L-C filter to get boost converter ripple lower - Page 1
Forum section: electronics / projects

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 11, 2021, 10:53:26 am
content: Hello,I am trying to get ripple a bit lower. The boost converter I am using has 3MHz switching frequency. Current load is between 10mA and 20mA.I was going to use ferrite bead, but after probing the result I realized that the frequency is way, way too high. Basically it is the same as having a 0R resistor for 3MHz switching frequency ripples...A.

author: bob91343
author_rank: Super Contributor
timestamp: March 11, 2021, 06:05:45 pm
content: What does the ripple look like? If it's short spikes, there is a lot of high frequency content. If it's a wave, the content frequency is lower. When you decide what part of the ripple you want to eliminate, you can decide what filtering method would work.Be advised that reducing ripple is not a simple task. Common mode signals as well as radiated energy become important.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 11, 2021, 06:58:23 pm
content: Hi,Thanks for the reply. The ripple is 40mV p-p wave. L-C filter using 10uH+10uF components does help, but very little. Based on LTSpice simulation it should work better, but as you said: ripple can be tricky. Well... hence the question on this forum.a.

author: bob91343
author_rank: Super Contributor
timestamp: March 11, 2021, 09:58:14 pm
content: Not enough information. We need to know the shape of the wave. A 10 uF capacitor is probably worthless at high frequency.

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 11, 2021, 10:35:18 pm
content: https://www.eevblog.com/forum/projects/is-it-ideal-to-place-a-ferrite-on-ldo-output-in-noise-sensitive-application/

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 12, 2021, 05:33:05 am
content: Hi,Good discussion in the linked thread. Let me give more information. The current filtering circuit is very standard starting point:<attached image>I have experiment using a ferrite bead, 10uH and 100nF inductors as L1. They all work and filters effectively high frequency spikes, but ground noise of 20-40mV p-p remains.This is the waveform after L1:<attached image>The boost converter is LT3461A. The attached circuit covers the boost converter circuit only. There is also LDO between payload and the boost converter IRL. However, LDO does not filter ripple basically at all. The ripple is not horrible, but there are 16-bit ADCs. I am trying to get the ripple as low as it is possible in scope of economical efforts. It also seems to be a fact that most ADCs can handle high frequency ripple/noise surprisingly well using techniques available.A.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 12, 2021, 07:08:52 am
content: A real scope trace with bandwidth to at least some 50-100MHz, properly triggered and zoomed to show a complete switching period would be very helpful.Also a photograph of the converter, showing the layout and components used.Quote from: bob91343 on March 11, 2021, 09:58:14 pmThis doesn't make any sense, it's an old myth that still lives strong. A 10uF capacitor is worthless at high frequency only if it is physically large, of high ESL and/or high ESR. An SMD 1210 10uF capacitor, OTOH, would be equally good at high frequency to any smaller value SMD 1210 capacitor, and only slightly worse than, say, a 0805 capacitor.This is also why I'm asking for a photograph. It may be the only thing needed is a better capacitor and layout. Spice screencap showing a polarized capacitor type in a 3MHz converter is alarming. If it is an elcap with long leads, it can't do much in the most important frequency range say 3MHz-50MHz, and the 100n in parallel is too small, it only helps at very high frequencies.If you increase the MLCC capacitance significantly, it starts working at lower frequencies,and still works at high frequencies as well, to the point you can remove the elcap as unnecessary.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 12, 2021, 09:01:25 am
content: Hi,C1 is a 0805 tant cap. The lowest ESR I found. 3461 and C1 packages are next to each other without any space in between. I've attached a photo of the layout. The same applies to a boost inductor, and actually, all the components around 3461. They are as close as I can get them and FB area is as small as possible. As instructed on the datasheet.Let me narrow what I am asking:I would be appreciated of suggestions what could be good values for C1 and L1 in practice when I try to minimize 3MHz switching noise. What else is needed, and what other noise components there are in the ripple, that is much more complex topic.The package of L1 is also 0805. The next inductor candidate I was going to test is 4u7uF with 5R serial resistance. The current L1 I am testing has very low serial resistance. At least in sim small (a few ohms) serial resistance improves filtering capability.A.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 12, 2021, 09:52:38 am
content: Don't use tantalum. Do a fully MLCC solution. You may need to parallel a few capacitors. Get some 4.7uF 0805 caps, X5R or better, you can relatively easily bodge two of them side-by-side in your current prototype C1 footprint, and still have room to solder two more on top of the existing and measure the difference 2 vs 4 caps.You could separate two different phenomena;1) high-frequency switching noise, this looks like sharp spikes twice every cycle, but if you zoom in to these spikes, you'll see they are sinusoidal oscillations where the frequency depends on your loop area (parasitic L) and switching component parasitic C, with such small converter and tight layout it's likely somewhere around 100-200MHz.To fix the switching noise issues, you optimize layout, slow down switching (increasing losses as a trade-off), add snubbers, and, finally, filter. This is the frequency territory where ferrite beads may be useful; filters can be small because frequencies are high.2) Ripple, this follows a clear pattern at your 3MHz switching period. Two reasons:* Capacitor charging and discharging during the cycle,* Capacitor ESR, inductor ripple current (which is inevitable and part of your design) is directly reflected as a voltage dV = dI * ESR.To fix the ripple issue, minimize capacitor ESR and maximize C. MLCC is often the only practical choice with near-zero ESR. Filtering is possible, but requires large filters; ferrite beads are of little use. You can create an additional LC stage but it will be of similar size to what the converter already has; after all, it's filtering the fundamental switching frequency.But first, getting an actual oscilloscope and measuring whether it's switching noise or ripple issue would be paramount. It would be beneficial to have at least 200MHz of BW, but even with a 50MHz scope you likely can separate the two issues, it may be just impossible to get the real amplitude of the switching noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2021, 01:11:28 pm
content: No ground plane under/around that layout? Can you zoom out, and also show where you are probing from?Tim

