Measures to suppress electromagnetic interference of power adapter

　　There are two forms of electromagnetic interference in the power adapter: common mode interference and differential mode interference. According to the electromagnetic interference sources analyzed above, combined with their coupling paths, the interference can be suppressed from the aspects of EMI filter, absorption circuit, grounding and shielding, and the electromagnetic interference can be attenuated to the allowable limit.

　　2.1 European standard power adapter AC input EMI filter;

　　Filtering is a method of suppressing conducted interference. Connecting a filter to the input end of the power supply can suppress the noise from the power grid from invading the power supply itself, and it can also suppress the interference generated by the power adapter and fed back to the power grid. As an important unit to suppress the conduction interference of the power line, the power filter plays an extremely important role in the electromagnetic compatibility design of the equipment or system. The power input terminal usually adopts the EMI filter circuit shown in Figure 1. This circuit can effectively suppress low-frequency differential mode disturbance and high-frequency common mode disturbance at the input end of the AC power supply. In the circuit, the differential mode capacitors Cx1 and Cx2 (also known as X capacitors) connected across the power supply are used to filter out differential mode interference signals. Generally, ceramic capacitors or polyester film capacitors are used, and the capacitance value is usually 0.1~0.47F. The common mode capacitors Cy1 and Cy2 (also known as Y capacitors) connected to the ground in the middle are used to short-circuit the common mode noise current, and the value range is usually C1=C2#2200pF. The suppression inductance L1 and L2 are usually 100~130H. The common mode choke coil L is composed of two identical coils wound on a magnetic core in the same direction. The inductance L#15~25mH is usually required. When the load current passes through the common mode choke, the magnetic lines of force generated by the coils connected in series on the live line and the lines of magnetic force generated by the coils connected in series on the neutral line are in opposite directions, and they cancel each other out in the magnetic core. Therefore, even in the case of a large load current, the magnetic core will not saturate. As for the common mode interference current, the magnetic fields generated by the two coils are in the same direction, which will present a larger inductance, thus playing the role of attenuating the common mode interference signal.

　　2.2 Utilize absorption circuit

　　The main cause of EMI in the power adapter is the sharp change of voltage and current, so it is necessary to reduce the rate of change (du/dt and di/dt) of the voltage and current in the circuit as much as possible. The absorption circuit can suppress EMI, and its basic principle is to provide a bypass when the switch is turned off, absorb the energy accumulated in the parasitic distribution parameters, thereby suppressing the occurrence of interference. The RC absorption circuit shown in Figure 2(a) can be connected in parallel at both ends of the switch tube. The reverse spike current and spike voltage generated in the switch tube or diode during the turn-on and turn-off process of the tube can be buffered. get over. The buffer absorption circuit can reduce the amplitude of the spike voltage and reduce the rate of change of the voltage waveform, which is very good for the safety of semiconductor devices. At the same time, the buffer absorption circuit also reduces the spectral components of radio frequency radiation, which is beneficial to reduce the energy of radio frequency radiation.

　　The clamp circuit is mainly used to prevent the danger of semiconductor devices and capacitors from being broken down. Taking into account the protection of the clamp circuit and the efficiency requirements of the power adapter, the breakdown voltage of the TVS tube is selected to be 1.5 times the induced voltage of the primary winding. When the voltage on the TVS exceeds a certain amplitude, the device is quickly turned on, thereby discharging the surge energy and limiting the amplitude of the surge voltage to a certain amplitude. A saturable magnetic core coil or microcrystalline magnetic beads can be connected in series on the drain of the switch tube and the positive lead of the output diode. The material is generally cobalt. When the normal current passes through the magnetic core is saturated, and the inductance is very small. Once the current is going to flow in the reverse direction, it will produce a very large back EMF, so that the reverse surge current of the diode can be effectively suppressed.

　　2.3 Shielding measures

　　An effective way to suppress radiated noise is to shield. Materials with good conductivity can be used to shield the electric field, and materials with high magnetic permeability can be used to shield the magnetic field. In order to prevent the magnetic field leakage of the transformer and make the primary and secondary coupling of the transformer good, a closed magnetic ring can be used to form a magnetic shield. For example, the leakage flux of the pot-shaped magnetic core is significantly smaller than that of the E-type. The connection line of the power adapter and the power line should use shielded wires to prevent external interference from coupling into the circuit as much as possible. Or use EMC components such as magnetic beads and magnetic rings to filter out high-frequency interference from power and signal lines. However, it should be noted that the signal frequency cannot be interfered by EMC components, that is, the signal frequency must be within the passband of the filter. The shell of the entire power adapter also needs to have good shielding characteristics, and the joints must meet the shielding requirements specified by EMC. Through the above measures, it is ensured that the power adapter is neither interfered by the external electromagnetic environment nor interfered with the external electronic equipment.

