Which Current probe you use for SMPS testing ? - Page 1
Forum section: electronics / testgear

author: maxpayne
author_rank: Regular Contributor
timestamp: June 26, 2018, 05:15:04 am
content: I am planning on buying an oscilloscope current probe. Will be mainly used for small to medium power SMPS (AC-DC and DC-DC) testing.Can you suggest or share your experience of what you are using now ?

author: Dubbie
author_rank: Unknown
timestamp: June 26, 2018, 05:20:22 am
content: I too am interested in this.

author: tautech
author_rank: Super Contributor
timestamp: June 26, 2018, 05:33:46 am
content: All switched current paths are actually AC so there's no need for DC capability and therefore cheaper AC only probes are suitable for SMPS providing you're measuring above the minimum rated probe frequency.Typical mains SMPS are under 100KHz but many DC-DC switchers run into many 100's of KHz.Of course the better the probe BW the more accurate risetime measurements will be and that's important for magnetics design and build so to be able to observe saturation current rise while while experimenting with switch ON time.I have a couple of Tek P60** 60 MHz AC probes.

author: maxpayne
author_rank: Regular Contributor
timestamp: June 26, 2018, 05:59:09 am
content: I was looking it at Keysight N2820A , but looks like it is no good for SMPS works. and I can make that probe using current shunt and opamp.

author: Hydron
author_rank: Super Contributor
timestamp: June 26, 2018, 08:43:15 pm
content: Quote from: tautech on June 26, 2018, 05:33:46 amI would mention that while the AC part of the waveform is probably the item of interest, there is likely to be a DC offset flowing in many places of interest. You'll need to make sure that any DC present won't saturate the probe if you're measuring using an AC only probe.You should think about what sort of things are you likely to want to measure and how it will help you - in smaller or higher frequency supplies in particular it may not be easy to get access (i.e. cut a track to insert a probe or similar) to what you want to measure without changing circuit operation.All that said, good current probes are extremely handy for some stuff - I have a couple of AC/DC Tek probes plus a variety of AC-only Pearson "Current Monitor" wideband CTs.

author: tautech
author_rank: Super Contributor
timestamp: June 26, 2018, 08:57:52 pm
content: Yes, circuit insertion is problematic in SMD but often there's a current sense resistor that can be measured across instead of needing a current probe. CT's are fine but you still have to break a connection to insert them although using traditional current probes one must still solder in a loop in order to use them anyway. Six of one, 1/2 dozen of the other.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 26, 2018, 09:09:25 pm
content: I have a pair of Tek A6302 probes with AM503B amplifiers for clamp-on use. They are modestly priced on the used market. 30Mhz is pretty good for anything that I have done so far.I also use a pair of Preamble/LeCroy 1855 differential amplifiers and a few custom sense resistor PCB's for smaller signals or higher frequencies. These gems require a lot more work and perhaps more fragile but can see tiny details.

author: Hydron
author_rank: Super Contributor
timestamp: June 26, 2018, 09:42:35 pm
content: Quote from: rx8pilot on June 26, 2018, 09:09:25 pmUh, this may be true of the US market, unfortunately not elsewhere! If you can get some at a good price they are very good probes though - old but performance has not improved with time.

author: rx8pilot
author_rank: Super Contributor
timestamp: June 26, 2018, 11:22:19 pm
content: Quote from: Hydron on June 26, 2018, 09:42:35 pmTrue enough.....eBay deals in the USA are rather good, shipping is cheap. I feel fortunate for sure.These units are reasonable, but far from amazing. Big, heavy, hot, and tend to drift. The newer Keysight probes are awesome....but still out of my price range. Especially true since I need two. Continuing to dream and save my money.

author: Hydron
author_rank: Super Contributor
timestamp: June 27, 2018, 06:13:54 am
content: The AM503 I have does drift pretty badly, I'm not sure how much the probe does though, as I've normally let them warm up together. This is one area where the more modern stuff is better, that's true. Still usable though if you let it warm up for a substantial period of time and check occasionally when you're using it.Might experiment one day to see if I can tell how much a probe alone drifts. Making my own amplifier is on the to-do-one-day list - should be reasonably practicable unlike repairing or building a probe.

author: nctnico
author_rank: Super Contributor
timestamp: June 27, 2018, 09:45:26 pm
content: Quote from: tautech on June 26, 2018, 05:33:46 amThat is not true! In a typical DC-DC converter the storage inductor also carries a significant amount of DC. Just look at the waveforms here:https://en.wikipedia.org/wiki/Buck_converterShameless plug for my dual channel current probe measuring the current through the low side MOSFET of a synchronous step-down DC-DC converter:

author: tautech
author_rank: Super Contributor
timestamp: June 27, 2018, 09:51:49 pm
content: And why might you want to measure a switcher where there might be DC current ?You can see the behavior of the components in the circuit with an AC probe IF it's in the right place and you understand the waveform that you see.

author: nctnico
author_rank: Super Contributor
timestamp: June 27, 2018, 09:59:31 pm
content: Quote from: tautech on June 27, 2018, 09:51:49 pmFor starters: without DC you can't see whether an inductor is close to it's saturation current or not. I used to use current transformers (AC) but quickly started missing the DC component of the signal.

author: tautech
author_rank: Super Contributor
timestamp: June 27, 2018, 10:22:53 pm
content: Quote from: nctnico on June 27, 2018, 09:59:31 pmAs ON time of the switch is increased significant current rise indicating saturation becomes plainly visible with an AC probe. If the switch is OFF there's NO constant DC component. All this can be calculated in theory and/or checked in practice with adjustments of ON time.For 'off the shelf' DC-DC converters there's a specified component list and recommended layout to save the designer much time and effort but to 'know your tools' and use them appropriately is the key when taking advanced (for some) measurements. Insertion of measurement equipment into the correct part of the circuit is paramount to obtaining meaningful results.

author: CopperCone
author_rank: Super Contributor
timestamp: June 27, 2018, 10:30:16 pm
content: the P6042 would be a classic for the hobbiest for this kind of work so long the current level peaks are low, like low power regulators.Advanced testing of DC DC converters would use injection transformer and a VNA probably.