How to reduce ripple of power adapter

1. PCB routing and layout

The feedback circuit shall avoid magnetic components, switching tubes and power diodes. The placement and routing of the output filter capacitor is very important to the ripple noise. Each circuit is of the same length, that is, the high-frequency current will be evenly distributed to each capacitor. If the PCB is a multilayer board, the nearest layer to the main current circuit layer can be selected to be grounded, which can effectively solve the noise problem

2. Add RCD between FET level D and input positive

Generally, the reverse recovery time of the FET is 2~3 times slower than that of the diode D1 to avoid the formation of a through current, which will produce a strong magnetic field and increase the output noise interference. Therefore, the switching speed of the switch can be artificially slowed down through the grid resistance R4.

3. Parallel RC at DS end of FET

An RC circuit can be connected in parallel at both ends of the FET DS to effectively reduce noise interference. As shown in Figure 2, the capacitance C2 is generally about 100p. Excessive capacitance will increase the switching loss of the FET, and the resistance R2 is generally less than 10 Ω.

4. Reducing transformer leakage inductance

The sandwich winding method can make the primary winding more closely coupled and reduce the leakage inductance, so as to achieve the purpose of reducing noise.

5. Parallel RC at both ends of output diode

When the diode is turned on and off at high speed, the parasitic inductance and capacitance of the diode will generate high-frequency oscillation during reverse recovery. In order to suppress high-frequency oscillation, RC buffer circuit can be added at both ends of the diode, as shown in Figure 2. The resistance R3 is generally 1 Ω~100 Ω, and the capacitance C3 is generally 100pf~1nf. If the power supply operating frequency is low and the efficiency meets the requirements, diode D3 can choose a diode with slow reverse recovery time.

6. Output plus two-stage LC filtering

LC has obvious noise and ripple suppression effect. The appropriate inductance and capacitance value is selected according to the ripple frequency. However, due to the advantages of low price and small volume of cylindrical inductors, cylindrical inductors are generally selected in LC. However, cylindrical inductors are open magnetic structures, which will cause serious magnetic interference around. We can use two inductors side by side, and the current flow direction is opposite, That is, it helps to guide the magnetic flux from one pole to another, thus reducing the electromagnetic interference.