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 12, 2021, 01:35:51 pm
content: hello,Siwastaja:Thank you for the valuable and informative reply! I replaced the C1 tantalum by two X5R 10uF. Waveforms attached. The first one is 3461 output and the second one is output of the LDO (which is VDD for MCU/ADC/etc). Spikes are most likely coming from ADC (250kHz).T3sl4co1l:Around no. Opposite side of the PCB. Yes. All over.br,A.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 12, 2021, 05:26:41 pm
content: If I read your timescale right, I think I clearly see the 3MHz (period = 333ns) ripple, but no HF switching noise. What is the BW of your scope?In any case, reducing the ripple is now all about decreasing ESR and increasing C of the capacitors. L can also be increased; this reduces ripple current and eases the job of the capacitors. But there will be practical limits, it's not a good idea to go physically massive inductor. You could pick the largest L available in that footprint, with high enough saturation current, and low enough ESR not to cause excessive power loss.You output current seems very low to me. This suggests a larger L than in appnote might be desirable, to keep ripple current some percentage (typically like 40%) of the output current, not some percentage of the maximum the converter could provide, as usually shown in suggested appnote circuits.A higher bandwidth LDO may also solve the ripple issue. Some modern types have quite PSRR ratings still around 10MHz. Look at the PSRR curve. Although, it seems to me, your current LDO is already doing good job.

author: bob91343
author_rank: Super Contributor
timestamp: March 12, 2021, 06:11:14 pm
content: I have measured a fair number of electrolytic capacitors and none of them are effective above 100 kHz or so. My selection of parts was not orderly, simply parts scavenged from defunct equipment. (That's where I get most of my stuff.)So presumably there are electrolytic capacitors better than these but I haven't encountered them. I would like to see some test results with a more organized selection to refute my findings.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 12, 2021, 06:53:24 pm
content: Thanks everyone.For the record, LDO is MIC5225br,A.

author: fourfathom
author_rank: Super Contributor
timestamp: March 12, 2021, 07:43:00 pm
content: Re MLCC frequency characteristics, they are truly much better than electrolytics. Here's a chart for a Samsung 10uF cap (X5R, 25V, 0805):(well, that chart looks like crap. The upper line is |Z| and the lower ESR)

