Suppression of ripple in switching power supply
For switching ripple, both in theory and in practice must exist. There are usually five ways to suppress or reduce it:
1. Increase the inductance and output capacitor filter
According to the formula of 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 capacitor 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 output capacitor value can reduce the ripple.
Usually, for output capacitors, aluminum electrolytic capacitors are used to achieve 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 very well, usually close to the current input terminal (take the BucK type as an example, near SWITcH), and connect a capacitor in parallel to provide current.
The effect of the above method on reducing ripple is limited. Due to volume limitations, the inductance will not be too large; if the output capacitance increases to a certain level, it will have no obvious effect on reducing the ripple; increasing the switching frequency will increase the switching loss. So when the requirements are stricter, this method is not very good. For the principle of switching power supply, etc., you can refer to various switching power supply design manuals.
2. Two-stage filtering, that is, adding an additional stage of LC filter
The LC filter has a more obvious suppression effect on the noise ripple. According to the frequency of the ripple to be removed, an appropriate inductance and capacitance are selected to form a filter circuit, which can generally reduce the ripple very 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, resulting in a drop 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. But this introduces an inductance and a capacitor inside the power system, which may cause system instability. Regarding system stability, a lot of information has been introduced, so I won’t write in detail here.
3. After switching power supply output, connect to 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 method with the highest cost and highest power consumption. Any LDO has an indicator: the noise rejection ratio. is a frequency-dB curve.
To reduce ripple. The PCB layout of the switching power supply is also very critical, which is a very difficult problem. There are dedicated switching power supply PCB engineers. For high-frequency noise, due to the high frequency and large amplitude, although the post-stage filtering has a certain effect, the effect is not obvious. There is special research in this area, and the simple way is to connect capacitance C or RC, or series inductance on the diode.
4. Connect capacitor C or RC on the diode
When a diode is turned on and off at high speed, parasitic parameters must be considered. During the reverse recovery period of the diode, 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 needs to be connected in parallel across the diode. The resistance is generally 10Ω-100Ω, and the capacitance is 4.7pF-2.2nF.
The value of the capacitor C or RC connected in parallel to the diode can only be determined through trial and error. If it is not selected properly, it will cause more serious oscillations.
If the requirements for high-frequency noise are strict, soft switching technology can be used. There are many books dedicated to soft switching.
5. The diode is followed by an inductor (EMI filter)
This is also a commonly used method to suppress high-frequency noise. Aiming at the frequency of noise generation, selecting the appropriate inductance element can also effectively suppress the noise. It should be noted that the rated current of the inductor should meet the actual requirements. The relatively simple method will not be explained in detail.
