Measuring/Calculating Voltage Across an Isolation Barrier - Page 1
Forum section: electronics / projects

author: TimNJ
author_rank: Super Contributor
timestamp: September 07, 2023, 09:54:38 pm
content: Greetings,I'm having a tough time thinking about this. Many safety standards reference the "working voltage" across an isolation barrier , i.e. the voltage across the barrier during normal operation, to determine the requirements for creepage/clearance, hi-pot, etc. I'm mainly talking in the context of an isolated switch-mode power supply.How can you go about (accurately) measuring this? To me, this seems like a difficult measurement, considering things like common-mode voltages/currents, setup/DUT coupling to earth, measurement equipment CMRR(?), input impedance, and so on.I don't even care so much about measuring it actually. How about theoretically? Logically, I would think that, at worst case, the voltage across the isolation barrier can be as large as (the greatest peak voltage on the primary + the greatest peak voltage on the secondary). As an example, flyback power supply, 600V peak drain-source voltage on the primary, 20Vdc output --> 620V max possible voltage stress across isolation barrier?? I'm not sure if it actually works out that way.If the primary-side is mains, and ultimately earth referenced, does the worst case stress across the insulation occur when the secondary is connected to earth?This one is driving me a bit mad, although I'm probably just being a dummy.Thank you,Tim

author: Konkedout
author_rank: Frequent Contributor
timestamp: September 08, 2023, 03:51:40 am
content: You are not being a dummy. I have designed AC-DC power supplies and it IS a bit complicated.First of all; what sort of circuit or power converter are we discussing?Second of all, do you have something to test (even with approximate resemblance) and an oscilloscope with which to observe it? Assuming that a power supply output is not connected to anything hazardous, it ought to be OK to connect the ground clip of a 10X or 100X oscilloscope probe to the output and the probe tip to (let's say bulk negative) of a power supply and see waveform as it is. I think that will be educational......In a small flyback power supply with optocoupler feedback, much of the waveform is controlled by capacitance in the transformer as well as added capacitors connected between mains, ground, and output. When the mains is ground referenced, the input rectifier is conducting, and the output is ground referenced you then have something a bit more solid dictating the immediate potential.

author: jonpaul
author_rank: Super Contributor
timestamp: September 08, 2023, 06:51:53 am
content: Bonjour Tim: How is NJ?1/ The safety regulations specify the barrier mains><secomdary parameters for insulation, eg transformers, wires, electrical devices PSU, optos.See UL/IEC/VDE/TUV, etc.2/ Industry testers may apply AC or DC via current limited variable supply and meters or go/no go in insualtion testers, see Associated Research, Danbridge, ec.3/ mains transients are defined by waveform, energy and peak V in IEC 61000-4-5 for various classes of protection.https://en.wikipedia.org/wiki/IEC_61000-4-5https://www.ti.com/lit/an/slva711/slva711.pdf4/ One seldom needs to actually measure the voatage across the barrier as its easy to calculate or estimate for the non transient mains 120/240VAC and DC highest V on the secondary.Measurement of the real transients is very hard, it requires statistical collection and recording in the field, thats why the IEC made 61000.5/ Design of the transformers is simplified by use of split bobbins rated for safety isolation.We used both split/multiple bobbin and 3M 56 mylar tape UL rated . On large trsf, we used specail HPTZ wire eg #8 sq or #4 rect.6/ Most optocouplers come in a UL rating, eg 1500V..2500V isolations.7/ Easy to setup an HV PSU and sense incipient breakodwn by corona noise current and current sudden spikes and exponential increase.The classic is the Danish Danbridge 30 kV (rare) insulation tester JP-30.Beuatiful, the best but costly.....https://danbridge.com/product_detail.php?indexs=46Hope this is useful to you!HAVE AN ABSOULTELY FANTASTIC DAY!Jon

