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I am using my company's new oscilloscope and I don't want to mess it up. I have searched all of the topics, but nothing tells me exactly what I want to know so I thought I would ask in case somebody had the same question.

The team designed a switch-mode power supply and an audio amplifier. I'm supposed to test both, but I am a little embarrassed that I don't know what will and what won't blow-up the DUT, the scope, or the probe. We have a normal 115VAC/60Hz outlets, powering the oscilloscope and the AC power supply. We also have an isolation transformer if I want to plug it directly into the wall. I am using a differential probe, but I only have one of them and need to look at 2 signals at the same time. I have lots of single ended probes.

  1. Can I use a single probe anywhere if the equipment is on an isolated outlet (we have isolated outlets on the front of our AC supply).
  2. If that doesn't work, can I put the DUT on the isolation transformer and then use the single ended probe anywhere?
  3. If the DUT is on the isolation transformer, can I put the alligator clip anywhere I want? if I want to measure the high-side fet of a half-bridge LLC circuit, can I tie the ground clip to the switching net and it just behaves like a differential probe?
  4. Is there any danger in using signal ended probes on the primary side of the high-frequency transformer in the LLC and the secondary side of the high-frequency transformer at the same time?

I appreciate any help you can help me to learn how to do this correctly!

Jeff

DEFAULT SET-UP Set-up with the AC supply which provides isolated AC outlets on the front panel.

ALTERNATE SET-UP IF NEEDED Alternate set-up if needed, using an isolation transformer.

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    \$\begingroup\$ Re: Q3 above. Connecting the oscilloscope ground lead to a high frequency / high impedance net on the DUT is likely to (adversely) influence the operation of the DUT due to capacitive loading effects. A high voltage differential probe is the correct probe to use in cases like this. A transformer does not allow the same range of measurements as a good diff-probe. An example of such probes is found at tek.com/probes-and-accessories/high-voltage-differential-probes \$\endgroup\$
    – elchambro
    Commented Jun 22, 2020 at 1:53
  • \$\begingroup\$ If all else fails, consult the scopes documentation: It may support to gang two single ended probes&inputs to show a differential reading. Be aware the load on the nodes may be way higher than with a differential probe. \$\endgroup\$
    – greybeard
    Commented Mar 25, 2023 at 10:14

2 Answers 2

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  1. If you're asking whether you can touch just the probe tip (and not the ground clip) the answer is yes within the voltage limits of your scope regardless of whether the DUT is isolated or not.

  2. Similar to 1 except note the comments in 3 about isolation currents which would try to flow through your scope tip in this case.

  3. Yes in theory however noteall isolation barriers are imperfect with C and R across them. If your alliator clip is holding an isolated node to earth it will carry any parastic currents that flow through that isolation barrier.

Consider for example if you earth the primary side switch node to measure the high side fet you'll impart the switching waveform onto your primary isolation (xfrm or AC supply). You may want to check their isolation rating and any parasitic currents will have to go through your alligator clips. It might be fine, or you may find one is 10X worse than the other and messes up all your measurements.

You may want to directly characterize the parasitics of your two options with an LCR meter and choose the primary isolation solution with the least parasitic C.

  1. Yes this should be fine since both primary and secondary are isolated you're generally free to ground each with a single aligator clip safety.

Note these are good questions which even experienced engineers screw up (with bad results). Sensing high side gates is a real problem. Don't count on differential probes - their CMRR isn't amazing and usually no match for 400V switch nodes. A poor man's solution is a battery scope that's truly floating - just avoid touching it while the circuit is operating...

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With the majority of oscilloscopes, the ground of the input channels is connected to the AC supply ground pin.

If you connect the probe to a signal that is not referenced to ground you risk getting a contaminated signal or worse causing damage to the device under test or the oscilloscope. With both of your examples you will be shorting out either the LLC isolation or the isolation transformer in the alternate set-up.

If you only use a single channel at a time then some of the arrangements may be usable, but if you are using two channels the probe grounds could for example short the PFC and the amplifier even if the scope is on an isolation transformer.

You could power the oscilloscope from an isolation transformer or use a battery powered scope.

I have occasionally lifted the ground on the power lead of the scope to perform successful measurements but that is potentially dangerous as the front panel controls on the scope could be at dangerous voltages.

The best approach is to use an oscilloscope with isolated input channels designed for such situations such as the Tektronix TPS2024B. Each channel is isolated from the power ground and from each other.

It simplifies the problem greatly if you only look at one side at a time.

However, one non-obvious issue with modern power supplies is that commonly there is a bridge rectifier at the input followed by the PFC; all parts of the PFC will be 'live' as the bridge diodes will be alternately connecting the positive output to one side of the incoming AC line then the other and the same for negative output.

The circuit can be safely probed if fed from an isolated AC supply as you have you. When the ground of the scope is connected these voltages will be forced on the incoming AC. you need to always be aware that even the neutral side of the incoming power to the PFC is live.

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  • \$\begingroup\$ Hi @kevin-white, Thank you for the feedback. Well, I would happily give up the primary/secondary probe scenario (i.e. one probe on the primary side, one probe on the secondary side- inadvertently tying primary and secondary sides of the power supply together). Also, I actually intend to use a 4 channel scope, but wanted to simplify the drawing. So, if I could have all four scope probes on the primary side and use the ground reference on the primary for ALL the probes, this would enable a lot of testing and I could just not prime anything on the primary side. Thoughts? \$\endgroup\$
    – AudioTech
    Commented Jun 22, 2020 at 5:41
  • \$\begingroup\$ @AudioTech - I've edited my answer with additional information. \$\endgroup\$ Commented Jun 22, 2020 at 14:48

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