Measuring switch current in HF power converter - Page 1
Forum section: electronics / testgear

author: sandalcandal
author_rank: Unknown
timestamp: August 20, 2021, 01:30:22 pm
content: Hello Test EquipmentI'm looking for solutions to measure current through a switch (MOSFET) being used in a high frequency, high power resonant converter (LLC). Some specs are:DUT package:TO-247-4Current:20 to 30 Arms ~43 ApeakFundamental Frequency:0.1 Mhz to 1 MHz (mostly 200kHz to 500kHz for now)Frequency of Harmonics:~10 MHzMinimum sensitivity:<0.2 AAccuracy/Error:<10 % FS (could go with 50% if limiting to qualitative or relative measurements)Current waveforms are mostly half sinusoidal to half triangular has seen herehttps://www.eevblog.com/forum/projects/1kw-resonant-converter-analysis-design-build-and-validation/25/but our main interest is in catching parasitic behaviour which seems to be up at~10MHz.So far we've been bending a leg out and soldering in a wire loop so we can get a Tektronix TCP312A probe in and make measurements using that. The issue with this method is that the added loop has a noticeable effect on circuit performance and behaviour at the levels we are trying to measure. There also might be issues with maximum current as we push to higher power levels (we might have access to a TCP305A though).An alternative we've been considering is using a coaxial current shunt. The issue here is that creating space for a coaxial current shunt to fit in will require changes to PCB layout which will likely affect circuit behaviour negatively (again) and the coaxial shunts rated for 30Arms are also just huge relative to the available space.https://powertekuk.com/coaxial-shunt-landingCTs are ruled out due to inductive loading and poor bandwidth/response.The other alternative we've been considering is a Rogowski coil probe from PEMhttp://www.pemuk.com/products/cwt-current-probe.aspxBeing able to just loop the coil between the legs of the switch seems really attractive but I'm concerned there might be some difficulty doing this with the close leg spacing for the source pin on a TO-247-4 package however. We shouldn't have any need to detect DC currents in our application. I've also never used one of these before so I'm not sure of any quirks of these Rogowski coil probes might have which could cause us issues.We have tried measuring current indirectly from other connections in the circuit but direct probing of the device legs seems required to get full parasitic behaviour.Looking for experience using PEM Rogowski coil probes or other alternatives I haven't considered yet.

author: Grandchuck
author_rank: Super Contributor
timestamp: August 20, 2021, 04:07:00 pm
content: The Little Bee current probe works with or without a toroid sensor attached.https://github.com/westonb/little-bee-B1#:~:text=Little%20Bee%20B1%20-%20A%20high%20performance%20current,based%20on%20an%20Anisotropic%20Magneto-Resistive%20%28AMR%29%20magnetic%20sensor.I have one and have verified basic performance and can report it meets specifications. I wonder if the flat end pressed against some circuit point might serve your needs?

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 20, 2021, 04:11:33 pm
content: Given the low speed a coaxial shunt is not really needed, these are fine when frequencies in the range of 10-100MHz are to be measured. Besides their greater freq response they also lack magnetic field interference which is typical in other shunts.A simple resistor shunt takes very little space but then the problem becomes making a floating low level measurement on that.So its either a low valued resistor shunt or a smallish rogoswki probe.

author: sandalcandal
author_rank: Unknown
timestamp: August 20, 2021, 04:19:17 pm
content: Quote from: Grandchuck on August 20, 2021, 04:07:00 pmThanks, haven't seen that before. The 10MHz bandwidth might be a bit of a concern and the max current isn't enough. Maybe we could do sort of magnetic attenuation work around but would prefer something "professional" so we don't have to dick around too much with calibration. Looks like a decent non clamp current probe similar to the I-PROBER-520 though.

author: sandalcandal
author_rank: Unknown
timestamp: August 20, 2021, 04:27:21 pm
content: Quote from: MasterTech on August 20, 2021, 04:11:33 pmThanks for the suggestions. I didn't think a small shunt resistor would work. How small of a shunt can you get for 30Arms? Is the bandwidth on a non-co-axial shunt >10MHz? I'm trying to find solutions specifically which have the minimum invasiveness with the PCB layout/current paths and won't affect parasitic behaviours which are the issues with our existing solution.PEM seems to make some nice and very small Rogowski probes but I'm hoping to hear experiences or potential alternatives.