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2023, 05:29:39 pm
content: Quote from: Konkedout on September 08, 2023, 03:51:40 amThank you.I guess the simplest case is a dumb 50/60Hz step-down transformer. But, as it pertains to me specifically, I am really talking about some AC/DC power supply: flyback, forward, LLC, etc.A fundamental question is: Is there voltage developed across a transformer (or across an isolation barrier in general) if there is no capacitance (intentional or not) across the barrier?To my feeble brain, I keeps coming back to a simple capacitive divider model. Some thought examples:1. 1nF across isolation barrier, 100pF secondary-to-earth coupling --> Isolation barrier sees 10% of the voltage, secondary-to-earth sees 90% of the voltage (Hmm, in practice, I don't think you'd ever see this. Why?)2. 1nF across isolation barrier, secondary direct link to earth --> Isolation barrier sees 100% of the voltageMy problem is: How can I measure the actual voltage across the isolation barrier without significant contribution from the measurement setup? I took a few measurements with the scope floating, and scope powered through an isolation transformer to try to get...uh, something of a true differential mode scope? 100:1 passive probe.It is a 100W flyback with a 390Vdc PFC front end. Input voltage is 230Vac. Drain-source voltage across primary MOSFET is ~610Vpk.There is a half-wave 60Hz component. Does that make sense? Should I really see that? Or is this some sort of common-mode artifact? (You can see when the coupling capacitance to earth is low, the 60Hz component is less.)And, from the perspective of the primary-to-secondary insulation, is it actually possible to see asymmetric voltage stress, as in the case of "output earthed, DC coupled"? I guess the answer is "yes", meaning there is some sort of additive effect of the switching voltages on the primary side and the mains.

author: TimNJ
author_rank: Super Contributor
timestamp: September 08, 2023, 05:59:53 pm
content: Quote from: jonpaul on September 08, 2023, 06:51:53 amNew Jersey is all I know, so it's...good?Thanks for your inputs. I am not so interested in measuring the dielectric strength of a given isolation barrier, nor am I really concerned with transients. Indeed, there are already standards which define "over-voltage categories" (OVC) of different electrical installations, and requirements for isolation based on OVC. And testing insulation strength is "easy enough" with a hi-pot tester and/or megger.But, many standards require the isolation barrier be designed according to the actual stress it sees (working voltage across the barrier). So, a barrier with 400Vac across it may need additional creepage/clearance and higher dielectric strength test compared to a barrier with only 250Vac across it. The question is: What are some ways to measure this correctly?

author: jonpaul
author_rank: Super Contributor
timestamp: September 09, 2023, 06:21:19 am
content: Tim: Yes lived in NJ in 1950s, Teaneck, Bergenfield..Cousins in Framingdale....lessons at Teterboro...Understand your question better now.1/ Indeed, your scope shots are typical mix of HF and mains.Probes can be 10x to 400V or 100x. probe C is 10..20pF not an issue. We need a diff measuennt, use CH1-CH2 or diff amps.Consider to separate the LF mains /2x mains and the HF SMPS components: Just a LP and HP filter before the scope? Or use FFT/digital filter if scope has one?2/ The P-S barrier capacitances are easily measured with a good RLC/bridge. We like the HP4275A, 4332A, Wayne Kerr 2345 (?) and old Teltronic 130 LC meters.If shield is present make a guarded capacitance measurement. We can easily see 0.3 pF on our lab equioment if the guards are used.3/ Transformer capacitance dominates over the few pF of an opto, typically 20..500 pF P-S.A Faraday shield properly used can greatly reduce the effective C.4/ Other PSU capacitance to cosider is CM EMI filter caps at mains entry (if any) eg VDE Y caps to earth5/ We can estimate the max barrier stress as peak rectified mains plus peak SMPS bus Veg 240 V * 1.41 + 360V ~ 720V peak, easily handeled by any mains rated (1500V) opto and most transformer tapes and wire coatings.6/ For the optos see the mfg specs and app notes.for transformers same if commercial part,7/ For DIY or designs of trsf, see specs forInsulating tape eg 3m56 MylarMagnet wire insulation eg Belden Formvar, HPTZ, Solderease, Nylease, etc.(use double/heavy/triple if needed)Bobbins: See core supplier reccomeded bobbins, eg split bottin, and concentriuc (VDE) for lowest Cp-s and best barrier isolation.Bon ChanceJon in Paris 6(PS found my first trsf, a 12V ><400V invereter toroid from 1960, solid vlnil bell wire pri and magnet wire sec potted in red RTV silicon rubber...)

