1. Increase inductance and output capacitance 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 and output capacitance can reduce ripple.
Similarly, the relationship between the output ripple and the output capacitance: vripple=Imax/(Co×f). It can be seen that increasing the value of the output capacitance can reduce the ripple.
Typically, for output capacitors, aluminum electrolytic capacitors are used to achieve large capacitance. However, the effect of electrolytic capacitors in suppressing high-frequency noise is not very good, 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 input terminal voltage Vin does not change, but the current changes with the switching. In this case, the input power supply will not be able to supply the current well. Normally, it is close to the current input (or SWITcH in the case of BucK type) and a capacitor is connected in parallel to supply the current.
The above methods have a limited effect on reducing ripple. Due to volume limitations, the inductance will not be too large; The output capacitance is increased to a certain extent, which has no obvious effect on reducing ripple. Increasing the switching frequency increases the switching losses. Therefore, this method is not very good in the case of more stringent requirements. The principle of switching power supply can refer to various switching power supply design manuals.
2. Two-stage filtering, that is, adding a one-stage LC filter
The effect of the LC filter on the noise ripple is more obvious. Depending on 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.
If the sampling point is selected before the LC filter (Pa), the output voltage drops. Because any inductor has a DC resistance, when there is a current output, there will be a voltage drop across the inductor, causing the output voltage of the power supply to drop. And this voltage drop varies with the output current.
Select the sampling point after the LC filter (Pb) so that the output voltage is the voltage we want. However, this introduces inductors and capacitors inside the power supply system, which can lead to system instability. A lot of information has been introduced about the stability of the system, so I won't go into detail here.
3. The output of the switching power supply is connected to the LDO filter
This is the most effective way to reduce ripple and noise. The output voltage is constant, which does not require changes to the original feedback system, but is also the most costly and power-hungry method. There is one metric for any LDO: noise rejection ratio. It is a frequency-dB curve, as shown in the figure on the right for Linear Technology's LT3024.
to reduce ripple. The PCB layout of the switching power supply is also critical, which is a very serious 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 is a special study in this area, and the simple way is to connect a capacitor C or RC in parallel on the diode, or an inductor in series.
4. The parallel capacitor C or RC on the diode
Parasitic parameters should be taken into account when the diode is turned on and off at high speeds. During diode reverse recovery, the equivalent inductance and equivalent capacitance become an RC oscillator, producing high-frequency oscillations. To suppress this high-frequency oscillation, a capacitor C or RC buffer network must be connected in parallel at both ends of the diode. The resistance is generally 10Ω-100Ω, and the capacitance is 4.7pF-2.2nF.
The value of the capacitance C or RC connected in parallel on the diode can only be determined by trial and error. If not chosen correctly, it can cause more severe oscillations.
If there are strict requirements for high-frequency noise, soft switching technology can be used. There are a lot of books dedicated to soft switching.
5. Inductor followed by diode (EMI filter)
It is also a commonly used method of suppressing high-frequency noise. Selecting the appropriate inductive element for the frequency of the noise can also effectively suppress the noise. It should be noted that the rated current of the inductor must meet the actual requirements. The easier way will not be explained in detail.
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