author: bdunham7
author_rank: Super Contributor
timestamp: August 20, 2021, 04:38:15 pm
content: What level of absolute accuracy and linearity do you need?

author: sandalcandal
author_rank: Unknown
timestamp: August 20, 2021, 04:50:33 pm
content: Quote from: bdunham7 on August 20, 2021, 04:38:15 pmFair point.Ideally we'd want to be able to integrate and get a decent idea of the energy in the parasitic oscillations but being able to clearly see them should be enough to be minimally useful and allow us to measure the effects of changes to circuit and components. Probably only50% linearityfor just seeing the oscillations clearly. Absolute accuracy can be 50% too doesn't need to be too great either just consistent across full scale hopefully. At least 10% accuracy should be enough for energy calculations to be useful. In bad cases the parasitic oscillations can have an amplitude 20% of the main fundamental current but <5% is where we're getting to. Sonoise limited sensitivity <1% or <0.2Awould be most likely needed.Check me on my reasoning there.

author: bdunham7
author_rank: Super Contributor
timestamp: August 20, 2021, 05:17:34 pm
content: Quote from: sandalcandal on August 20, 2021, 04:50:33 pmActually those sound like reasonable expectations. I'm not an expert in the area and I've only dabbled with current measurements in sensitive circuits, but it seems your pick two triangle (like good/fast/cheap--pick two) is accuracy, circuit loading and cost. My concerns have almost always been circuit loading--not interfering with a circuit you are trying to analyze for instability or parasitics or both is the main issue for diagnostics. I can't give you any direct advice on the PEM products as I haven't used them, but at $1200 I can't think of anything offhand better than the CWT Ultra Mini for what you are doing.I have an older version of the Tek probe you are using and it doesn't get out much because it it too cumbersome to set up in most cases, something you seem to have discovered as well. Things like the I-prober and Little Bee seem handy, but I don't know how well they work in close quarters.

author: sandalcandal
author_rank: Unknown
timestamp: August 20, 2021, 05:35:43 pm
content: Quote from: bdunham7 on August 20, 2021, 05:17:34 pmThanks for having a look. I can go a bit heavier towards expensive in this case since it's a commercial project but again, checking my options here.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2021, 12:20:43 am
content: Quote from: sandalcandal on August 20, 2021, 01:30:22 pmIf an oscilloscope current clamp has too much inductance, then I would use a resistive current shunt even if it has to be built in. The problem with a current shunt is that it does not isolate the common mode voltage like a current clamp does, which means it is best used in the source lead unless you are prepared to make the measurement with large common mode voltages present.Attachment to the current shunt is most safely done using a high voltage differential probe. An isolated oscilloscope probe is even better but more expensive unless isolation is built into the oscilloscope. For reasons of access and to cancel out magnetic flux induced errors, a small length of twisted pair wire can be used between the current shunt and probe.When I have designed circuits which might require current measurement, I sometimes include the current measurement circuits in the design and layout which might include signal conditioning as well as a current shunt.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2021, 02:36:50 am
content: If that's all you're doing, you're being way too picky -- qualitative probing from a simple loop (H probe) will do, just keep adjusting until the squiggles settle down.You give up specificity (of the enclosed current), but gain sensitivity to orientation, location, etc. It's as good, I'd say, for this purpose. And it's not hard to infer real values, if you've looked at the waveforms already, and know the relevant RLC networks, it's all just voltage and current sloshing around, you know the step energy input and impedances, go from there.Tim

