How much do you know about how to deal with the ripple of a switching power supply?

What is the ripple of a switching power supply? What impact does it have? This article is based on how to suppress or reduce the ripple of a switching power supply. The ripple must exist in theory and in practice. There are usually 5 ways to suppress or reduce it, let’s interpret these methods one by one!

What is the ripple of a switching power supply? What impact does it have? This article is based on how to suppress or reduce the ripple of a switching power supply. The ripple must exist in theory and in practice. There are usually 5 ways to suppress or reduce it, let’s interpret these methods one by one!

1) Increase 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 value of the output capacitor can reduce the ripple.

The usual practice is to use aluminum electrolytic capacitors for the output capacitors in order 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 will be 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 is working, 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, it is close to the current input terminal (in the case of BucK type, near SWITcH), and parallel capacitors are used to provide current.

The above method has limited effect on reducing ripple. Because of the volume limitation, the inductance will not be too large; the output capacitance will increase to a certain extent, and there will be no obvious effect on reducing the ripple; increasing the switching frequency will increase the switching loss. Therefore, this method is not very good when the requirements are relatively strict. For the principle of switching power supply, you can refer to various switching power supply design manuals.

2) Two-stage filtering, that is, adding one-stage LC filter

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

If the sampling point is selected before the LC filter (Pa), the output voltage will decrease. Because any inductor has a DC resistance, when there is a current output, there will be a voltage drop across 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 hope to get. But this introduces an inductor and a capacitor inside the power system, which may cause the system to be unstable. Regarding the stability of the system, many materials have been introduced, so I won’t write them in detail here.

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 most costly and power-consuming method. Any LDO has an indicator: noise rejection ratio. It is a frequency-dB curve, as shown on the right is the curve of Linear Technology’s LT3024.

To reduce ripple. The PCB layout of the switching power supply is also very critical, which is a very powerful problem. There are dedicated switching power supply PCB engineers. 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 way is to parallel a capacitor C or RC on the diode, or a series inductor.

4) Parallel capacitor C or RC on the diode

Parasitic parameters should be considered when the diode is turned on and off at high speed. During the diode reverse recovery period, the equivalent inductance and equivalent capacitance become an RC oscillator, generating high-frequency oscillation. In order to suppress this high-frequency oscillation, a capacitor C or RC snubber network must be connected in parallel across the diode. The resistance is generally 10Ω-100 Ω, and the capacitance is 4.7pF-2.2nF.

The value of the capacitor C or RC connected in parallel on the diode can only be determined after repeated trials. If the selection is improper, it will cause more serious oscillation.

If the requirements for high-frequency noise are strict, soft switching technology can be used. There are many books dedicated to introducing soft switches.

5) Connect the inductor after the diode (EMI filter)

This is also a commonly used method to suppress high-frequency noise. In view of the frequency of the noise, selecting the appropriate inductance element can also effectively suppress the noise. It should be noted that the rated current of the inductor must meet the actual requirements. The simpler approach will not be explained in detail.

Summary: The above is about the ripple of the switching power supply, some of the contents are summarized, if you can add some waveforms, it would be better. Although it may not be complete, it is sufficient for general applications. Regarding noise suppression, not all applications are in practice. It is important to choose an appropriate method according to your own design requirements, such as product volume, cost, development cycle, etc. The above is the analysis of the ripple of the switching power supply, I hope to help everyone.

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