How to suppress the interference of switching power supply?

　　introduction

　　As a power supply device for electronic equipment, switching power supply has the advantages of small size, light weight, high efficiency, etc., and has been widely used in digital circuits. However, due to working in a high-frequency switching state, it is a strong interference source, and the interference generated by itself directly endangers the normal operation of electronic equipment. Therefore, suppressing the electromagnetic noise of the switching power supply itself and improving its immunity to electromagnetic interference to ensure that electronic equipment can work safely and reliably for a long time is an important topic in the development and design of switching power supply.

　　1 Generation of switching power supply interference

　　The interference of switching power supply can be generally divided into two categories: one is the interference caused by the internal components of the switching power supply; the other is the interference caused by the switching power supply due to external factors. Both involve human factors and natural factors.

　　1.1 Internal interference of switching power supply

　　The EMI generated by the switching power supply is mainly caused by the high-order harmonic current interference generated by the basic rectifier and the peak voltage interference generated by the power conversion circuit.

　　1.1.1 Basic Rectifier

　　The rectification process of the basic rectifier is the most common cause of EMI. This is because the industrial frequency AC sine wave is no longer a single frequency current after rectification, but becomes a DC component and a series of harmonic components with different frequencies. Harmonics (especially high-order harmonics) will generate conducted interference and radiated interference along the transmission line, causing the front-end current to be distorted. On the one hand, it distorts the current waveform connected to its front-end power line, and on the other hand, it generates radio frequency interference through the power line.

　　1.1.2 Power conversion circuit

　　The power conversion circuit is the core of the switching power supply , which has a wide bandwidth and rich harmonics. The main components that produce this pulse interference are

　　1) Switching tube There is distributed capacitance between the switching tube and its heat sink and the casing and the leads inside the power supply. When a large pulse current (roughly a rectangular wave) flows through the switching tube, the waveform contains many high-frequency components. At the same time, the device parameters used in the power supply, such as the storage time of the switching power tube, the large current of the output stage, and the reverse recovery time of the switching rectifier diode, will cause the circuit to be short-circuited instantly and generate a large short-circuit current. In addition, the load of the switching tube is a high-frequency transformer or energy storage inductor. At the moment the switching tube is turned on, a large inrush current appears in the primary of the transformer, causing spike noise.

　　2) The transformer in the high-frequency transformer switching power supply is used for isolation and voltage transformation, but due to leakage inductance, it will generate electromagnetic induction noise; at the same time, under high-frequency conditions, the distributed capacitance between the transformer layers will transfer the high-order harmonic noise on the primary side to the secondary, and the distributed capacitance of the transformer to the casing forms another high-frequency path, making the electromagnetic field generated around the transformer more easily coupled to other leads to form noise.

　　3) When the secondary side rectifier diode is used for high-frequency rectification, due to the reverse recovery time, the charge accumulated by the forward current cannot be eliminated immediately when the reverse voltage is applied (due to the existence of carriers, current still flows). Once the slope of the reverse current recovery is too large, the inductance flowing through the coil will produce a spike voltage, which will generate strong high-frequency interference under the influence of the transformer leakage inductance and other distributed parameters, and its frequency can reach tens of MHz.

　　4) Capacitors, inductors and wires Since switching power supplies operate at higher frequencies, the characteristics of low-frequency components will change, thereby generating noise.

　　1.2 External interference of switching power supply

　　External interference of switching power supply can exist in "common mode" or "differential mode". The interference type can vary from short-duration spike interference to complete power failure. It also includes voltage change, frequency change, waveform distortion, continuous noise or clutter, and transients. The types of power supply interference are shown in Table 1.

　　Among the several types of interference in Table 1, the main ones that can be transmitted through the power supply and cause damage to the equipment or affect its operation are electrical fast transient pulse groups and surge shock waves. As for interference such as electrostatic discharge, it will not cause any impact on the electrical equipment caused by the power supply as long as the power supply equipment itself does not produce vibration stop, output voltage drop and other phenomena.

　　2 Switching power supply interference coupling path

　　There are two ways of switching power supply interference coupling: one is conduction coupling and the other is radiation coupling.

　　2.1 Conductive coupling

　　Conductive coupling is one of the main coupling paths between the disturbance source and the sensitive device. Conductive coupling requires a complete circuit connection between the disturbance source and the sensitive device. Electromagnetic disturbance is transmitted from the disturbance source to the sensitive device along this connection circuit, generating electromagnetic interference. According to its coupling mode, it can be divided into circuit coupling, capacitive coupling and inductive coupling. In the switching power supply, these three coupling modes exist simultaneously and are interconnected.

　　2.1.1 Circuit Coupling

　　Circuit coupling is the most common and simplest conduction coupling method. It has the following types:

　　1) When the direct conduction coupling wire passes through an environment where interference exists, it picks up the interference energy and conducts it along the wire to the circuit, causing interference to the circuit.

　　2) Common impedance coupling: Since two or more circuits have a common impedance, when the currents of the two circuits flow through a common impedance, the voltage formed by the current of one circuit on the common impedance will affect the other circuit, which is common impedance coupling. Common impedance coupling interference is caused by the output impedance of the power supply, the common impedance of the ground wire, etc.

　　2 1.2 Capacitive coupling

　　Capacitive coupling is also called electrical coupling. Since the peak voltage generated between two circuits is a narrow pulse with a large amplitude, there is parasitic capacitance between the frequencies, so the charge of one circuit affects the other branch through the parasitic capacitance.

　　2.1.3 Inductive coupling

　　Inductive coupling is also called magnetic coupling. When there is mutual inductance between two circuits, when the interference source appears in the form of power supply , the magnetic field generated by this current interferes with the adjacent signals through mutual inductance coupling.

　　2.2 Radiative coupling

　　The interference coupling caused by the radiation path is called radiation coupling. Radiative coupling is the transmission of electromagnetic energy from the interference source to the receiver through space in the form of electromagnetic field. There are usually four main coupling paths: antenna coupling, wire inductive coupling, closed loop coupling and aperture coupling.