Boost PFC current  sense resistor inductance. - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: August 18, 2022, 01:02:02 pm
content: Hi,The company have refused to pay for my 1kW L4981B (200-265VAC) CCM Boost PFC PCB (prototype) because i have used five Wirewound 1W Axial resistors in parallel (0.22R each) for current sense.They note that the (below) resistors are not quoted "non inductive"However, the inductance is minimal, and in the PFC application, AYK, the current sense does not go into a comparator, but into a compensated opamp (current error amplifier), and as such, will not cause problems.The RC feedback compensation network of this error amp effectively filters away any minor inductive spikes.I will also put a 1n on the opamp input pin.Please might you back me up here?0R22, 2W resistor, Axialhttps://uk.farnell.com/multicomp/mcknp02sj022ka10/res-0r22-5-2w-axial-wirewound/dp/1903758?st=mcknp02sj022

author: jonpaul
author_rank: Super Contributor
timestamp: August 18, 2022, 01:38:39 pm
content: we avoid WW resistance in CS, the inductance will affect the signals. Check on a high frequency RLC bridgeEasily finding low inductance 4 term CS 0.01 Ohm, just searchexpect to pay a reasonable pricej

author: mc172
author_rank: Frequent Contributor
timestamp: August 18, 2022, 02:11:08 pm
content: This is just nuts, what kind of reality is this? Why can't you just fit a different resistor, one that is non-inductive? I think it's more likely that, assuming you are in person how you are here, they've heard what you have to say and respond to engineering discussion and won't buy your power supply because they don't want their building burning down, or any of their customers properties suffering the same fate.Why is everything so difficult with you that we need to see umpteen threads about really simple engineering decisions every day?As we know, you're just going to like this post and not reply to any of this, and would you agree that you'll probably start another tangentially related thread about it if you don't get responses that match your narrative?

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: August 18, 2022, 04:04:10 pm
content: Hey Treez, how's it hanging?I don' t have my scrying mirror here nor a good answer, so I'll just leave a music video with questions about you that I ponder every time you post. Hope it resonates with your tastes.

author: mzzj
author_rank: Super Contributor
timestamp: August 18, 2022, 04:17:38 pm
content: Quote from: mc172 on August 18, 2022, 02:11:08 pmI think "Faringdon" is some advanced AI autoroute routine trying to gain consciousness. Impressive as AI yet makes no sense at all.Or broken recording from Indian tech support operating with -30dB SNR voice link

author: jonpaul
author_rank: Super Contributor
timestamp: August 18, 2022, 07:56:40 pm
content: Hobbiest/désigner/ manufacturer/ "treez" aka " farrington" is notorious to use many posts,per day on he power forum, modérateurs should count his posts per day.he will not do his own work, just pleads for free advice from the forum.Perhaps a foreign trool deployed to waste our time?perhaps a psychological issue ?j

author: Gyro
author_rank: Super Contributor
timestamp: August 18, 2022, 09:21:00 pm
content: Quote from: Faringdon on August 18, 2022, 01:02:02 pmYou really think that a company, just looking for an excuse not to buy some suspect (if they just glance at your other threads on this forum) prototype from you, is going to be swayed by some random 'back up' reply on this same forum?You have already made your reasoning clear and they (presumably their own engineers for a technical detail like this) are not satisfied. Unless you employ some third party who the company recognises as more 'trustworth' than its own engineers, it sounds like stalemate.

author: coppercone2
author_rank: Super Contributor
timestamp: August 18, 2022, 09:36:59 pm
content: 1) buy correct resistor (you should be happy you have a interested customer)2) accept money3) spend money on economyif all they want is a god damn parts swap, thats LUCKY. What you could have heard is that they want a different topology.Thats the subassembly game. Do you know how many custom engineering requirements real engineering companies have for certain customers? Yes people rant behind closed doors that its not maximizing cash flow but who the fuck really cares. If you were selling a full unit then you might have a point but this sounds like some subassembly. Like I said, lucky someone is interested. Lucky you don't have to do a full PCB redesign or have to re-spec mechanical shit like heat sinks or bus bars etc.And its better because if the RC compensation network fails then the minor inductive spikes won't be there in the first place. Unless you can prove that non inductive resistors have some kind of a serious reliability problem, you don't have a case.

