Noisy DC-DC output under load - Page 1
Forum section: electronics / projects

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 26, 2015, 09:52:52 pm
content: Hello!Today is started to build parts for a thingy i am attempting to build for myself.The first step is one part of the power supply, a Microchip MCP1640 boost converter.http://ww1.microchip.com/downloads/en/DeviceDoc/22234B.pdfIt should provide 5V with at least 300mA (500mA peak), here is the schematic.Dont ask why there is a 0R resistor in there populated with a 10K on the board,it serves no purpose.The board:The PCB is not my best work... and the input cap is at the wrong place... :-/But i think it looks worse then it is...And the test conditions:The board was cleaned with IPA after soldering with a Ersa Analog 60 at 300°.Every connection was checked for shorts, high impedance and defects.The board was checked with high magnification before soldering, it was a bit under exposed as you can see.In the above picture you can allready see the problem, the voltage drops slightly with a connected load and becomes very noisy. There are some pretty bad spikes on there.To keep it simple and because i do not have another load a 22 Ohm power resistor is used.Thats about 220mA load on the output at 5VI do not suspect this to be a normal behaviour, i tried to increase the output and input capacitance with no change.The inductor has a value of 10µH, the maximum stated in the datasheet.Since i do not have that much experience, what could be the most likely culprit?Wrong inductor, bad layout or perhaps no failure at all and this is normal for a cheap DCDC converter like this?Greetings,Peter

author: rx8pilot
author_rank: Super Contributor
timestamp: February 26, 2015, 10:54:29 pm
content: consider.....Moving the output caps and rotating them 180 to get the output of the controller pin feeding directly into the caps. Essentially swapping them with the feedback resistors. Move the 'useless' enable resistor closer to the input so it is out of the way. Rotate the output connector to match the new position of the output caps. On the output caps. connect EACH one to GND, dont chain them together and tie only at one point.For measuring, to get a useful indication you really need to use a VERY short ground lead. I use GND springs and measure across the last output cap. Using a clip-on ground lead is very mis-leading because it picks up all the radiated noise from the SW node.

author: dom0
author_rank: Super Contributor
timestamp: February 26, 2015, 11:06:08 pm
content: At 100 mV/DIV you already need to consider common-mode noise. Also, much shorter leads, forget the alligator clip. You might also want to use shorter power leads and twist them to avoid inductive coupling from that. Connect your load with short leads and twist those, too.If the noise persists you need to find out what's the noise source. Make sure it's not the supply and not common-mode noise. Check if it's (harmonically) related to the switching frequency.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2015, 01:32:46 am
content: Also, measure the noise in steady state conditions, under load, at an appropriate time/div setting. If you wish to observe noise superimposed on a slow signal, use Peak Detect mode.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: February 27, 2015, 02:43:35 am
content: Example of a low-inductance SMPS measurement.

author: jaxbird
author_rank: Frequent Contributor
timestamp: February 27, 2015, 07:53:34 am
content: As already mentioned, try optimize the layout, more ground, less isolation. And use ground spring for measurements.Quote from: rx8pilot on February 27, 2015, 02:43:35 amPlease cut your nails

author: Mad ID
author_rank: Regular Contributor
timestamp: February 27, 2015, 07:57:24 am
content: This is not how it's done..you are seeing noise which isn't there.Try this:I suspect the noise will decrease in worst estimate by 10x.Cheers.

author: tautech
author_rank: Super Contributor
timestamp: February 27, 2015, 08:53:34 am
content: Follow the layout in the datasheet as close as you can.There is also a AN1311 from Microchip.Be sure to getset it up inPFM mode version. (MCP1640C)In one I used for a project, I had to go to 100uF SMD Tant on the output cap to get ripple low.I used the reccommended input values and a spec'ed Coilcraft inductor.I'd add that your GND pours are not up to it IMO.EditDo any tests powered from the battery you intend to use.Short leads.RTFM

