Today, when evaluating a power supply demo board, I came across a small problem. It was that the main side of the transformer could not use oscilloscope quantity, and the power board stopped when connected to it. When my colleague took the oscilloscope probe point, he was afraid to measure the main side circuit again. At the beginning there is really a little confused, according to very simple, no matter what input is negative voltage or positive voltage, only know the reference point for the chip, this as long as there is no problem, and this is the power supply manufacturers demo board!?
In doubt did a few small tests, only connected to the oscilloscope probe in the input port of the negative pin, add electricity, the phenomenon is the same.I'm going to get something that I don't know.
Since it seems to be related to the ground link of the oscilloscope, check from this aspect.Found the oscilloscope probe and oscilloscope is through the sites/power input fields, demo board also common to power supply and the oscilloscope, and power supply input and output is isolated, that is to say the demo board and power supply system is, as a result, access to the oscilloscope is forced to connect the power supply input and output together, will, of course, we have a problem.
Figure this out, oscilloscope connected to the problem is not clear, so put on another input and output of the dc voltage source, and then test demo board, all OK!
Here is a small lesson, is in the test of the power supply, must first make clear the connection relationship, otherwise easy to go wrong and make inexplicable.Because switch power supply is used more on the system, and there is isolation and non-isolation mode, in addition, in order to debug including official use will use external power supply, and external power supply also has a lot of kinds, plus the test equipment itself, if there is no prior consideration of these factors, abnormal testing is inevitable!