Measuring ripple current in PFC capacitor - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: March 28, 2023, 04:37:48 am
content: Hi,Please help to measure ripple current in an electrolytic capacitor in our SMPS?Its PFC’d, so we want to measure the ripple current in this capacitor over an n x 10ms interval. (n = 1 or 2 or 3 or ……)So anyway, we are checking out a novel 150W offline , isolated single stage PFC converter.I have no schem or BOM as we haven’t payed for it yet. “We are just sampling it” from the supplier.Its kind of like a Half Bridge, but each switching cycle “stroke” switches from a different capacitor.One capacitor that gets drawn from is a 33uF, 500V electrolytic in the primary side. Hence we wish to measure the ripple current (RMS) in this capacitor.I am sensing the ripple current in this capacitor with a 100:1 current sense transformer. Secondary of which is simply a 10R burden resistor. So its sensing at 0.1V/A. (I used a 10:1 probe to measure the burden resistor voltage, and the scope was on 10:1)I had the scope on 2ms/div, and there’s 10 divisions on the scope window.At 100VAC input to the converter, and 150W output loading, the “cycle RMS” feature on the scope showed “0.317V”.…since we are at 0.1V/A, may I assume that this means an AC RMS ripple current of 3.17A? This would seem odd since the capacitor only felt at room temperature (20degC) to touch after I’d switched off.Anyway, at 240VAC, the “cycle RMS” feature on the scope showed “0.103V”….so again, may we assume this means 1.03A AC RMS ripple current?Anyway, I wasn’t happy with these readings, so repeated them but this time with the MATH function…So I did the integral with respect to time (dt) of (C1*C1), where C1 was channel 1, the scope probe channel that I was reading the burden resistor voltage with.Anyway…at 100VAC and 150W, the integral over the 10 divisions of the scope window, (at 2ms/div) gave “167uV^2.sec”.So we divided 0.000167 by 20ms….giving 0.00835. The square root of this was then taken giving 0.091. But since our system is at 0.1V/A, then this comes out as 910mA AC RMS ripple current?This is not the same as the 3.17A that was gotten using the “cycle RMS” feature.How would you interpret these different scope readings?I must admit, i'm surprised the scope just doesnt let you do......integral with respect to time of i^2 between two cursor points, and use the cursor-to-cursor time interval to do the division....then the answer could be square rooted , and job done. (cursor-to-cursor time being one 10ms period).The SDS2104x scope has a memory depth of 28MPts.....so for the actual measurement done today, thats one sample every 20ms/28e6 seconds = every 714ps (since there was 10 divisions on the scope screen and it was set to 2ms/div)...that surely should have been enough samples?The scope used was an Siglent SDS2104x…SDS2104x User Manual:https://int.siglent.com/upload_file/user/SDS2000X/SDS2000X_UserManual_UM0102X-E02B.pdf

author: jonpaul
author_rank: Super Contributor
timestamp: March 28, 2023, 04:54:25 am
content: As always FTTS, oddball and useless questions that can never be answered.We Use Pearson CT, wideband TRMS meterPearson 410,411 CTFluke 8920A, or Clarke Hess 255.Trivial to sketch the ckt topology, no mystery.Suggest to avoid cheap ChiCom instrumentsj

author: ejeffrey
author_rank: Super Contributor
timestamp: March 28, 2023, 05:11:25 am
content: Can you post a screenshot of the scope capture? Along with either the cycle-RMS or the computed integral or both?

author: Faringdon
author_rank: Super Contributor
timestamp: March 28, 2023, 05:30:51 am
content: S(C1*C1)dt(where "S" is the integral sign.C1 is channel 1, the channel we have the burden resistor measured onSorry i dont have a scope shot at the moment.....as you can imagine, it just looks like a scope window with 20ms's worth of "fuzz".....since the switching pulses are at 50khz to 200khz.As you know, the calculation done is done by taking the instantaneous current samples.....then squaring them...then adding these up over the 10ms (or 20ms) interval....then dividing the result by the time interval.....then taking the SQRT of that...but it doesnt appear to work.I admit that i am only measuring the high frequency stuff, since my CT is too small for the 100Hz.QuoteThanks, that Pearson 410 is £600 even second hand.....may i ask if there are cheaper ways round this?Our peak currrent only some 40A, and RMS will be less than 5A.Pearson 410 wideband current monitorhttps://www.ebay.co.uk/itm/155246334800?_ul=UK&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338353466&toolid=20006%26customid%3Ds%253AGS%253Bgc%253ACjwKCAjw_YShBhAiEiwAMomsEMmKtzOuneIkyFA_b7cfOL-2kNKOSO2kzqToz6Ixyl5CP4YqYmFlFhoCQo0QAvD_BwE%253Bpt%253A1%253Bchoc%253A2&customid=s%3AGS%3Bgc%3ACjwKCAjw_YShBhAiEiwAMomsEMmKtzOuneIkyFA_b7cfOL-2kNKOSO2kzqToz6Ixyl5CP4YqYmFlFhoCQo0QAvD_BwE%3Bpt%3A1%3Bchoc%3A1&gclid=CjwKCAjw_YShBhAiEiwAMomsEMmKtzOuneIkyFA_b7cfOL-2kNKOSO2kzqToz6Ixyl5CP4YqYmFlFhoCQo0QAvD_BwEThe Fluke 8920A is true RMS, but will sting us for £800......i think we would have to put the Peason 410 into the scope, and just do the RMS calculation there....either by MATH, or by the RMS function.....it unfortunately doesnt have inter-cursor measurements.Maybe LEM have something...Most of the LEM ones dont have the wideband range required.

