Correct use of oscilloscope in power supply development

Oscilloscopes are commonly used tools. How to use them correctly in power supply development? Here is a summary. Please correct me if I find any mistakes. Some pictures are quoted from the Internet.

This post only talks about how to measure correctly. How to improve the measurement accuracy is not within the scope of discussion.

The figure below shows the commonly used rectifier circuit and oscilloscope power supply circuit.

The oscilloscope needs to be connected to the ground wire when it is used normally, that is, the oscilloscope casing, the BNC ground shell of the channel, and the signal ground of the probe are all connected to the ground wire PE of the power supply circuit.

If you use the positive pole of the probe and the signal ground (alligator clip) to measure directly, the place where the alligator clip touches will form a loop with the ground wire PE, which may cause the circuit breaker to trip or burn the circuit board or oscilloscope. As shown in the figure below

Some people directly disconnect the ground (PE) of the oscilloscope for measurement, as shown in the following figure

This measurement method is called "floating measurement". The metal end of the oscilloscope casing (such as chassis, casing, connector) and the probe reference ground (negative end, alligator clip) have the same electrical potential, floating relative to the earth, and have a certain voltage. Once the human body accidentally touches the oscilloscope casing, the current will flow through the human body into the earth, which may cause serious personal injury or even death!

This method is strongly not recommended.

How to measure correctly? There are three common methods.

1. Use a high voltage differential probe;

2. Use pseudo-differential measurement, i.e. "AB" measurement.

3. Add an isolation transformer or use an isolation probe;

Let's talk about it in detail below

1. Use a high-voltage differential probe, as shown in the figure below. However, differential probes are often expensive, and not all bosses are willing to spend a lot of money to purchase them.

2. Use pseudo differential measurement, i.e. "AB" measurement. Use two channels of the oscilloscope, connect the probe tip (positive end) of one channel to the neutral line, and connect the probe tip (positive end) of the other channel to the live line. The difference between the two channels is the AC waveform. Open Math operation and perform subtraction on CH1 (VA) and CH2 (VB).

This method requires perfect consistency between the two channels and the probes. Even so, the different noise generated on the two channels will affect the measurement results.

The measurement principles of the two figures below are the same.

3. Add an isolation transformer or use an isolation probe;

An isolation probe is sometimes more expensive than an oscilloscope, so an isolation transformer is added. Many posts simply say to add an isolation transformer, or directly add an oscilloscope to an isolation transformer. Is this correct?

As can be seen from the figure below, usually the isolation transformer does not isolate the ground wire (PE), so adding an oscilloscope to the isolation transformer will definitely not work, and the measurement point will form a loop through the negative pole of the oscilloscope probe and the ground wire.

The correct method should be to add an isolation transformer in front of the power supply device, as shown in the figure below. Then the measurement point will not form a loop to the ground wire through the oscilloscope probe.

You can also use an oscilloscope with channel isolation, but I haven't used it, so I won't explain it in detail.