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 27, 2015, 03:41:14 pm
content: Hello and thanks everyone for the reply!Quote from: rx8pilot on February 26, 2015, 10:54:29 pmI tried to keep as close to the reference layout in the datasheet as possible, that is whay the caps and feedback resistors are oriented that way.But i shuffled a few components around and gave it another try.Quote from: tautech on February 27, 2015, 08:53:34 amRTFM only gets you so far, i have to learn this stuff by mself i dont have annyone helping me out except the internet and datasheets. I have no electronics training ;^^But i am more then willing to learn, lets toss everything in the bin and give it another go.The battery is a genuine, new, almost fully charged (3.9V) panasonic NCR18650PF LiIo cell.I started with the bench supply because it shows me the load and if i short something i got a bit of a saefty net. The fairly big LiIo cell will do bad things when shortedThe ground pour is not that good, yes, i cant do very fine pitch stuff at home.This regulator is a late add-on, i dit not order a proper board yet.The board is double sided but i dont have my drill press at the moment, i only shorted the top and bottom at one point. The backside is one solid copper plane.Quote from: T3sl4co1l on February 27, 2015, 01:32:46 amNo load connected, powered from the LiIo cell, measured as shown in the picture above:Closeup of the spikes:And this is how it looks with the 200mA load connected (button pressed):From what you told me, i would say my board/layout is insufficient.I build a few DCDC converters with a LM2575 buck converter and dit not have such problems.It seems this boost converter is more sensitive?I have read a good deal about this stuff, but... well...Greetings,Peter

author: dom0
author_rank: Super Contributor
timestamp: February 27, 2015, 03:51:48 pm
content: What switching frequency does that converter have? Those spikes look very dubious to me at 10 MHz repetation rate and a bandwidth somewhere in the 80 MHz region... might be radio pickup.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2015, 04:10:48 pm
content: Those look quite typical for the transients you should observe due to coupling through the filter network, and induction due to strays. I would say your layout and probe technique are adequate.Do take the time to ground the backside, though. Take a drill, poke a bunch of holes in the ground around the circuit, and short front to back with some sort of via (a short stub of wire, soldered on both sides, or proper rivets). Priority to locations near large current flows: beside the controller chip, the input and output bypass caps, and on both sides of long traces to stitch the resulting split in the top side ground. That should improve the transients somewhat, and more importantly, allow you to filter the residual by isolating common mode voltages.You can also bring the traces on top and use Manhattan style construction over solid ground plane.Finally, by bringing the input and output terminals some distance from the switcher, you bring them outside of the noisy zone. Now you can apply a few stages of LC filtering to remove the noise, and since it's filtering against solid ground plane, at a common point, you have no common mode voltage to worry about.Tim

author: rx8pilot
author_rank: Super Contributor
timestamp: February 27, 2015, 06:20:04 pm
content: Quote from: jaxbird on February 27, 2015, 07:53:34 amThose are not nails, those are multi-tools. Chisel, tweezers, screw drivers, manual pick-n-place machine.Quote from: T3sl4co1l on February 27, 2015, 04:10:48 pmI would agree. The ringing you see is from the very fast rising and falling edges of the SW node interacting with any parasitic inductance and capacitance in the PCB and the devices. With edges that rise that fast, tiny amounts of inductance and capacitance is enough to induce oscillations - nearly all SMPS designs will have this. It looks like the ringing is very high frequency. Turn on the bandwidth limiter on your scope to 20Mhz to see how much that attenuates the ringing. it will probably read much lower, in which case you can simply add some small value ceramics on the output to damp the very high frequencies - maybe .1uf or so. You could also add a somewhat lossy RC snubber across the SW and OUT pins. This would be a small value resistor and a small value cap to damp the ringing at the source. A snubber can slightly damp or over damp the oscillations depending on how much efficiency you are prepared to lose.http://www.ti.com/lit/an/slva255/slva255.pdfIs a snubber primer.

author: tautech
author_rank: Super Contributor
timestamp: February 27, 2015, 07:03:48 pm
content: Much, much better layout and performance.Try just a couple of wire links soldered to the backplane.Engage AC coupling on your scope for ripple measurement.Use BW limit and turn on some averaging.DC trigger.That should get displayed ripple to real world values.Top work for at home.

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 27, 2015, 09:56:23 pm
content: Hello and thanks for the replys, everyone, you rock!Edit: Mixed up AC and DC coupled, all measurement taken AC coupled.Second edit: I will have to study what common mode noise is as well, forgot about that.Quote from: T3sl4co1l on February 27, 2015, 04:10:48 pmQuote from: rx8pilot on February 27, 2015, 06:20:04 pmQuote from: dom0 on February 27, 2015, 03:51:48 pmAnd we have a winner, i just got home and went straight to the bench an scope.I had a closer look at the output, the "spikes" are fairly consistent but not uniform in amplitude.Both channels are AC coupled, i took a measurement and had a look in the datasheet, the switching frequency of the converter is:And a closeup of the adjacent two:And a even closer look of those two spikes:And that is were i thought to myself "Damn, you have seen something likt this before somewere".I got the second probe out, AC coupled both channels tweaked a few knobs and took a big bite out of the table edge.That is the inductor switch on channel one and the output on channel two.And for a closer look:Now i will have a read about output ringing, filters, snubbers, bandwith limit, averaging and so forth.That will keep me occupied for a good amount of time.I will get back to you when i have modified the circuit.I can not drill holes for the vias right now. I will not put one of my tiny carbide drills in the cordless drill nor ruin one of my HSS on the FR4.But i might have some cheap diamond mills for the dremel... will have a look tomorrow, can not dremel at night. Well i can but lets just say, i will not do it again ;^^Quote from: tautech on February 27, 2015, 07:03:48 pmThanks

