Introduction The high switching frequency of power switching devices is the main cause of electromagnetic interference (EMI) in switching power supplies. While increasing the switching frequency reduces the size and weight of the power supply, it also leads to more serious EMI problems. How to reduce EMI in products and ensure they pass EMC standard certification tests such as FCC or IEC1000 has become an urgent problem to solve. 1 EMI Analysis The input is 220V AC, which is converted to DC by a power diode rectifier bridge to serve as the input of the flyback converter. The output consists of three DC outputs: +5V, 15V, and 12V. There is also an auxiliary 5V power supply to power the optocoupler PC817. The control circuit uses feedback control and selects the TOP223Y chip from the TOPSwicth series. When the switching power supply is operating, its internal voltage and current waveforms rise and fall within a very short time; therefore, the switching power supply itself is a noise source. Interference from switching power supplies can be classified into two types according to the noise source: spike interference and harmonic interference. The fundamental way to prevent power supply interference from harming electronic systems and the power grid is to weaken the noise source or cut off the coupling path between power supply noise and electronic systems/power grids. In this circuit, the AC input voltage Ui is rectified into a sinusoidal pulsating voltage by a power diode rectifier bridge, and then smoothed into DC by capacitor C12. However, the waveform of the capacitor current is not a sine wave but a pulse wave. The current contains high-order harmonics. A large amount of current harmonic components flow into the power grid, causing harmonic pollution to the power grid. In addition, because the current is a pulse wave, the power factor of the power supply input is reduced. 2 EMI Suppression 2.1 Suppression of High-Order Harmonics A common-mode choke L11 is used in the circuit to suppress high-order harmonics. For the two input lines of the switching power supply, there are common-mode interference and differential-mode interference. Under the action of differential-mode interference signal, the current i generated by the interference source produces a magnetic flux Φ in the opposite direction in the magnetic core. There is no magnetic flux in the magnetic core, and the coil inductance is almost zero. Therefore, differential-mode interference signal cannot be suppressed. Under the influence of common-mode interference signals, the magnetic flux generated by the two coils is in the same direction, which reinforces each other, and the inductance of each coil is twice that of the coil when it exists alone. Therefore, the electromagnetic coils connected in this way have a strong suppression effect on common-mode interference. A common-mode choke is inserted between the mains and the rectifier bridge in the circuit. This choke presents extremely low impedance to the differential-mode grid-side current at the mains frequency, so the voltage drop to the mains is extremely low; while it has a high equivalent impedance to the high-frequency common-mode noise generated by the power supply, so the desired insertion loss can be obtained. 2.2 Choke L11 and C11 form a low-pass filter. The equivalent inductance of choke L11 is L, with the power supply terminal as the input and the mains direction as the output. Its transfer function is G(s) = = (1) The amplitude is A(ω) = |G(jω)| = (2) The phase is L(ω) = 201gA(ω) = -201g (3) In the low frequency range when ω<< , A(ω)≈1, L(ω)≈0 In the high frequency range when ω>> , A(ω)≈ , L(ω)≈ -401gωLC11 is shown in Figure 5. It can be seen that the low-pass filter composed of the above LC network can filter out high-order harmonics above ω0=1/LC11. 2.3 Common-mode and differential-mode filter scheme The main EMI problem of this circuit is the transmission of power supply noise into the power grid. The original common-mode choke L11 and the filter circuit composed of capacitors C11 and C12 are changed to a circuit. L1, L2, C1 can remove differential-mode interference, and L3, C2, C3 can remove common-mode interference. L1 and L2 are materials that are not easily magnetically saturated; C1 can be a ceramic capacitor; L3 is a common-mode choke; given C=C2=C3 and the cutoff frequency fo, L3 can be calculated according to L3=1/[(2πfo)2C]; given C1 and the cutoff frequency fo, L1 and L2 can be calculated according to L1=L2=1/[2(2πfo)2C1]. 2.4 Buffer Circuit Due to the rapid switching, the switching current and voltage waveforms are in pulse form, generating noise pollution, increasing the power supply output ripple, and affecting the power supply performance. In the circuit, the input is AC 220V, and after rectification, the voltage on the capacitor is about 1.2 to 1.4 times the AC effective value, that is, the maximum is Ucm=220×1.4=308V. In addition, the voltage Up=Un×88/9 of the transformer secondary side refracted to the primary side, where Un is the voltage of the first winding of the secondary side, which is generally about 9V, so that the regulated output is 5V. Then Up=88V. Therefore, the total voltage that the switch must withstand when it is turned off is Ut = Ucm + Up = 308 + 88 = 396V. It is evident that overvoltage protection for the switch is necessary. The TOPSwitch switch chip used in the circuit has internal overvoltage protection and buffer circuitry. For safety, an external buffer circuit consisting of resistors R and capacitor C is added. The waveforms of the switch voltage ut, current ic, and turn-off power dissipation pt are shown before and after adding the buffer circuit. After adding the RC buffer circuit, the switching voltage rise rate slows down, noise is reduced, EMI is suppressed, and the switching power dissipation is reduced, preventing the transistor from being damaged due to overcurrent and overheating. R in the buffer circuit limits the current when the switch is turned on and capacitor C discharges, preventing impact on the switch transistor. For the current surge when the switch is turned on, no current-limiting inductor is added to the circuit due to the current limiting of the transformer primary winding Np. 2.5 Opto-isolation: The Flyback circuit uses a PC817 optocoupler to isolate the main circuit and the control circuit. In power supply circuits, switch control is crucial, requiring high precision and stability. The control circuit is also sensitive to noise; noise can disrupt the control signals, severely impacting the power supply's operation and performance. Therefore, a PC817 is used to isolate the two parts of the power supply, preventing noise from being conducted into the control circuit. 3. Conclusion Through EMI analysis and the adoption of appropriate suppression methods, the designed switching power supply exhibits strong electromagnetic interference resistance and high power supply stability. In the sewing machine servo control system, it satisfies the drive control requirements of the Mitsubishi IPM module, ensuring safe, reliable, and stable motor operation.