Ⅰ. The PCB trace and layout of the power adapter
The feedback circuit should avoid magnetic components, switching tubes and power diodes. The placement and routing of the output filter capacitor is very important for ripple noise. Each loop has the same length, that is, the high-frequency current is evenly distributed to each capacitor.
If the PCB of Chinese power adapter is a multi-layer board, you can choose the layer closest to the main current loop layer to cover the ground, which can effectively solve the noise problem.
Ⅱ. Add RCD between the field effect tube of the power adapter and the input positive.
Generally, the reverse recovery time of the FET is selected to be 2~3 times slower than the diode D1 to avoid the formation of a through current. This current will generate a strong magnetic field and increase the output noise interference, so it can pass through the grid artificially Resistor R4 slows down the switching speed of the switch tube.
Ⅲ. The DS end of the power adapter's field effect tube is connected in parallel with RC
An RC circuit can be connected in parallel to both ends of the FET DS, which can also effectively reduce noise interference. The capacitor C2 is generally about 100P. Too large capacitance will increase the switching loss of the FET. The resistance R2 is generally less than 10Ω.
Ⅳ. The power adapter reduces the leakage inductance of the transformer
The sandwich winding method can make the primary and secondary windings more tightly coupled, so that the leakage inductance can be reduced, so as to achieve the purpose of reducing noise.
Ⅴ. The output diode of the power adapter is connected in parallel with RC at both ends
When the diode is turned on and off at high speed, the parasitic inductance and capacitance of the diode will produce high-frequency oscillation during the reverse recovery period. In order to suppress high-frequency oscillation, an RC snubber circuit can be added to both ends of the diode. The resistance R3 is generally 1Ω~100Ω, The capacitor C3 is generally 100pF~1nF. If the operating frequency of the industrial power adapter is low and the efficiency meets the requirements, the diode D3 can be a diode with a slower reverse recovery time.
Ⅵ. Power adapter output plus secondary LC filter
LC has a more obvious effect on noise and ripple suppression. Choose the appropriate inductor capacitance value according to the ripple frequency. However, due to the advantages of low price and small size of cylindrical inductors, cylindrical inductors are generally selected for inductance in LC, but cylindrical inductors It is an open magnetic structure, which will cause serious magnetic interference to the surroundings. We can use two inductors to be placed side by side, and the China power adapter current flows in the opposite direction. That is to help guide the magnetic flux from one magnetic column to another magnetic column, which can reduce electromagnetic interference.
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