors (but again, not huge, because that costs switching loss).Tim

author: dzseki
author_rank: Frequent Contributor
timestamp: December 15, 2018, 02:47:39 pm
content: So if ogden suggests 100uH coil Tim is suggesting a 10uH coil, then my 33uH coil sounds about rightSeriously, in this circuit we want to control the duty cycle with 1% resolution (between 0 and 100% basically), so with higher switching frequency this gets a little problematic. I may consider a different output filter stage, but this wasn't the scope of this thread.But you made me curious... So I throw in the circuit diagram of my first generation circuit, I am interested about your opinion.This was a lower current version of the circuit (~2A), it is synchronous, a PWM driver with heavy output filtering (fc=50Hz).The problem with this circuit was that when the control pulled back from high duty cyle (say 90%) down to 50%, the energy stored in the output kicked back in a way that Vin voltage got elevated beyond its rated value, in extreme cases up to 60V (!), this obviously made the (24V 600W) power supply upset on that end. In one ocassion the LT1158's bootstrap circuit alsoy died...The solution I found for this was replacing Q1 with a diode (giving up syncronous mode). This is interesting, because Q1 already had a bulk diode, also the first attempt was to add an other diode parallel to Q1, but that did not help, Q1 had to be removed altogether.My idea was the problem originates from the shot-through dead time of the LT1158, and therefore this defeats somewhat the synchronous scheme in this application

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2018, 07:05:53 pm
content: Quote from: dzseki on December 15, 2018, 02:47:39 pmProblematic? A voltage regulator is continuously variable. Drive it with a DAC, use a PWMDAC at any frequency (with adequate input filtering -- use an active filter if necessary), get a programmable one, whatever you like.QuoteThis looks way better! (Except for the absolute fuckoff massive output filter values, as mentioned.)Weird chip. Driver, with current limit, with controlled dead time. Not very much drive strength though. Expensive as hell, as all specialty-ish LT chips are.But if cost is no object, you can easily toss in another LT chip, a TimerBlox, and generate continuously variable PWM that way. Same idea, just a different way to get there.QuoteWell, yeah, that's precisely what you've done! That's what a synchronous inverter does, it works both ways: buck and boost are identical, down to the direction of current flow.Or if you don't need the output voltage changing so rapidly, just don't slew it so hard! This hints at another rarely-noted but key design principle: Always Limit Bandwidth. If you don't need it, filter it!If you had gone with a switching regulator instead -- those often have a freewheeling mode, i.e., the synchronous diode is turned off when inductor current goes negative. So they act as an ideal diode, and don't discharge the output. With a different choice of controller, you might've never discovered this property.Or, if you had reduced the stored energy, breaking up the filter into a number of small stages.QuoteWell yeah, you only get backflow into the supply when the output voltage is greater than the input (that's not going to happen), or when inductor current goes negative (which can only happen if Q1 stays on too long).I'm not sure how obvious all of this is, or should be -- obviously, it's obvious to me, because I've done this before; I hope I'm not being condescending as a result. That doesn't help. I just want to give you the insight that leads you to the correct conclusion.If nothing else, the underlying lesson should be: you've discovered a deeper generalization of switching circuits, and that insight should guide you to improve the controller, not worsen the inverter!QuoteI'm not sure what you'd base that on -- if nothing else, shoot-through is a very short duration phenomena, so it can't possibly affect the inductor's voltage or current by more than a small fraction. But the evidence shows that slamming the input up and down, causes a HUGE dip or surge in the supply, not a tiny change!Also, shoot-through is current flowing through both transistors,notthe inductor -- it should have no effect at all, actually, instead causing the transistors to heat up excessively.So we can safely rule that out.HTH,Tim

author: dzseki
author_rank: Frequent Contributor
timestamp: December 15, 2018, 07:58:11 pm
content: Quote from: T3sl4co1l on December 15, 2018, 07:05:53 pmI mean at 200kHz 1% duty cycle corresponds to a 50ns pulse, that is not that trivial to make.QuoteWe are on the same page here, I already use the LTC6992 for PWM modulator.QuoteI also figured this out (and actually did), but one annoying thing still remained: the same bad thing happens when the pulse by pulse current limiting of the driver kicks in, and there was no way to avoid it, so I dropped Q1 instead of the current limiting.Thanks for the explanation otherwise!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2018, 10:41:18 pm
content: Quote from: dzseki on December 15, 2018, 07:58:11 pmAnd 0.1% corresponds to 5ns, and so on; it's arbitrary if you aren't using a fixed-threshold digital counter to generate it.Even if you are, you can always delta-sigma modulate the threshold (or count, or timer-compare, or whatever it's called on the chip). This costs more software if it's on an MCU, but it shows it's at least possible, if not easy.Can also adjust the total count (MAX_VAL or whatever), which gives you control of both numerator and denominator, which is finer control for certain values (the worst case step size is still 1/b though). These can be calculated relatively easily with the method of convergents. The downside is, now the frequency changes, but it can still be held within a modest range, say 2:1, that still meets whatever ripple spec after whatever filtering.Uh, but I digress...QuoteAh good, AFAIK those are continuous-time so you're set!QuoteAh, so its protection is also synchronous, it doesn't go to open circuit? Yeah, weird chip...Tim