author: TimNJ
author_rank: Super Contributor
timestamp: September 09, 2023, 06:03:40 pm
content: Quote from: jonpaul on September 09, 2023, 06:21:19 amYes I’m quite familiar with those places!NJ has an interesting engineering past, RCA, Bell Labs, DuMont, and so on.Thanks. Your assessment about Vbus + Vpk(AC) seems to be correct, taking a look at the 715V peak seen in the “earthed” scopeshot below. 230V * 1.414 + 390V = 715V.That’s interesting to me because there *is* ~600-630 peak across the MOSFET but somehow that does not show up across the transformer? Hmmph. Does that make sense?And the half-wave waveform I guess makes sense since (at least in the US) neutral is tied to earth, so for one half of the cycle line swings below neutral, and yada yada yada something to do with the diode bridge being reverse biased.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 09, 2023, 08:25:09 pm
content: I once worked at a factory where they had a Hi-Pot tester. They called it the "Phone Booth", because it looked like one. I am not exactly sure how it worked. I guess the booth itself was earthed, and it also earthed both of the mains input wires of the "Device under Test" and then applied a few kV to the "earthing" wire of the DUT and measured the (leakage) current.Because you only have to generate enough power to overcome the leakage, your HV power supply does not need much power. I never really understood why Hi-Pot testers cost around EUR5000. Probably because it is:1. Specialized equipment for a relatively small market.2. High voltage, so potential safety risks.3. Their target market is for equipment manufacturers. They don't care about costs, as long as the gadget is reliable.But in principle it is quite easy to make a 1kV (or 2) low current power supply, connect it with a shunt to "earth", and then use it to apply some high voltage potential over your isolation barrier. Isolation testers (which work with upto 5kV or so) are not very expensive. You can get them from Uni-T for a few hunded EURo's and even Brymen has one built into a DMM (the BM887 and BM878) which are both below EUR300, but these only go upto 1kV. You need high voltages, because things like ESD mats react non-linearly to high voltages. They can have leakage at a high voltage, while measuring them with a normal DMM shows > 20MOhm.

author: Alti
author_rank: Frequent Contributor
timestamp: September 09, 2023, 09:07:23 pm
content: Quote from: TimNJ on September 07, 2023, 09:54:38 pmI think this has to be set in some context.So "working voltage" term must be somehow defined in requirements you try to fulfill.Quote from: IEC 62368-1, 3.3.14.8You are from UL world so please post relevant quote and standard number and then we can discuss about it.

author: TimNJ
author_rank: Super Contributor
timestamp: September 10, 2023, 04:45:12 pm
content: Quote from: Alti on September 09, 2023, 09:07:23 pmIEC62368-1 is relevant, sure. Other standards with similar verbiage are also relevant, e.g. IEC60601-1, IEC60335-1 and so on. I would not say the US is “UL world” at this point. There are not too many standards (that people actually care about) that are solely UL, at least at the finished product/subassembly level.For the sake of my question, I don’t think the actual standard matters too much. I’m just asking how to make a *good* measurement of voltage stress across an isolation barrier when the system is mains powered and the test equipment is also coupled to mains somehow. At this point, my measurements seem plausible, just asking for a sanity check or any additional tips.I think I’m probably seeing this measurement as a special case somehow, when in reality it’s not that different from most other measurements I’ve made on mains powered SMPS.