author: f4eru
author_rank: Super Contributor
timestamp: March 28, 2023, 05:40:28 am
content: Yeah. Doing electronics without a scope is like to try to drive blind.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 28, 2023, 05:51:57 am
content: Oh I missed the time scale. Can you zoom into something like 2 us / division so we can see a small number of cycles? For that matter does the scope math change when you zoom in?

author: jonpaul
author_rank: Super Contributor
timestamp: March 28, 2023, 02:24:45 pm
content: FTTS,a Chinese scope math Calc is incapable of a wideband TRMS measurementBus ELKO cap ripple current is the TRMS from low frequency ( Mains) and mains harmonics up to any significant HF switch currents, eg 5..10x SMPS frequency eg 500 kHz...20 MHz.Hall Effect Lemo will give some indication but not at HF.With a 4 terminal sense R and HF differential amp, you may get the current, sense ARt cap low side.Another method is A TEKTRONIX current probe eg P6021 with amp eg 134.We found many Fluke, Clark Hess TRMS HF meters and Pearson's decades ago at the Silicon Valley AARL ham radio Fleas.Without the proper equipment to measurement technique, you get useless or,mislead data.JonPS... try a 1 ohm power resistor 10w non inductiveApply small types K TC.Measure resistance Trise after stabilization at max worst case scenarioRemove R from DUT, place in same thermal environnement ( Shielded, no draft same Tamb) and run a lab DC supply up in current.Adjust lab supply till stabilization T of R 1 ohm is same.Voilà, à very basic TRMS current by transfer!Where to send my consulting invoice (:-:)Bon SoiréeJon

author: Faringdon
author_rank: Super Contributor
timestamp: March 28, 2023, 05:32:48 pm
content: Thanks P6021 and Tek 134 is here for £300, i wonder if it will be ok?https://www.ebay.co.uk/itm/325529603894?hash=item4bcb138f36:g:KBoAAOSwg3hj5Phu&amdata=enc%3AAQAHAAAAwG793D4OjPLJgvtUsLf5FAbbduqYIDUJ7zhuWi75gKxuGmxrUHpKqcVBi82Mmuc8u6BMxDhixlDvL95eJUMNmACyD0IPIbc7Y6NvLS3Ms2RArFwggf6DYBVStkKSFObpWbiN1Q0mRjd6vnkKAum1dw9LLqnuKn%2BPujQ8lhPbPh8m4Kxrr1qujWe%2FAlwbZbhC4jsPoPraZdbBmJQA9b3kElZElOokmzRG0xY3qD2MJmCVzQFSZo7MTCIYvFzbVgzP8w%3D%3D%7Ctkp%3ABk9SR8KgrZTlYQ#SHIPPING_SECTION_MODULEQuoteThanks, if i ever make it to 300000 Euros, then i will send you 200 Euros.

author: Faringdon
author_rank: Super Contributor
timestamp: April 03, 2023, 07:16:13 pm
content: So,If you want to measure the ripple current in the output capacitor bank of a Boost PFC, you can either buy a Pearson 410 Wideband current sense transformer (£600) and a Fluke 8920A True RMS meter (£800)…..OR….as in the attached, (LTspice and jpeg) you can settle for a little less accuracy and use the MATH function on a scope….and do an integral w.r.t time (10ms) of the i^2…….and to get the “i” you can use…(1)….A $3 current sense transformer (eg CST2010-100L) with eg a 10R burden resistor…use this to get the switching frequency ripple current(2)….A $1 sense resistor of a few milliOhms or so…..Filter its signal with say 1k and 47nF……use this to measure the 100Hz ripple.When you have the above two readings, then find the total capacitor ripple current by doing the SQRT{(1)^2 + (2)^2}.Which way would you choose? £1400 or $4 ?