Products > Test Equipment
HV differential probe versus isolation transformer
AVGresponding:
--- Quote from: PwrElectronics on June 28, 2023, 05:21:43 pm ---At my employer, we've been working on motor drive inverters and DC-DC converters running off 700VDC for a number of years.
We use differential probes for nearly all HV measurements and for looking at gate signals, etc. Scopes are grounded to earth via the normal AC power plugs.
In the early days of working on the 700V stuff, we didn't have much in the way for diff probes and I recall trying to get a look at the high side gate signals using a TEK THS720A scope. With isolated inputs, it was not rated for that sort of voltage but since it is battery powered, I figured just run off battery with only that 1 probe connected and use a wood stick to press the control buttons, etc. Basically, the scope was floated.
For reasons unclear, as the DC voltage increased, the gate waveform I was looking at became more and more distorted. Later was able go get a diff probe and that issue was not present.
Nearly 30yrs ago I worked at a plant that made power supplies (mostly off-line types). The line repair techs (repaired test failures) used to chop off the earth pin on their AC powered bench scopes and float the scope to whatever the internal DC link voltage on the power supply was when troubleshooting things. That could often be 200V. They got away with it but for sure was not a safe practice. That place at that time didn't really have any sort of safely rules or training....
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Yeah, in fairness, the highest voltage we worked with was 450VDC or so, and nowadays I don't usually poke anything more exciting than an SMPS, so no more than 350VDC or so.
The THS700 is only rated for those kind of voltages with the P5102 probe, and the probe at frequencies below about 2 or 3 MHz, as its impedance drops quite a bit once you go past that.
HarryDoPECC:
These are not too pricey and can be quite good as long as you keep in mind that anticipated rise times must also fit in the bandwidth.
https://w140.com/tekwiki/wiki/A6902
Luxor92:
I've left the thread for a while, thought it will be dead soon, but noticed so many answers when back to here. Actually to the moment I explored the situation with attention (thanks to tgzzz for provided articles). Basically there is nothing better than diff probe. Isolation amp will be more expensive + bulky. Indirect grounding - too exotic :) and also expensive, I guess. Other options even't don't wanna consider (isolated channel osc etc). Recently found a video, part of it explains how to calculate error related to CMRR when measuring hight side gate signal of half-bridge. So we can work with that now) Anyway, it's interesting to read about your experience :-+
tggzzz:
Glad to be of some help, and it is good to see you have been able to reach a conclusion as to what suits your purposes :)
Thanks for reporting back; too few new posters do that!
Luxor92:
--- Quote from: scoper007 on June 28, 2023, 07:22:03 am ---Looks like Micsig has new series HV diff probe! (Chinese web) :popcorn: https://www.micsig.com.cn/MDP/
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I like that it has shorter cables.. but wonder why 1:10 ratio is so unpopular. If we look at new MDP1500 and it has 2% accuracy in 1:50 mode, and 150 diff volts maximum. So we have ±3 volts which is for MOSFET or IGBT driving 20 volts signal already ±15%. Not good :/
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