My power supply is squealing!! make it stop!! - Page 1
Forum section: electronics / beginners

author: Mighty Burger
author_rank: Regular Contributor
timestamp: January 09, 2022, 02:27:36 am
content: Boards arrived and I assembled and tested them. My step-up converter (12V to 170V) is making an annoying, constant very high-pitch noise when under load. (Testing with a 18mA load). It is quiet when not under any load.This is for a Nixie Tube clock. It's not incredibly loud but it would be very annoying to have next to your bed when you're trying to get some sleep. I don't want to develop tinnitus from this thing. This project never seems to stop offering learning opportunities. Sometimes they are quite frustrating.Here's the schematic and PCB. I attached the full schematic at the bottom of the post if you'd like to view it. The power supply is contained entirely within the outline on the PCB, I intend to throw a shield over it.Part of my confusion is the fact the power supply has a switching frequency of around 40KHz. Confirmed this with my oscilloscope. This should be outside the audible range of any human, right?Here's what I suspect are potential causes.1) Inductors. If the inductors are the source of the squeal I'd have no idea how to change the design to fix it.2) Ceramic capacitors. All the SMD capacitors are ceramic. Some googling says they can have a piezoelectric effect and make audible noise.3) The output LC filter isn't designed correctly. This is something I only just realized after one of my coworkers mentioned that as a potential issue. I don't know where I got the values for the components, but after a little research trying to understand this stuff I think it's not effectively filtering out the power supply.I did a simulation in LTSpice to see the filter response. I've never done this before so I could easily be doing something wrong. I didn't know what a Bode plot was until yesterday.This is what I got:Funnily enough, the 3.3uF capacitors in the simulation seem to make no difference whatsoever ..The GREEN lines are the original circuit, i.e. the circuit that I have built up that's squealing audibly. (Solid line is the gain). It's not attenuating effectively at the supply's switching frequency of 40KHz.The BLUE lines are one potential alternative I thought of. I would just use another 3.3uF capacitor instead of the two smaller ceramic capacitors. Threw in 1-Ohm as a guess to its ESR (datasheet doesn't state it). At 40KHz that's around ~20dB attenuation. I think I can increase the supply's switching frequency up to a maximum of 100KHz, at which point this filter would give ~35dB attenuation. Increasing the inductor value would probably help with the filtering too.I did a little testing by messing with the capacitors.C8 and C19 removed (no filtering basically)- About 15V spikes on the output. Audible squeal present.C8 and C19 installed (original, potentially incorrect filter) - About 2V spikes on the output. Audible squeal present.C8 and C19 removed, but 3.3uF capacitor installed where C19 was (potential alternative)- About 150mV spikes on the output. Audible squeal still present, though it might be a little quieter?From these experiments I think it's clear the next boards I order should have the larger capacitor to replace C8/C19, maybe even a larger inductor to replace L3. But the SQUEAL is still there!What do you think is causing this? How could I fix it?

author: Psi
author_rank: Super Contributor
timestamp: January 09, 2022, 03:26:41 am
content: Adjust C9 to change the switching frequency up a bit.Looks like you have 680pf atm, which is around 40khz. Try 330/470pf etc.You maybe getting a harmonic of the switching frequency causing the inductor/cap to oscillate/resonate at an audible frequency.So if you move the osc freq a bit you may come out of resonance. It probably wont stop the sound, but you may find the volume goes down a lot at some frequencies vs others.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: January 09, 2022, 04:25:07 am
content: Mighty Burger: Dicking about with the switching frequency can have unexpected results by changing the duty ratio so that the regulator runs out of elbow room to regulate the voltage. I think its fairly certain that the inductor is the one that's doing the squealing. I can see that it is made of several mechanical components and they are probably rattling against each other, driven by the magnetic forces. If you can goop it up with some kind of hard epoxy that might quiet it down. I'm also not surprised that an audible sound is possible even though the switching frequency is well above audible range. The duty ratio of the switcher will have a random frequency/phase modulation on it of some amplitude. This modulation spectrum will extend all the way down to 0 Hz and up to fs/2. It will seem to change character when the digits change since the load on the circuit will change with it. I have heard it myself with a similar booster I made some time ago. It is very hard to beat down.

author: Mighty Burger
author_rank: Regular Contributor
timestamp: January 09, 2022, 04:39:06 am
content: Quote from: Psi on January 09, 2022, 03:26:41 amBullseye.I replaced C9 with a 330pF cap and the volume dropped pretty significantly. Still audible but you kinda have to really lean in and try to hear it, it's hard to hear over the fan of the lab power supply. Before it was pretty obvious whether it was powered on based on the sound it made.While I wish I could completely resolve the squealing, I'm fairly happy with the current audio volume after that switch. Maybe replacing the ceramic caps with other types would further reduce the buzzing.For fun I tried 470pF. Louder squeal, probably worse than originally with the 680pF cap.I think I'll have to try a variety of values and see which switching frequency results in the lowest volume. To be more accurate I might see if I can get some kind of microphone setup working to measure the volume amplitude, could be helpful to get an exact frequency of the sound, not sure if any of my mics can pick up that high a frequency sound though. Maybe.Thanks for the advice Psi!Quote from: basinstreetdesign on January 09, 2022, 04:25:07 amThanks for the advice. I didn't see your response until just now, after I tried changing the frequency. The power supply seemed to work still, it maintained the same voltage, despite the different switching frequency. I'll have to do some more extensive tests to verify it still functions over a variety of loads.Maybe I could try epoxying down those inductors to see if that reduces the noise all the way. It's kinda at the point though where it might be hard for me to tell a difference.

