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Forum Index : Electronics : Accurately measuring isolated gate voltages
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Haxby Guru  Joined: 07/07/2008 Location: AustraliaPosts: 423 |
Hi all, I think it's fair to say that most of the problems we face with inverters blowing up are due to not driving MOSFET gates correctly. I monitor my gate voltages through a HV probe (micsig dp10013) but because it's a HV probe, I'm not getting the resolution I want. Quite a bit of noise too. I'm looking for a low voltage but isolated probe to really get a good idea of what's going on. Trouble is that I only see two types of probes on the market. Low voltage differential probes or high voltage isolated probes. There are some Tektronix solutions, but ah, the price tag is staggering. Any ideas? |
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InPhase Senior Member  Joined: 15/12/2020 Location: United StatesPosts: 178 |
Can you isolate the scope itself from the electrical supply? Maybe an isolation transformer. |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 423 |
I'd prefer not to... I'm working with 400v DC so I'd prefer the danger to start and end somewhere very close to the PCB under test. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Peter bought one of these, and seems pretty pleased with it: https://www.globalmediapro.com/dp/A01LT0/Pintek-DP-25-Differential-Probe-25MHz-1000V/ Its on my eventual ultimate shopping list, but a long way down. Cheers, Tony. |
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poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1418 |
Yes, the DP-25 is "high voltage" but it has 3 attenuation settings. 20x 50x and 200x I use 20x and it is pretty good. There is more to measuring than the diff probe unit. The connections you make between the Gate and Source pin and the diff probe unit can be simple foot long banana clip leads. These of course are not shielded. This makes for a bit of noise. But the signal we are interested is a lot larger than the EMI picked up by the two unshielded leads. Tektronix also make the A6902B Isolator. About $250 from Ebay. This is a 2 channel fully floating pre-amplifier. It has a wide range of input attenuation settings. I bought one used. It needed a bit of fiddling to obtain reasonable calibration and accuracy. They take 1/2 hr to warm up properly and stop continually changing the zero offsets. If you are in Melbourne, come by and I can loan you either the DP-25 or A6902B The DP-25 easily exceeds the signal to noise performance of the 6902B when used with MOSFET Gate drive probing. Nothing to cry about but the traces are much cleaner. You can reduce the noise on the trace to a large degree with the DSO "averaging" function. Maybe you have also a so called "high resolution" aquisition mode. I find this works wonders too. Finally, the 6902B manual shows use cases based on Triacs and 400V Edited 2021-04-27 12:09 by poida wronger than a phone book full of wrong phone numbers |
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| Haxby Guru Joined: 07/07/2008 Location: AustraliaPosts: 423 |
Thanks for the offer Poida, I might take you up on that. For now it sounds like my 50x HV probe will do. It's interesting that you can buy a pretty decent DSO for $200 these days but the various probes, which are arguably much simpler, start at around $300. Supply and demand I suppose. |
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| nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458 |
Oh, I completely missed this thread... Peter, would you recommend the Tektronix above the pintek? The pintek could only be bought from China for 250eur. the dp50 could be sourced directly(EU) but costs 430eur. Micsig DP10013 or Dp10007 differential probes could be bought for 170eur but Haxby was not satisfied with those, so I'm a bit anxious to buy 1 or 2 of those. But I don't think we can expect the same accuracy or clean signal if we use banana leads, right? For the a6902a or b I only find USA based sellers so I need to pay taxes and transport costs on top. So, it will probably cost me between 300-500eur to get me a complete working unit. The a6902a can be found cheaper, is there a difference with the a6902b? I can't find the difference. Not knowing completely what I could get I would prefer something new at this stage. I did sent some questions about transport costs to some sellers. It's all a big confusing but in theory I should be able to use 2 diff probes on a 2 channel scope? With 2 different reference "grounds"? Or is it possible to just use one differential probe and use the other scope probe to measure from another reference, is there a delay between the normal scope measurement and with the differential probe? (I always prefer to do my scope measurements isolated from the earth, even with earth connected you always need to be carefully when working on a DUT) Edited 2021-05-16 16:38 by nickskethisniks |
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