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 12, 2021, 08:31:16 pm
content: And the BW of the scope is 100MHz

author: Siwastaja
author_rank: Super Contributor
timestamp: March 13, 2021, 08:48:32 am
content: Quote from: axel15 on March 12, 2021, 06:53:24 pmThis part is unsuitable for the purpose of high-frequency filtering; PSRR is only defined up to 1kHz. Well obviously it does work; even the input and output capacitors of said regulator circuit, a piece of wire, some series resistance would filter as well. And looking at your traces, it really does filter.But there are much better suited modern linear regulators that do have PSRR ratings in your frequency region of interest (say up to 10MHz).Quote from: bob91343 on March 12, 2021, 06:11:14 pmDepends on the package size and lead length. They can be useful for higher frequencies than that, very widely and effectively used in some 200-300kHz converters, but definitely not much use for a 3MHz switcher. Besides, a tiny 3MHz switcher doesn't need that much capacitance, you simply don't have to go electrolytic. Except maybe for a parallel damping circuit which doesn't take the bulk of ripple current and doesn't hence contribute to the ripple voltage.Are you sure you are not misunderstanding the "effectiveness" as: "part is not effective beyond the SRF"? This is anextremelycommon mistake to make, even supposedly experienced designers do this brain fart so I don't blame anyone, just sayin'. SRF is just the point of lowest supply impedance. You need what you need. If you need Z < 0.1 ohms, and SRF is at 0.01 ohms, the whole region where Z < 0.1 ohms is "effective" for you, and this goes on both sides of SRF.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 13, 2021, 09:11:54 am
content: Siwastaja:Can you suggest an option to replace MIC5225?What material and type works for L1 (the filtering inductor)? Like Multilayer, Wirewound, Ferrite, Metal and so on...br,A.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 13, 2021, 09:15:39 am
content: >Don't use tantalum. Do a fully MLCC solutionReally? The board is full of tantalums. Replacing them all by ceramic ones? I have used ecaps/tantalums for 40 years... a big change in the mindset... :-)br,A.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 13, 2021, 10:50:00 am
content: Don't use components due to mindsets/feelings, use engineering practices like calculations; sometimes you can handwave without calculation but that requires deep understanding. Also don't do random replacements. Capacitors are notoriously difficult, you can't buy ideal capacitor, not even close, all types have some specific use cases (well obviously they wouldn't be manufactured otherwise), tantalums included so they are not "bad".The #1 reason nowadays to use tantalum caps tends to becontrolled high ESR. Another, maybe less significant today than 40 years ago, is large capacitance in small package, and fairly stable capacitance value. Hence they are good for analog circuits requiring large C values, like very long time constant timing / filtration circuits, at least when compared to aluminum electrolytics.So tantalums are used for example to stabilize a control loop of an old-style LDO linear regulator which is unstable with too low ESR (minimum ESR and recommended output cap type is specified in the regulator datasheet). For small tantalums, ESRs are available in about 0.5 ohms to some 10 ohms. It's typically specified, and it's fairly accurate and stable!But your boost converter requires absolute lowest ESR possible, tantalum output cap is the polar opposite of what the circuit requires. Hence don't use tantalum for the boost converter output. With MLCC, you'll easily get down to 20 mOhms, resulting in less voltage ripple and less heating in the capacitor.Also note tantalums are tricky, need a lot of voltage derating and blow up from current transients. Boost converter output has harsh current ripple as well. Tantalum could be safely designed in but requires extra effort and probably makes little sense.After filtering, andafterthe LDO, a tantalum is possibly still a very good choice in parallel to all those small 100nF ceramic bypass caps, for dampening. The LDO would also then take care of current limitation, protecting the tantalum from inrush current.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 13, 2021, 12:27:48 pm
content: Thanks Siwastaja. So in summary, I can go safe and just use low ESR ceramics replacing all the tantalumsin the circuit like this.To be honest, I didn't know that there are LDOs that can handle MHz level. I thought that I need to take care of noise before LDO and that what I was doing - hence the initial question. I noticed that even when those LDOs do exist it is not common that they have MHz-level PSRR. Actually, I only found two that suits to my design. On order... lets see what happens.I am a little bit uncertain which capacitor type is best for the L1. A few ohms serial resistance, WireWound, Ferrite.... Does this look something that is not completely based on feelings?https://www.digikey.com/en/products/detail/taiyo-yuden/CBL2012T470M/957991br,A.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 13, 2021, 12:28:35 pm
content: BTW: great comments and advice!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 13, 2021, 12:58:59 pm