author: tautech
author_rank: Super Contributor
timestamp: February 27, 2015, 11:18:11 pm
content: That looks better.Yep, until you see the 1640 advertised frequency, (500 KHz) the DSO is just telling lies.For your project and presumably longer wires to it, plus the added bulk capacitance on the rails of the powered curcuit the ripple will be much less.Engage averaging and BW limit on your DSO to that the displayed waveform more accuratly reflects "real world" use.Probeing more remotely will also help.Consider moving the load resistor to the end of a cable and add some bulk capacitance there too.Probing there will reflect end usage.

author: tautech
author_rank: Super Contributor
timestamp: February 27, 2015, 11:51:38 pm
content: The only 1 or 2 holes you need to drill will not ruin a HSS bit.Or solder a few links over the edge to link GND planes.That should quieten things down a bit more.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2015, 01:17:14 am
content: I've used carbide PCB drills by hand before... but I wouldn't recommend it unless you have a set of them on hand.Or use the HSS and accept the dulling. Typically you'll get about 100 holes from an HSS bit before it's too dull to use on FR4, or much of anything else. So, a few here and there, not a big deal, but it's definitely more wear than drilling metal.Those spikes are definitely pushing the BW of your scope, so expect they are actually quite a bit sharper, more squiggly, and probably a little taller than you're setting here. If you zoom the rising/falling edge of the switching node, figure it looks like that, but the full slope, differentiated and attenuated, not just a rounded hump.Tim

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 28, 2015, 01:52:01 pm
content: Hello everyone and a big thank you again for all the replys!Quote from: tautech on February 27, 2015, 11:51:38 pmQuote from: T3sl4co1l on February 28, 2015, 01:17:14 amI use the big carbide bits in the cordless drill on occasion, but the 1mm or 0,6mm bit will shatter in a million pieces as soon as it hits the FR4 when used in a cordless drillBut i found an old HSS bit, i keep the good ones very sharp, dialed the dremel to 11 and...And then the moment of truth, AC coupled i am down to 20mV/Div:Thats from >700mV Vpp yesterday down to 90mV Vpp after drilling a few vias to the bottom groundplane and adding a 0,1µF ceramic close to the output.That is with the BW limit on:And with BW limit and averaging:And here is were i run in a bit of a problem, there was a reason behind this beyond "looks bad" on the scope.For now i am only designing a mobile power supply, i do not know what in perticular it will power in the future.https://www.eevblog.com/forum/projects/one-lithium-battery-to-rule-them-all/msg617779/I have a few analog circuits wich may run of this at some point and in the past ran into a problem.I was trying to detect a short impulse from a opamp, wich looked fine on the scope.But the circuit was triggering the edge interrupt on the µC when it should not.It took me some time to realise it was caused by voltage spikes on the power supply (USB), they were that narrow they dit not show up on the timebase i was using. I ran the circuit only from the USB supply because i was programing it.This is why i was one reason i was concerned over these big spikes.I have a problem understanding how/why/when those could produce a problem in the future.Looks like i have to dwell on this some time, i have a problem understanding the bigger picture at the moment. Too much information at onceBut thanks again to everyone, you helped me out a lot!

