What are the five main sources of output ripple of switching power supplies?
Using a 20M oscilloscope bandwidth as the limiting standard, the voltage is set to PK-PK (effective values can also be measured). Remove the clamp and ground wire on the oscilloscope control head (because this clamp and ground wire will form a loop, like an antenna receiving noise, introducing some unnecessary noise), and use a grounding ring (it is also possible to not use a grounding ring, but the error generated should be considered). Parallel a 10UF electrolytic capacitor and a 0.1UF ceramic capacitor on the probe, and directly test with the oscilloscope probe; If the oscilloscope probe does not directly contact the output point, it should be measured using twisted pair cables or 50 Ω coaxial cables.
The output ripple of switching power supplies mainly comes from five aspects: input low-frequency ripple; High frequency ripple; Common mode ripple noise caused by parasitic parameters; Ultra high frequency resonance noise generated during the switching process of power devices; Ripple noise caused by closed-loop regulation control.
Ripple is an AC interference signal superimposed on a DC signal and is an important criterion in power testing. Especially for power supplies used for special purposes, such as laser power supplies, ripple is one of their fatal weaknesses. So, the testing of power ripple is extremely important.
There are roughly two methods for measuring power ripple: one is voltage signal measurement method; The other clock is the current signal measurement method.
Generally, for constant voltage sources or constant current sources with low ripple performance requirements, voltage signal measurement method can be used. For constant current sources with high ripple performance requirements, it is best to use current signal measurement method.
Voltage signal measurement ripple refers to using an oscilloscope to measure the AC ripple voltage signal superimposed on the DC voltage signal. For a constant voltage source, testing can directly use a voltage probe to measure the voltage signal output to the load. For testing a constant current source, it is generally done by using a voltage probe to measure the voltage waveform at both ends of the sampling resistor. During the entire testing process, the setting of the oscilloscope is the key to whether real signals can be sampled.
