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.
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, 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.
2. Two-stage filtering, that is, adding a first-stage LC filter
The LC filter has obvious suppression effect on noise and ripple. According to the frequency of the ripple to be removed, select the appropriate inductor and capacitor 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 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.
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 the highest power consumption. Any LDO has one indicator: the noise rejection ratio. is a frequency-dB curve.
4. On the diode and capacitor C or RC
When the diode is turned on and off at high speed, parasitic parameters must be considered. During the diode reverse recovery, the equivalent inductance and equivalent capacitance become an RC oscillator, producing high frequency oscillation. In order to suppress this high frequency oscillation, a capacitor C or RC snubber network should be connected in parallel across the diode. The resistance is generally 10Ω-100Ω, and the capacitor is 4.7pF-2.2nF.
The capacitor C or RC connected in parallel on the diode can be determined by trial and error. If it is not selected properly, it will cause more serious oscillation.
5. The diode is followed by an inductor (EMI filter)
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.