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 21, 2021, 06:10:38 am
content: Quote from: sandalcandal on August 20, 2021, 04:27:21 pmThe smaller (in size) the resistor the lower the self-inductance and the higher the bw, but it will dissipate less power. The smaller (the value) the lower the measured voltage, so you will hit measurement problems with the common mode, as in differential probes the higher the rated common mode also the higher the attenuation in the differential signal. This does not happen with fiber isolated probes where you can measure 1V signal and it doesn't matter if the common mode is 100V or 1000V.My advice would be to use smallish SMD resistors in parallel since these lower the self-inductance and increase the total power. Then from the resistor solder a thin coaxial cable with as small area loop as possible to remove the magnetic interference in that area

author: sandalcandal
author_rank: Unknown
timestamp: August 24, 2021, 03:59:58 pm
content: Quote from: T3sl4co1l on August 21, 2021, 02:36:50 amThanks for the suggestion Tim. If I only wanted really "qualitive" probing then I'd probably be fine sticking with what we have now instead of considering a specialised $1200 probe (albeit "cheaper" than our current solution).My hope is to find better alternatives.Quote from: MasterTech on August 21, 2021, 06:10:38 amI'm not really seeing how this is going to be much better than a coaxial shunt in terms of reducing invasiveness.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 29, 2021, 06:09:30 am
content: Quote from: sandalcandal on August 24, 2021, 03:59:58 pmNothing beats the size of 3 or 4 smd resistors soldered in parallel, it is something you can fit in one pin of a through hole package

author: fourtytwo42
author_rank: Super Contributor
timestamp: September 10, 2021, 03:11:09 pm
content: Hi sandalcandal,I used to guard my old tek current probe with my lifeso many secrets revealedHad you thought of SENSEFETS even as a temporary measure (pun) so you can get an idea of whats really going on albeit not with the final mosfet. In case you have never heard of them app note from ON herehttps://www.onsemi.com/pub/Collateral/AND8093-D.PDFI am afraid I don't know what the current state of play concerning availability is, it might even be one of those things that have dissapeard in which case my apologies.Roger

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2021, 07:39:23 pm
content: The subject of SENSEFETs came up a couple months ago on the forum and we could not find any still in production.

author: Slh
author_rank: Regular Contributor
timestamp: September 10, 2021, 09:04:38 pm
content: I've used the Pem Rogowski probes quite a lot. They're good for high frequency AC like that. If you get a little one then they're also great for sticking anywhere in your power supply. Much easier to fit in awkward places than the Tek hall effect probes and they never complain about degaussingthey even come with rechargeable batteries built in so you don't have to have a socket for them if you're doing a quick test.I did notice that the phase and accuracy at high frequency isn't specified on the datasheet. For best accuracy you need to make sure that the loop is pressed home in its holder. Also, at low current levels it can be hard to trigger on the current as there is a tiny bit of ripple at a few Hz. It's not something I generally find a problem but good to know about.I think their voltage rating is pretty good. I can't imagine that you'd have a problem with T0-247 packages, not sure about a 4 pin but I expect that they can advise on their smallest coil.

author: sandalcandal
author_rank: Unknown
timestamp: December 15, 2021, 09:28:57 pm
content: Only just saw the responses. Came back to the thread because I was looking to finally make the order.Thanks for the suggestion 42. The MOSFETs are pretty critical to the circuit performance so I don't think we can go substituting and do performance and behavioural measurements.Thanks for sharing your experience Slh. Good to know people have had positive experience with PEM probes.

author: KevinA
author_rank: Regular Contributor
timestamp: December 16, 2021, 04:39:50 pm
content: Here is a paper I found years ago, it served me well when we started using Class D amplifiers.

author: sandalcandal
author_rank: Unknown
timestamp: December 16, 2021, 11:33:10 pm
content: Quote from: KevinA on December 16, 2021, 04:39:50 pmThanks for the paper KevinA. I'm surprised they managed the performance they did considering other writing I've seen labelling ferrite core current transformers as inadequate for this purpose. Looks like the performance might be just a bit low for our needs (designed for 50A in 100ns) but a slightly more optimised design with newer magnetics might be feasible.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 02, 2022, 12:26:30 pm
content: Bit late but I haven't been on for a while. The only other method not to be entirely sneezed at but certainly very time consuming is simulationCertainly used it myself when the circuit topology both electronic & physical is simple enough and the component characteristics at the frequencies of interest are known, it's not absolute but it can give you a good feeling from where intuition can take overSeasons greetings everyone