author: Christopher
author_rank: Frequent Contributor
timestamp: February 28, 2015, 02:24:18 pm
content: The real noise is probably something closer to the 2nd or 3rd screen capture. This is due to the output capacitors charging, and the other crap on there looks like pickup. 20mVpp looks more than good enough for your applicationWell done. See any mistakes like this as learning experiences! Now you know for next time what a good power supply ripple should look like.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2015, 02:45:54 pm
content: The main reason power line spikes can cause problems (like false triggering) is, the sensitivity of the circuit is very different at high frequencies -- usually much worse.The most basic is, high frequencies don't like to stay in wires, so they tend to radiate and re-radiate throughout your circuit. You can have a noisy circuit that's putting out 100mV spikes like that, and measure the same noise literally everywhere in the circuit: even with the probe ground clipped to the probe tip. The proof is that, with the "grounded" probe, you see nothing when it's completely disconnected from the circuit, but you see the signal when you touch ground to ground. (The signal is physically present on circuit ground, as a common mode signal -- it's getting into the probe across the ground clip inductance.)Riddle: when is ground not "ground"? Answer: all the time! Ground is a convenience of definition; the real world has no need of our silly conventions.Once you get common mode noise, it can couple into all sorts of nodes, internal or external. So a little noise on the supply can directly or indirectly affect signals as well.The cure for common mode noise is to find any source of unbalanced noise, or cables spanning a noisy ground loop (yes, even a solid ground plane can have a voltage drop at RF). Some remedies include bypassing both leads to local ground (say if you have the PCB on a supporting metal frame: take advantage of the mounting screws, and use that local ground with some capacitors and inductors on the leads), or using ferrite beads to reduce the amount of CM current going down the cable (but, ferrites are not a cure-all -- they're a finite impedance, and do the best job against a low impedance -- between bypass caps, say).Analog circuits are susceptible three ways:- Really strong noise causes the input protection diodes to conduct, rectifying it into a DC offset. This applies for anything from ~MHz to GHz+. If you've added input protection diodes, take the time to add EMI filtering (usually a series R and parallel C, or if you can't tolerate much or any R, use a ferrite bead or chip).- Even if you haven't added input protection diodes, there is typically a rectifying effect in the circuit. Bipolar amplifiers are notorious for rectifying RF in a similar manner. LM358s -- and TL431s -- often need extra filtering to work accurately within noisy circuitry.- The PSRR (power supply rejection ratio) of an op-amp typically drops -20dB/dec past some point, sometimes actually achieving positive gain (i.e., negative PSRR) somewhere in the MHz. Another way to put it: such high frequencies are basically passed around untouched by the amplifiers. If you need clean outputs, make sure you have clean inputs. Power supply noise could also cause rectifying effects on internal junctions, still more reason to filter.Digital circuits can trigger on events much faster than intended. MCU interrupts should have a couple cycle pin debounce, but maybe you were using one that was true pin change only, without this feature. Power supply noise, or anything coupled to the input, can push the pin over the threshold, especially if it's being biased near the threshold, say by an insufficient pull-up or something. (Needless to say, don't use pin-change interrupts with small voltage swings, say if you wanted to use one as part of a sigma-delta ADC.) A rapid change in VCC/VDD can also potentially corrupt internal states, double-tapping gates or flip-flops.Tim

author: Kalvin
author_rank: Super Contributor
timestamp: February 28, 2015, 03:02:31 pm
content: It seems to me that you have nice extra space on your PCB for a LC filter connected to the output of your current design.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 28, 2015, 07:32:28 pm
content: There are inductors available in 1206 and 0805 that could help reducing your overall footprint so that a second filter fits about the same area. Not sure if that'll work wth 500 kHz, but on 2 Mhz they work fine. Low current and inductance of course, don't expect Amps or 10's of uH.If you do not have a lot of extra space, a series ferrite choke can help you with reducing the energy of the high frequency peaks seen in the captures.

author: PeterFW
author_rank: Frequent Contributor
timestamp: March 02, 2015, 08:57:10 pm
content: Quote from: T3sl4co1l on February 28, 2015, 02:45:54 pmThank you for the explanation, slowly i am geting thereSome of the stuff i had to research is a bit over my head but i think for the most part i got a grasp on why, how and from were the problem occurs and what to do to get rid of it ^^Quote from: Kalvin on February 28, 2015, 03:02:31 pmQuote from: Jeroen3 on February 28, 2015, 07:32:28 pmI replaced the inductor with a shieldet and slightly smaller one for the final board iteration and addet three 0805 pads for either caps, incuctors or ferrite beads in a t-filter configuration.That way i have all fronts coveredGreetings,Peter

