Single ended scope probes on equipment under test which ARE on an isolation transformer

Question

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

ALTERNATE SET-UP IF NEEDED

Answer 1

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.