author: uer166
author_rank: Super Contributor
timestamp: August 18, 2022, 10:00:06 pm
content: Quote from: Faringdon on August 18, 2022, 01:02:02 pmYeah, no.. Even a 1206 film resistor can be too inductive if you use peak current mode control, or have cycle-by-cycle current limit as you should, and are running at a decent speed. The fact that they're 12mm across means thateven ifyou have the non-inductive version, you'll still likely run into issues unless your switching is slow as molasses. Wirewound? Forget about it.If I were you I would stop splitting hairs on trivial stuff that doesn't matter, and concentrate on absolute basic SMPS concepts (such as the above), which involves more prototyping and testing and less LTSpice and forum posts.

author: temperance
author_rank: Super Contributor
timestamp: August 18, 2022, 10:29:41 pm
content: @ jonpaulBeware of his laughing potatoes.@Faringdon aka mentiroso, bugiardo, Lügner, lögnareWhat a nonsense again. If you where to come up to me with a new design containing an L4981B, I would not pay you either. THAT CHIP IS OBSOLETE.Dummkopf, next time, make up a better story.Everything you post in here and countless other boards is all nonsense and perhaps people should stop replying to you.

author: Faringdon
author_rank: Super Contributor
timestamp: August 19, 2022, 06:42:47 am
content: QuoteThanks,This is the crux of the matter, the L4981B PFC, as you know, is "average current mode controlled". It has a peak current limit, (with a separate, dedicated comparator just for that) but its set well above the maximum running peak current, and would rarely get tripped under normal case use.As such, when i've reverse engineered Offline 800W power supplies, i've actually seen 3W axials used as the PFC current sense resistor.If we look at the PWM comparator in the L4981B, we wee that the current sense resistor signal does not go directly into it......instead, the output of the current error amplifier goes into it, as well as the synthesized ramp from the oscillator.There is a separate comparator for peak current limit, but as discussed, the trip point of this is set well above normal running peak current.When you look at the operation with average current mode control......the current limit level into the current error amplifier is changing over the 10ms half cycle.....it depends on the multiplier output current...and you can set the maximum of the multiplier output current with various resistors etc. The current error amplifier doesnt actually "see" the peak current....due to its compensation network, it filters the peaks down, and just "sees" the short term average of the inductor current....say the average over about 10 switching cycles or so.QuoteThanks, yes thats right, but its in plenty of stock at the moment, and is cheap,....and its (average current mode) method is identically used by a large number of average current mode PFC chips, eg UC3854, LT1248 etc. This is just a proto PCB and will be easy to carry over to these other chips.L4981Bhttps://www.st.com/resource/en/datasheet/l4981.pdfThe attached shows an LTspice sim with an LT1248 Boost PFC controller (identical method to L4981B).......there is 1uH put in series with the sense resistor, and as you can see, it makes no bad effect on the operation of the Boost PFC, and doesnt result in the separate peak current comparator getting tripped. AYK, 1uH is way more inductance than you'd get with five wirewound 2W resistors in parallel.

author: jonpaul
author_rank: Super Contributor
timestamp: August 19, 2022, 11:06:50 am
content: since 1975, never had an issue on CS resistors.Non inductive wirewound are fineMetal film /carbon film power resistors are readily availableEntire discussion seems moot and yet another time sink.Suggest that Farrington/Treeze/whatever consider a change carreer...try law or politics, not electronics....Cheers! Bon Chancej

author: Faringdon
author_rank: Super Contributor
timestamp: August 19, 2022, 12:39:47 pm
content: Inductive wire wound resistors are much cheaper, and since their inductance is insignificant for this purpose, it is believed that they are perfectly acceptable to use here.Would you agree?