author: Siwastaja
author_rank: Super Contributor
timestamp: March 03, 2015, 01:28:01 am
content: Snubbers can be great in reducing the noise at its very source. In some cases, you can do without snubbers. Sometimes, you'll need a small one to prevent your MOSFETs from blowing from voltage spikes. Even a very small one, with practically no power loss, can do wonders. Sometimes, you can go quite far in reducing EMI, if you don't mind the efficiency loss.(I may state the obvious, but for me, it took time to understandwhat a snubber IS, there's a lot of false information. It has absolutely nothing to do with: switching (rise/fall) time, switch losses, switching frequency, inductor inductance / spikes... It really is there to dampen the energy circulating in theparasiticinductances in layout and capacitors, which is happening much beyond switching frequency and independend of it (typically at something like 30 - 500 MHz)).You may need to add a snubber over both switches (diode and MOSFET), so one between GND and switch node and another between V+ and switch node.The attachments show what a snubber can do (pink = switch node, yellow = analog current measurement signal). This almost perfect snubber resulted in a 1% extra loss at full power (it's a 100W synchronous bi-directional converter). Without a snubber, it sometimes blew a FET; with a snubber, switch node overshoot went from 60% to practically zero. The amount of EMI also distrupted the analog control side; snubber solved that, too.Boost converters have the problem of diode reverse recovery, causing "unlimited" short circuit currents, and a huge EMI spike when the short circuit suddenly disappers. Snubber helps over the diode, but what you really need is a diode which is specified as both ultra mega giga fast reverse recoveryANDsoftreverse recovery (snubber may still be beneficial.)You can find instructions how to properly size the snubber components by experimenting with the circuit. I have found that the best way is just to try out different values until you reach the optimum point. In this case, I used a certain sizing algorithm to find "optimum" snubber of 2.2nF + 4.1 ohm, and while it did remove some of the horror going on (to the point of stopping the FET blowage), I finally settled for 47 nF for almost perfect waveforms, because I don't mind a bit of extra loss in an otherwise very efficient circuit, if I can increase reliability, and the accuracy of the analog side. R doesn't affect the amount of power loss, but the loss happens in the resistor so make sure it doesn't overheat. I experimented with different values and found the optimum result at 2 ohms. The power loss is 0.5*C*Vp-p^2 * switching frequency.So just put something like 2.2-4.7nF + 4.7 ohm in series over the switches and start experimenting. Increase C until you have what you need or you are over your snubber power loss budget. Then play around with the R value a bit. It may do absolute wonders (or may not).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2015, 04:31:29 am
content: Quote from: Siwastaja on March 03, 2015, 01:28:01 amThere are also places where youcan't. It would be nice to add some supply inductance to those "integrated switch" chips (synchronous or otherwise), but sadly as they draw VCC from the same node, you don't have much choice but to follow the same tired advice upon which those chips were designed... "minimize inductance".On the upside, they're usually compact enough that this is okay, but -- they keep getting faster and faster, too, making their own problems worse.QuoteThat's not true; snubbers serve all three purposes. Much increase in rise/fall time is usually avoided, but you'll still see it on large IGBTs where the switching speed is low anyway (e.g., large VFDs).Voltage/current peak clamping networks fall under the domain of snubbers. Often, all three are effectively combined, for example using a dV/dt snubber on a flyback supply to increase risetime (giving the transistor more time to turn off, reducing turn-off losses), clamp the peak overshoot (due to leakage inductance), and dampen ringing (due to both LL and Lp).QuoteYup. The general idea is to dampen the stray inductance (or snubber inductance, if you're tuning it to the application). Which, if you don't know where it's concentrated (or you know, and it's distributed, so it's impossible to dampen the full thing at any one point), one across each device is necessary.QuoteIf you can't avoid the recovery (and sometimes, even schottky diodes are bad -- because their sharp C(V) curve ends up looking an awful lot like reverse recovery, anyway!), you need to add loop inductance (which reduces dI/dt, recovery charge and peak recovery current). Which, in turn, has to be snubbed. You can avoid some if you use a saturable reactor (old school!), which simply adds some delay between switching and recovery without storing too much energy.Also works for interference type synchronous switches, which is good for bidirectional conduction where you can't tolerate a body diode being accidentally forward biased for 10 nanoseconds (and maybe bringing snap recovery with it!).QuoteI have quite good luck estimating inductance by layout, and sizing the RLCs according to that. Design equations: C = 2 to 5 times Cjo, R = sqrt(L/C). Cjo being the average capacitance of whatever junction is open at the time / what the R+C is connected across. For a dV/dt snubber, these values should still be typical, but the L may be a different value (e.g., Lp instead of LL). For a peak clamp, use much larger C (enough to charge by perhaps 10% of total supply?), and R = Tcyc / (10*C) (i.e., a few R*C time constants to allow the capacitor to mostly discharge during the idle period).Fine tuning is done by adjusting R and C up and down by 1.5-2x per step; not usually worth getting any more particular than that. A good approach is to use an overly large capacitor and find the correct damping, then drop C until it just starts getting worse, then fine tuning the resistor one last time. You'll usually be left with some overshoot (going for a rock solid edge usually costs too much dissipation, but can be done).In situations where two inductances are at work (like the flyback example), you can use two R+Cs, to dampen LL and Lp seperately. If not using a dV/dt or other snubber, of course.Tim