2.2 using absorption circuit The main cause of EMI in switching power supplies is the sharp change in voltage and current, so it is necessary to reduce the rate of change of voltage and current in the circuit (du/dt and di/dt) as much as possible. The absorption circuit can suppress EMI. The basic principle is to provide a bypass when the switch is turned off, and absorb the energy accumulated in the parasitic distribution parameters, thereby suppressing the occurrence of interference. The RC snubber circuit as shown in Figure 2(a) can be connected in parallel across the switch. During the turn-on and turn-off of the switch or diode, the reverse spike current and spike voltage generated in 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 beneficial for the safety of the semiconductor device. At the same time, the buffer absorption circuit also reduces the spectral composition of the radio frequency radiation, which is beneficial to reduce the energy of the radio frequency radiation. Clamp circuits are primarily used to prevent the risk of breakdown of semiconductor devices and capacitors. Taking into account the protection of the clamp circuit and the efficiency requirements of the switching power supply, 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 turned on quickly, thereby discharging the surge energy and limiting the amplitude of the surge voltage to a certain amplitude. A saturable core coil or a microcrystalline magnetic bead may be connected in series on the positive electrode lead of the drain of the switch tube and the output diode. The material is generally cobalt. When the normal current is passed, the core is saturated, and the inductance is very small. Once the current is flowing in the opposite direction, it will generate a very large back EMF, which effectively suppresses the reverse surge current of the diode. 2.3 Shielding measures An effective way to suppress radiated noise is to shield. The electric field can be shielded by a material having good electrical conductivity, and the magnetic field can be shielded by a material having a high magnetic permeability. In order to prevent the magnetic field leakage of the transformer and make the primary and secondary coupling of the transformer good, the closed magnetic ring can be used to form the magnetic shielding. For example, the leakage flux of the can core is much smaller than that of the E type. For the connection line of the switching power supply, the power line should use the shielded wire to prevent external interference from being coupled into the circuit. Or use EMC components such as magnetic beads and magnetic rings to filter out high-frequency interference from power supplies and signal lines. However, it is important to note that the signal frequency cannot be disturbed by the EMC component, ie the signal frequency is within the passband of the filter. The outer casing of the entire switching power supply also needs to have good shielding characteristics, and the joints must meet the shielding requirements specified by EMC. Through the above measures, the switching power supply is protected from external electromagnetic environment and does not interfere with external electronic equipment. 2.4 Winding of the transformer The leakage inductance must be minimized when designing high frequency transformers. Because the greater the leakage inductance, the higher the peak voltage amplitude generated, the greater the loss of the drain clamp circuit, which inevitably leads to a reduction in power supply efficiency. To reduce the leakage inductance of the transformer, measures such as reducing the number of turns of the primary winding, increasing the width of the winding, and reducing the insulation between the windings are generally employed. The main parasitic parameters of the transformer are leakage inductance, inter-winding capacitance, and cross-coupling capacitance. The cross-coupling capacitance between the transformer windings provides a path for common mode noise to flow through the entire system. Faraday shields are used in the winding process of the transformer to reduce the cross-coupling capacitance. The Faraday shield is simply wrapped between copper and aluminum foil between the primary winding and the secondary winding to form a surface shield isolation region and grounded, wherein the primary winding and the secondary winding are interleaved to reduce the crossover. Coupling capacitor. In the installation procedure, the heat sink is generally required to be grounded. The parasitic capacitance between the drain and the heat sink of the switch provides a path for common mode noise. A copper foil or aluminum foil can be added between the drain and the heat sink and grounded. Reduce this parasitic capacitance. 2.5 Application of Grounding Technology The switching power supply needs to pay attention to the connection of the ground wire. The ground wire bears the heavy responsibility of the reference level, especially the reference ground of the control circuit, such as the ground level of the current detecting resistor and the ground level of the voltage dividing resistor without the isolated output. (1) Signal grounding of the device. The signal ground of the device may be a ground reference point for the signal at a point or piece of metal in the device, which provides a common reference potential for all signals in the device. Such as floating ground and hybrid grounding, in addition to single point grounding and multi-point grounding. (2) The device is connected to the earth. In engineering practice, in addition to seriously considering the signal grounding inside the equipment, the signal ground of the equipment, the casing and the earth are usually connected together, and the earth 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. This grounding method separates the noise source from the sensitive circuit. In addition, the ground wire should be as wide as possible, and the blank area can be filled with copper to reduce the ground level error and EMI. Vent spacer,stator core vent spacer lamination,stator core vent spacer Henan Yongrong Power Technology Co., Ltd , https://www.hnyongrongglobal.com