EMC Problem Analysis of Power Adapter

　　Power adapter is a power supply conversion device for small portable electronic equipment and electronic appliances. It is generally composed of shells, transformers, inductors, capacitors, control ICs, PCB boards and other components. Its working principle is to convert AC input to DC Output; According to the connection method, it can be divided into wall type and desktop type. Widely used in security cameras, set-top boxes, routers, light bars, massagers and other equipment. The adapter is essentially a switching power supply, so the EMC problem is getting more and more attention.

　　In switching power supplies, the rapid changes in current and voltage caused by the high-frequency turn-on and turn-off of power devices are the main causes of EMI.

　　Magnetic and electric field noise is directly related to varying voltages and currents and coupling channels such as parasitic inductances and capacitances. It is intuitively understood that reducing the voltage rate du/dt and the current change rate di/dt and reducing the corresponding stray inductance and capacitance values can reduce the noise generated by the above-mentioned magnetic and electric fields, thereby reducing EMI interference.

　　1. Reduce the voltage rate du/dt and the current change rate di/dt

　　(1) The reduction of the voltage rate du/dt and the current change rate di/dt can be achieved by the following methods: changing the resistance value of the gate and increasing the buffer suction circuit, as shown in Figure 2 and Figure 3. Increasing the resistance value of the gate can reduce the voltage change rate of the power device during turn-on.

　　2. Reduce parasitic inductance and capacitance values

　　(1) The switching device is one of the noise sources, and the stray inductance and parasitic capacitance of its internal leads are also channels for noise coupling. However, because these parameters are inherent characteristics of the device, electronic design and application engineers cannot optimize them. Parasitic capacitance includes drain-source capacitance and gate-drain Miller capacitance.

　　(2) The transformer is another noise source, and the leakage inductance of the primary and secondary, the interlayer capacitance of the primary, the interlayer capacitance of the secondary, and the coupling capacitance between the primary and the secondary are the channels of noise. The solutions are as follows:

　　The primary or secondary interlayer capacitance can be reduced by reducing the number of winding layers, and increasing the width of the transformer skeleton window can reduce the number of winding layers. Separate windings, such as primary winding, can reduce the leakage inductance of the primary, but due to the increase of the contact area between the primary and the secondary, the coupling capacitance of the primary and the secondary is increased.

　　The Faraday shield with copper can reduce the coupling capacitance between the primary and secondary. The Faraday shield is wound between the primary and secondary, and should be connected to the primary or secondary static points such as primary ground and secondary ground. The Faraday shield reduces the coupling coefficient of the primary and secondary, thereby increasing the leakage inductance.

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