author: bdunham7
author_rank: Super Contributor
timestamp: January 09, 2022, 05:36:27 am
content: This might seem off the wall, but could you remove L3 and just bypass it as a test?

author: jonpaul
author_rank: Super Contributor
timestamp: January 09, 2022, 09:05:44 am
content: Bonjour, Now a very common issue, as nowadays designers are not familiar with loop oscillations and subharmonic oscillations.The noise is evidence of poor loop design, a sub-cycle oscillation.If i varies with load or line, that is a clew. Neither indictor or other fixes mentioned address the root cause.Probe the control ckt servo and look for a subharmonic frequency of the main oscillator for the problem.Bon ChanceJon

author: John B
author_rank: Frequent Contributor
timestamp: January 09, 2022, 06:50:06 pm
content: I've found especially the MC34063 will do that, as it's a simple hysteric controller.

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2022, 07:06:21 pm
content: C10 on the feedback pin doesn't help. Its time constant with the feedback divider Thevenin resistance introduces enough lag in the feedback loop to permit sub-harmonic oscillation in the audio frequency range. It *may* need some capacitance there to suppress leading edge and reverse recovery spikes that may confuse the feedback comparator, but the time constant should be small compared to the oscillator's period, and IMHO due to the high value upper feedback divider resistor, stray capacitance will be sufficient.

author: Mighty Burger
author_rank: Regular Contributor
timestamp: January 09, 2022, 11:22:04 pm
content: Thanks for the advice everyone.bdunham7, I'll certainly try that once I get back to the lab this weekend. If that is the culprit, I'll have to find a suitable alternative as without that filter the output is just way too noisy.Perhaps a better controller would be more suitable for a future revision. For now though I'd like to see if I can get this one working nicely. This is my first step into SMPS design and to try to have a more gentle introduction I started off with someone else's schematic, which happened to use the MC34063.There's some talk about subharmonic oscillation. I've tried searching online to try to understand what it means but I can't wrap my head around it. There's just a little bit too much lingo I haven't become acquainted with yet for it to make sense.Now when I was measuring the output noise with my scope, looking at the spikes that correlated with the supply switching, I noticed something unusual. About one out of every four "spikes" were skipped. Could this be the subharmonic oscillation you guys are talking about? Now that I think about it, it would make sense that this would cause an audible noise.Scope picture attached to this post.I didn't measure with the scope what happens when I pumped the frequency up by changing C9. I'll have to look at that this weekend.Quote from: Ian.M on January 09, 2022, 07:06:21 pmI failed to mention I actually tried removing each ceramic capacitor one by one before making the post. When I removed C10, the whole circuit stopped working as the 500mA fuse at the 12V input blew, so I think it might be necessary to have some capacitance there. I can definitely throw a smaller one in there though and see if that puts a stop to the sub-harmonic oscillations. Not sure what value to use, maybe one such that the RC time constant is about a tenth or twentieth of the switching period?Edit: a couple rough calculations say that would be around 2 pF to have a time constant of 1uS. I'd say stray capacitance is already greater than 2 pF.. Not sure what to do here

author: Benta
author_rank: Super Contributor
timestamp: January 10, 2022, 12:33:45 am
content: I'm 100% with Ian.M.C10 is a really bad idea.Introducing an extra pole in the feedback loop is bound to make trouble.You say that when removing C10 you get a different problem? Well, let's look into that when you've done some more testing.

author: Brumby
author_rank: Unknown
timestamp: January 10, 2022, 01:30:44 am
content: (I'll leave quietly)

author: Terry Bites
author_rank: Super Contributor
timestamp: January 11, 2022, 02:25:45 pm
content: The Gods speakhttps://product.tdk.com/en/techlibrary/solutionguide/acoustic-noise.html

author: amyk
author_rank: Super Contributor
timestamp: January 11, 2022, 02:42:11 pm
content: A blob of silicone or similar dampening material can help quiet loud components.

author: David Hess
author_rank: Super Contributor
timestamp: January 12, 2022, 03:59:26 am
content: If the whole thing is laid out above a ground plane, then C10 is just adding to the existing parasitic capacitance at the feedback point so removing it will not matter. If anything, a small amount of capacitance should be added across R6 to cancel the parasitic capacitance and add some phase lead.Why is R14 there? What kind of capacitors were used for C5 an C6?

