Which five factors most frequently cause output ripple from switching power supplies?
With the 20M oscilloscope bandwidth as the limit standard, the voltage is set to PK-PK (effective value is also measured), and the clip and ground wire on the oscilloscope control head are removed (because the clip and the ground wire will form a loop, like an antenna receiving Noise, introduce some unnecessary noise), use a grounding ring (it is also possible not to use a grounding ring, but the error generated by it should be considered), connect a 10UF electrolytic capacitor and a 0.1UF ceramic capacitor in parallel on the probe, and use an oscilloscope The probe of the oscilloscope should be tested directly; if the oscilloscope probe is not directly in contact with the output point, twisted pair or 50Ω coaxial cable should be used for measurement.
The output ripple of switching power supply 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 switching of power devices; ripple noise.
Ripple is an AC interference signal superimposed on a DC signal, and is a very important criterion in power supply testing. Especially for power supplies for special purposes, such as laser power supplies, ripple is one of its fatal points. Therefore, the test of power ripple is extremely important.
The measurement method of power supply ripple is roughly divided into two types: one is the voltage signal measurement method; the other is the current signal measurement method.
Generally, the voltage signal measurement method can be used for constant voltage sources or constant current sources that do not require much ripple performance. For a constant current source with high requirements for ripple performance, it is best to use the current signal measurement method.
Voltage signal measurement ripple refers to measuring the AC ripple voltage signal superimposed on the DC voltage signal with an oscilloscope. For a constant voltage source, the test can directly use a voltage probe to measure the voltage signal output to the load. For the test of the constant current source, the voltage waveform at both ends of the sampling resistor is generally measured by using a voltage probe. Throughout the test process, the setting of the oscilloscope is the key to whether the real signal can be sampled.
