Switching ripple must exist in theory and in practice. There are usually five ways to suppress or reduce it:
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 inductance value and the output capacitance value can reduce the ripple.
Similarly, the relationship between output ripple and output capacitance: vripple=Imax/(Co×f). It can be seen that increasing the output capacitance value can reduce the ripple.
In general, for the output capacitor, aluminum electrolytic capacitors are used to achieve the purpose of large capacity. However, electrolytic capacitors are not very effective in suppressing high-frequency noise, and the ESR is also relatively large, so a ceramic capacitor will be connected in parallel next to it to make up for the shortcomings of aluminum electrolytic capacitors.
At the same time, when the switching power supply is working, the voltage Vin at the input end remains unchanged, but the current changes with the switch. At this time, the input power supply will not provide current very well. Usually, a capacitor is connected in parallel near the current input end (for example, near SWITcH for BucK type) to provide current.
The above method has limited effect on reducing ripple. Due to volume limitations, the inductor will not be made very large; when the output capacitance increases to a certain extent, there is no obvious effect on reducing ripple; increasing the switching frequency will increase the switching loss. Therefore, this method is not very good when the requirements are strict. For the principles of switching power supplies, etc., you can refer to various switching power supply design manuals.
Secondary filtering, that is, adding another LC filter
The LC filter has a more obvious effect on suppressing noise ripples. According to the ripple frequency to be removed, the appropriate inductor and capacitor are selected to form a filter circuit, which can generally reduce the ripple well.
The sampling point is selected before the LC filter (Pa), and the output voltage will decrease. Because any inductor has a DC resistance, when there is current output, there will be a voltage drop on the inductor, resulting in a decrease in the output voltage of the power supply. And this voltage drop varies with the output current.
The sampling point is selected after the LC filter (Pb), so that the output voltage is the voltage we want to get. However, this introduces an inductor and a capacitor inside the power supply system, which may cause system instability. Regarding system stability, there are many materials that introduce it, so I won’t write it in detail here.
After the output of the switching power supply, connect the LDO filter
This is the most effective way to reduce ripple and noise. The output voltage is constant and there is no need to change the original feedback system, but it is also the most expensive and power-consuming method. Any LDO has an indicator: noise suppression ratio. It is a frequency-dB curve.
To reduce ripple. The PCB wiring of the switching power supply is also very critical, which is a very difficult problem. There are special switching power supply PCB engineers. For high-frequency noise, due to the high frequency and large amplitude, the post-stage filtering has a certain effect, but the effect is not obvious. There are special studies in this area. The simple way is to connect capacitors C or RC in parallel with the diode, or connect inductors in series.
Connect capacitors C or RC in parallel with the diode
When the diode is turned on and off at high speed, parasitic parameters should be considered. During the reverse recovery of the diode, the equivalent inductance and equivalent capacitance become an RC oscillator, generating high-frequency oscillations. In order to suppress this high-frequency oscillation, a capacitor C or an RC snubber network is connected in parallel at both ends of the diode. The resistor is generally 10Ω-100Ω, and the capacitor is 4.7pF-2.2nF.
The value of the capacitor C or RC connected in parallel to the diode must be determined through repeated tests. If it is not properly selected, it will cause more serious oscillations.
If the high-frequency noise requirements are strict, soft switching technology can be used. There are many books specifically introducing soft switching.
Connecting an inductor after the diode (EMI filtering)
This is also a common method to suppress high-frequency noise. According to the frequency of the noise, choosing a suitable inductor component can also effectively suppress the noise. It should be noted that the rated current of the inductor must meet the actual requirements. It is a relatively simple approach and will not be explained in detail.