author: Mighty Burger
author_rank: Regular Contributor
timestamp: January 17, 2022, 12:11:53 am
content: NEW REPLY - Sorry, forgot to crop an image in the old post and it won't let me modify attachments.Right after I post this I'll delete my other response.Got back for the weekend and I was able to do some experimentation. I think you guys were right with C10 not helping.I reduced C10's value lower and lower, from 10nF to 1nF to 100pF to 10pF to eventually eliminating it altogether. Even a small reduction from the original 10nF to 1nF was enough to make it basically silent, for both a 40kHz and 80kHz switching frequency. I'm not sure why the fuse blew when I tried removing C10 that first time, I didn't experience it this time. Probably shorted something by accident.I did notice a strange pattern develop as the capacitance was lowered. The signal driving the gate of Q4 looked like a series of square waves, one short one and one long one. See attached image "1 Strange Waveform". The blue trace is the driving signal. I'm not sure if this is something to be concerned about. The signal changes shape a little bit with a varying load. Voltage seems to be holding steady.Quote from: David Hess on January 12, 2022, 03:59:26 amIt is laid out above a ground plane, but it's on a 2-layer PCB so the plane isn't as close to the traces as it would be on a 4+ layer board.I experimented adding some capacitance as R6 as you suggested, though unfortunately there were adverse effects. Capacitance across R6 did cause the driving signal to look like a very nice and regular square wave, but it also added what I can only describe as super low frequency oscillations on the output that resulted in large swings in output voltage. See images 2 and 3.With 100pF across R6, with C10 eliminated, and with no load, the output looked like a sawtooth wave with a frequency of 1 Hz and the output varying between 165V and240V!I thought it might help if I added C10 back.With 100pF across R6, and with C10 = 100pF, I saw basically the same thing, but with the output swing reduced to 165V to 200V.With 10pF across R6, and with C10 = 100pF, I again saw the same thing but with only around a 2V output swing around 170V. Still significantly worse than without any added capacitance across R6, but I'd say good enough for nixie tubes.With 10pF across R6, and with C10 eliminated, voltage swing increased to 18V.Strangely enough, having 10pF in series with 100K across R6, and with C10 eliminated, I got what I thought was a good result, but I didn't take down notes as to what it looked like.I think this is what I'll go with. I'll add a couple pads for a capacitor and resistor across R6, and I'll keep the pad for C10 but not populate it just in case I need it later.I'll be honest, it's a little frustrating to me to just do trial and error, trying to judge oscilloscope traces to find what I think looks best. I'm ignoring a whole world of complexity, and using random values I think allows for some instability where I don't realize there is. There has to be a smart way to figure out exactly what components should be used, but I have yet to take a control systems or power supply class so I'm in the dark. I've heard of gain and phase margin but I have absolutely zero idea how to calculate or empirically determine them with this circuit.QuoteR14 isn't intended to help with the filtering at all. It's to reduce the immediate inrush current. You can't see it in the cropped schematic but I have a MOSFET switching the 12V input to this power supply, and I think it requires a lower inrush current. I think C4 should have enough ESR where a resistor is not required.C5 and C6 are boring old aluminum electrolytic.Just a side note, R4 on the other hand was added to help with filtering. If you look at the frequency response of the filter without it, there's a spike right around the filter resonant frequency. R4 helps smooth that response.

author: xavier60
author_rank: Super Contributor
timestamp: January 17, 2022, 04:40:29 am
content: Instead of a conventional compensated error amp controlling PWM, the MC33063 uses a simple on/off method. Despite this, there should not be so much hysteresis. It can be caused by too much lag in the regulation feedback path which is ruled out now by C10 being removed, or maybe too much switching noise getting into pin 5.A small capacitor can help but is has to be placed as closes as possible to pins 4 and 5.It's also best to put the feedback divider close to the IC to minimizes the track length on pin 5, lessening noise pickup.

author: xavier60
author_rank: Super Contributor
timestamp: January 17, 2022, 06:28:29 am
content: Joining pins 1, 8 and 7 might also cause odd behavior because Gate drive current is being sensed as well as inductor current.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2022, 02:27:28 pm
content: Quote from: xavier60 on January 17, 2022, 04:40:29 amI had forgotten that aspect of the MC33063. It uses constant off time so it should be unconditionally stable, but coupling into the feedback pin can still cause problems. Checkold 78S40 application notesfor ideas.Something else to be considered is that proper operation of the MC33063/78S40 requires sufficient ripple voltage on the output to trip the comparator on every cycle. If the output capacitor ESR is too low, this could be a problem. The linked application note from National shows this.

author: Transconductance
author_rank: Newbie
timestamp: January 17, 2022, 05:02:31 pm
content: Probably subharmonic oscillation. This happens a lot in flyback and boost converters. Look for a controller with "slope compensation."What is the purpose of C10? Does it help to take this out? If you take it out and it gets a little better, then you could also try a small amount of shunt capacitance across R6. Like 100pF.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2022, 08:25:21 pm
content: Quote from: Transconductance on January 17, 2022, 05:02:31 pmThat is for current mode controllers. Slope compensation does not apply to constant-off time controllers.

author: Benta
author_rank: Super Contributor
timestamp: January 17, 2022, 08:47:29 pm
content: It's 30 years since I had a project with the 34063, but I found some notes from back then:1: C10 is a no-go.2: The device tends to "squegging" when used without load (squegging is the term we used for cycle-skipping, making the magnetics audible). Attaching a load solves it, as it increases the feedback ripple.