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How to Reduce the Ripple Wave of the Power Adapter?

1. PCB routing and layout of the power adapter


The feedback circuit should avoid magnetic components, switch tubes and power diodes. The placement and routing of the output filter capacitors are critical to the ripple noise, and each loop length is equal, that is, the high-frequency current will be evenly distributed to each capacitor.


If the PCB is a multi-layer board, you can choose the layer closest to the main current loop layer to cover the ground, and the ground cover can effectively solve the noise problem.


2. The field effect of the power adapter is to add RCD between the D level and the input positive


Generally, the reverse recovery time of the FET is selected to be 2 to 3 times slower than that of the diode D1 to avoid the formation of through current. This current will generate a strong magnetic field, which can increase the output noise interference, so the gate resistance R4 can be artificially passed to slow down the switching speed of the switch tube.


3. The field-effect tube DS end of the power adapter is connected in parallel with RC


A RC circuit can be connected in parallel at both ends of the field-effect tube DS, which can also effectively reduce noise interference. The capacitor C2 is generally around 100P. If the capacitance value of the China power adapter is too large, the switching loss of the field-effect tube will increase. The resistor R2 of less than 10Ω is generally selected.


4. The power adapter reduces the leakage inductance of the transformer


The sandwich winding method can make the coupling of the primary and secondary windings tighter, so that the leakage inductance can be reduced, so as to achieve the purpose of reducing noise.


5. The output diode of the power adapter is connected in parallel with RC


When the diode is turned on and off at high speed, during the reverse recovery period, the parasitic inductance and capacitance of the diode will produce high-frequency oscillation. In order to suppress the high-frequency oscillation, an RC snubber circuit can be added at both ends of the diode. The resistance R3 is generally 1Ω to 100Ω. Capacitor C3 is generally 100pF to 1nF. If the operating frequency of the power supply is low and the efficiency meets the requirements, diode D3 can be a diode with a slower reverse recovery time.


6. The output of the power adapter is added with a secondary LC filter


LC has obvious effect on the suppression of noise and ripple. Select the appropriate inductor and capacitor value according to the ripple frequency. However, due to the advantages of low price and small size of cylindrical inductors, most of the inductors in LC will choose cylindrical inductors. However, cylindrical inductors have the open magnetic structure, which will cause serious magnetic interference to the surrounding. We can use two inductors side by side, and the current flows in opposite directions, that is, it helps to guide the magnetic flux from one magnetic column to the other, which can reduce the electromagnetic interference.

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