For switching ripple, both theoretically and practically exist, there is periodic jitter in the output to form ripple. Ripple can be voltage or current ripple. The ripple of the switching power supply can come from 2 places: the input terminal (low frequency ripple); the cycle of switching. There are generally three ways to suppress or reduce it:

1. Increase the inductance and output capacitor filtering

According to the formula of the switching power supply, the current fluctuation in the inductor is inversely proportional to the inductance value, and the output ripple is inversely proportional to the output capacitance value. Therefore, increasing the inductor value and output capacitor value can reduce the ripple.  

The usual practice, for the output capacitor, use aluminum electrolytic capacitors to achieve the purpose of large capacity. However, electrolytic capacitors are not very effective in suppressing high-frequency noise, and the ESR is relatively large, so a ceramic capacitor is connected in parallel next to it to make up for the lack of aluminum electrolytic capacitors.

At the same time, when the switching power supply works, the voltage Vin at the input terminal does not change, but the current changes with the switch. At this time, the input power supply will not provide current well, usually close to the current input terminal, parallel capacitors to provide current.

2. Two-stage filtering, that is, adding a first-stage LC filter

The LC filter has obvious inhibitory effect on the ripple. According to the frequency of the ripple to be removed, the appropriate inductor and capacitor are selected to form a filter circuit, which can generally reduce the ripple well.

3. After switching power supply output, connect LDO filter

This is the most effective way to reduce ripple and noise, the output voltage is constant, and the original feedback system does not need to be changed, but it is also the method with the highest cost and highest power consumption. Any LDO has an indicator: the noise rejection ratio, which is a frequency-dB curve.

For high-frequency noise, due to the high frequency and large amplitude, although the post-filter has a certain effect, the effect is not obvious. There are special studies in this area, and the simple approach is to add capacitors C or RC to the diode, or series inductors.

4. Diode followed by inductor (EMI filtering)

This is also a commonly used method to suppress high-frequency noise. According to the frequency that generates noise, selecting an appropriate inductive element can also effectively suppress noise. It should be noted that the rated current of the inductor must meet the actual requirements.