　　2.4 Winding of the transformer

　　Leakage inductance must be minimized when designing high-frequency transformers. Because the greater the leakage inductance, the higher the amplitude of the peak voltage generated, and the greater the loss of the drain clamp circuit, which will inevitably lead to a decrease in power supply efficiency. Reducing the leakage inductance of the transformer usually adopts measures such as reducing the number of turns of the primary winding, increasing the width of the winding, and reducing the insulation layer between the windings.

　　The main parasitic parameters of the transformer are leakage inductance, capacitance between windings, and cross-coupling capacitance. The cross-coupling capacitors between the transformer windings provide a path for common mode noise to flow through the entire system.

　　In the winding process of the transformer, Faraday shielding is used to reduce the cross-coupling capacitance. Faraday shielding is simply to wrap copper or aluminum foil between the primary winding and the secondary winding to form a surface shielding layer isolation area, and ground, in which the primary winding and the secondary winding are wound in a staggered manner to reduce crossover Coupling capacitor. In the installation procedures, the radiator is generally required to be grounded, so the parasitic capacitance between the drain of the switch tube and the radiator provides a path for common mode noise. A copper or aluminum foil can be added between the drain and the radiator and grounded. Reduce this parasitic capacitance.

　　2.5 Application of grounding technology

　　The power adapter needs to pay attention to the connection of the ground wire. The ground wire is responsible for the reference level, especially the reference ground of the control circuit, such as the ground level of the current detection resistor and the ground level of the voltage divider resistor without isolation output.

　　(1) The signal of the equipment is grounded. The signal ground of the device may be a point or a piece of metal in the device as the signal ground reference point, which provides a common reference potential for all signals in the device. Such as floating ground and mixed grounding, as well as single-point grounding and multi-point grounding.

　　(2) Connect the equipment to the earth. In engineering practice, in addition to serious consideration of the signal grounding inside the equipment, the signal ground and chassis of the equipment are usually connected to the ground, and the ground is used as the grounding reference point of the equipment.

　　The ground level attenuation of the control signal should be as small as possible. Therefore, the control part is grounded at one point, and then the common connection point is connected to the power ground at a single point. This grounding method can separate the noise source from the sensitive circuit. In addition, the ground wire should be laid as wide as possible, and the blank area can be filled with copper to reduce the ground level error and EMI.

　　Use surface mount components as much as possible in the device to make the assembly density higher, smaller volume, lighter weight, higher reliability, good high-frequency characteristics, and reduce electromagnetic and radio frequency interference.

　　2.6PCB component layout and routing

　　The string between strip lines, wires, and cables in PCB is one of the most difficult problems to overcome in printed circuit board circuits [7]. The radiation disturbance of the power adapter is proportional to the product of the current in the current path, the loop area of the path, and the square of the current frequency, so the PCB layout design will directly affect the electromagnetic compatibility performance of the whole machine. When designing the printed circuit board of the power adapter, we must start with the optimized design of layout and wiring.

　　(1) The wiring of printed boards usually meets the following principles

　　1. The wires used for the input and output terminals should be avoided as far as possible. It is best to add ground wires between wires to avoid feedback coupling;

　　2. The printed board wires should be as wide as possible, especially the power wire and the ground wire;

　　3. The corners of the printed conductors generally adopt a circular arc shape;

　　4. The trace width of the dedicated zero-volt line and power line (1mm, the power line and the ground line should be as close as possible, etc.).

　　(2) The layout of components should generally comply with the following principles: 1. Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.

　　2. Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between the components.

　　3. For circuits operating at high frequencies, the distribution parameters between components must be considered. In general circuits, components should be arranged as equal as possible.

　　4. The distance between the components located on the edge of the circuit board and the edge of the circuit board is generally not less than 2mm.

　　3 concluding remarks

　　The size of power adapter is getting smaller and smaller, the power density is getting bigger and bigger, EMI/EMC problem has become a key factor of the stability of power adapter, more and more people pay more and more attention. There are many electromagnetic compatibility control strategies and control technical solutions for power adapters, such as suppression of interference transmission channels, space separation, time separation, frequency management, electrical isolation, and so on. In the design of the power adapter, only the comprehensive use of various electromagnetic interference suppression technologies can effectively improve the electromagnetic compatibility of the power adapter and truly meet the needs of various occasions.

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