content: If I understood you correctly, your L1 is the actual energy storage inductor of the switch-mode converter. Loss in unwanted here. At f_sw = 3MHz, the type is critical, otherwise it may have so much AC loss that the converter has hard time boosting at all, or at least has very crappy efficiency. So L1 needs to have low AC loss (core loss + AC copper loss), a type designed for high-frequency switch mode converters. It would be great if more manufacturers published AC loss calculators, but at least Coilcraft and Wurth Elektronik do. You can plug in your boost converter numbers in their web-based tool and get the DC and AC loss figures directly in W for their products. Based on this number, you can calculate efficiency.Failing to have that information, go for:* Ferrite core inductors (this isn't a guarantee though because lossy ferrite materials exist, the ones used in "ferrite beads" for example)* Shielded types with closed magnetic path, definitely not rods or like that. What you linked is likely kind-of a rod.* Inductors that mention switch-mode supplies in typical applications,* Physically small parts* NOT parts described for "noise filtering" etc.The inductor you linked is described like this by the manufacturer:"They are suitable for an anti-noise measure on the power supply circuit of DSC, DVC, HDD, LCD-TV, mobile phones,PC, game equipments, various communication equipments and etc."This means, they are power filtration inductors, where AC loss is a desirable feature. The converter inductor has the opposite requirement. I wouldn't use that type. You could use this type in the post-filter, though.Low-loss components in power conversion. Lossy components in high-frequency filtration. As a rule of thumb. Works for both inductors and capacitors.Note that for a robust design, the inductor saturation current should be at least the value of thecurrent limitof the controller IC. This sucks when designing very low-current converters because of the non-availability of converter ICs for such low currents. LT3461 is rated for switch current limit = 600mA max, this is on the low side of the market already so we'll have to live with that. If you want to guarantee no blown up converter under all edge cases, the saturation current of the inductor should be around 600mA. This stinks when your load current is 20mA so with dI_L = 50%, maximum current under steady-state is just 30mA, so you'd need so much extra saturation current rating just to guarantee no saturation under edge cases.You likely can do with a bit below that, though, in practice; saturation is gradual, and if the converter's current limit is fast, it can react properly even if you have just a tiny bit of inductance left. In other words, one example circuit shows a 4.7uH inductor with a 3V3 supply; the current limit clearly works with such little inductance. If your inductor is 47uH, even if it saturates and loses 90% of its inductance, you still have enough inductance for the current limit to prevent the IC blowing up. DC resistance of the inductor also limits the current. But this is still something to consider when picking an inductor with lower saturation current than the worst-case switch current limit.Inductor calculation itself is quite simple. For example, let's assume a voltage-doubling boost so D = 50%. Let's say it runs at 1MHz. Hence, period is 1us. D being 50%, inductor is charged during 0.5us and discharged during 0.5us. Let input voltage = 5V and output current = 100mA, and let inductor current ripple percentage = 40% of that = 40mA. This means, inductor current varies between 80mA and 120mA. To charge inductor by 40mA during 0.5us,dI/dt = V/L -> L = V/(dI/dt) = 5V / (40mA/0.5us) = 62.5 uH. You can repeat the same calculation for discharging the inductor during (1-D) and should get the same result, a great sanity check. Just be sure to use the correct equivalent circuit for on and off states so that you understand what voltage is applied over the inductor.For small output currents, you need higher L given the same ripple current percentage. Which wouldn't be a problem, because you can accept lower saturation current. Or could, if you had a driver with adjustable or small enough current limit. Oh well... Often you need to compromise a bit here.Some modern LDOs indeed can filter in the range of approx. 10MHz. This is great because filtering such "low" frequencies with passives more or less sucks. With passives, resonances are always a real concern (hence the recommendation of lossy types of inductors and capacitors).

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 13, 2021, 02:27:41 pm
content: Hi,Sorry for not being clear. You are giving such an comprehensive answers! L1 is indeed for the LC circuit and not for the 3461. I am referring to L1 as it was the filtering inductor on the image a few posts earlier.For the record, the inductor I am using with 3461 is this:https://www.digikey.com/en/products/detail/murata-electronics/LQH44PN100MP0L/2056193?s=N4IgTCBcDaIDIEUASAWFAFAcgRgAy4Fl1c4QBdAXyABut you answered to the actual question too: the linked 4,7uH one is a good choice for the L1. Just that I started to wonder how much different material and coil types matters how LC filter performs in this application and with the noise/ripple I am dealing with at the moment.(I would not use the linked 4,7uH with the 3461. I am not that far from the map :-) )br,A.

author: AccountRemovedPerUsersRequest
author_rank: Regular Contributor
timestamp: March 22, 2021, 07:13:27 pm
content: hi,* new (L-C) L1 filtering inductor* LDOs changed* full MLCCScreenshot after LDO.br,A.