Many power supplies will be a bit noisy when they are working, and the next step is to analyse them for you:

The first method that can effectively improve the output voltage noise of switching power adapters is that engineers can use a combination of chip three-terminal capacitors and ordinary electrolytic capacitors at the output of the adapter as a way to improve the high frequency characteristics of the filter. Before describing the specific mode of operation of this method, we need to first understand why the adapter noise voltage peaks. Without any additional measures, the output of the adapter usually contains a large noise voltage peak, which is caused by the imperfect characteristics of electrolytic capacitors at high frequencies. Because an electrolytic capacitor can be equated at high frequencies with a series connection of capacitance, resistance and inductance, the bypassing effect of capacitance on noise is not evident at high frequencies. Instead, the noise voltage is reflected in the output of the switching power adapter due to the presence of the resistor and inductor.

After understanding the cause of the output noise peak, in order to better suppress the output noise of the switching power adapter, there are usually two methods that can be used by product developers. Method one is to split the output electrolytic capacitor into several, that is, a large-capacity electrolytic capacitor using several small-capacity electrolytic capacitors in parallel to replace. This proposal can not inhibit the generation of noise voltage at all, but with the new approach to the letter generated by the peak-to-peak noise voltage is smaller than the original. Method two is in parallel with the electrolytic capacitors next to a small-capacity high-frequency ceramic capacitors, the use of high-frequency capacitors in the high-frequency under the embodiment of the low capacitive impedance, so that the output noise voltage to get a greater attenuation, of course, in the ceramic capacitors on the printed circuit board should be kept relatively short length of the wiring to maintain the smallest possible line impedance.

The second method that can effectively improve the output voltage noise of the switching power adapter is that engineers can use high-performance surface mount filters to improve the noise peak conditions. The internal circuit of the mounted filter can be equated to a π-type filter line, in the switching power adapter output string on a mounted high-performance filter, compared with the original output noise voltage peak will be greatly reduced, in the test, the noise voltage peak on the oscilloscope is almost a straight line, which indicates that the output voltage noise has been significantly suppressed, thus well illustrating the role of surface-mounted high-performance filter in this line. This is a good illustration of the role of surface-mounted high-performance filters in this circuit.