author: mc172
author_rank: Frequent Contributor
timestamp: August 19, 2022, 12:46:36 pm
content: No.

author: jonpaul
author_rank: Super Contributor
timestamp: August 19, 2022, 01:04:05 pm
content: Cost $0.02...0.15 per pcMetal film carbon film 2wWW normal layer one directionWW NI non inductive wind 1/2 CW 1/2 CCWJon

author: Faringdon
author_rank: Super Contributor
timestamp: August 20, 2022, 10:51:22 pm
content: I think this thread could even be a good one for EEVBLOG to advise on, if it was wished to do so.Since it is quite remarkable, that we have found an application (PFC current sense resistor for Average Current Mode PFCs), where an inductive wire wound resistor is perfectly satisfactory for SMPS current sensing.(.....Not perfect, but perfect is the enemy of "perfectly satisfactory")To think ....all the PFC App Notes where they recommend a way over-expensive PFC current sense resistor, when a plain wirewound would have been fine.....here's one that recomends the priceyFCSL90R005FER resistor.......https://www.infineon.com/dgdl/InfineonApplicationNote_PFCCCMBoostConverterDesignGuide-AN-v02_00-EN.pdf?fileId=5546d4624a56eed8014a62c75a923b05

author: Circlotron
author_rank: Super Contributor
timestamp: August 20, 2022, 11:45:05 pm
content: Inductive current sense resistors may cause issues with the measured current values but I think this may be less important than another effect. Any inductance in the source lead of the PFC mosfet will lead to slow turnoff. It basically becomes a source follower during the falling edge of the gate drive. As the gate voltage goes down the voltage at source lead goes down as well, courtesy of the energy stored in the inductance, even swinging below ground. Not talking about the voltage drop across the resistor decreasing because of the falling current but rather the stored energy in the inductance dumping out as unexpected negative going voltage. As a result the negative dv/dt between gate and source (not gate and ground) will be way slower than expected. That’s going to lead to one very hot mosfet. Might even pop.

author: coppercone2
author_rank: Super Contributor
timestamp: August 21, 2022, 02:10:06 am
content: Quote from: Circlotron on August 20, 2022, 11:45:05 pmyes thats it, I saw a resistor cause a problem before in a PFC a long long time ago and this is why (jogged my memory), the solution was non inductive. so much e-waste.. but fortunately not my problem. I assume this some what depends on MOSFET choice, so even if it works, you can have major problems later if you need to replace a part because of supply chain issues, I think the problem might be 'barely working'.

author: Faringdon
author_rank: Super Contributor
timestamp: August 21, 2022, 09:43:27 am
content: QuoteThanks,Your kindly supplied words do indeed apply to the standard BCM Boost PFC.....though in this post, it is referring to the Average current mode Boost PFC......The sense resistor for the Average current mode PFC, (eg L4981B of this post) goes "back from ground"......so AYK, the MOSFET source wont "see" the sense resistor at all, nor its inductance.Looking at eg the L4981B datasheet circuits shows this.The sense resistor voltage that gets sensed is a negative sense voltage

author: temperance
author_rank: Super Contributor
timestamp: August 21, 2022, 11:39:51 am
content: In case of a resistor with some inductance, what will be the voltage across that resistor when this power supply will go trough surge testing considering the bulk capacitor + 100nF or something to be a dead short for surges?

author: Faringdon
author_rank: Super Contributor
timestamp: August 21, 2022, 02:39:14 pm
content: QuoteThanks, ,that is a good question.....with Av Curr Mode PFCs, i always put two series DO201 diodes across the sense resistor......precisely for that reason, also, for inrush-with-hot-NTC, or indeed, restart-without-inrush-limiter -in-circuit.So the voltage across this resistor would be clamped to the two diode drop.