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The purpose of electromagnetic interference suppression of switching power supply is to make the product work normally without performance degradation or malfunction when subjected to electromagnetic interference in a certain electromagnetic environment, and at the same time, it does not pollute the electromagnetic environment.

One, the generation mechanism of electromagnetic interference of switching power supply

The interference generated by the switching power supply is classified according to the type of noise interference source, which can be divided into spike interference and harmonic interference. If divided according to the coupling path, it can be divided into two types: conducted interference and radiated interference. Now explain separately according to the noise interference source;

1. Interference caused by the reverse recovery time of the diode

The rectifier diode in the high-frequency rectifier circuit has a large forward current flowing when it is forward-conducting. When it is turned off by the reverse bias voltage, due to the accumulation of more carriers in the pn junction, the current For a period of time before the electron disappears, the current will flow in the reverse direction, causing the reverse recovery current of the disappearance of the carrier to decrease sharply and a great current change (di/dt) occurs.

2 Harmonic interference generated when the switch tube is working

The power switch tube flows through a larger pulse current when it is turned on. For example, the input current waveform of the forward type, push-pull type and bridge type converter is approximately rectangular wave when resistive load, which contains rich high-order harmonic components. When using zero-current, zero-voltage switching, this harmonic interference will be small. In addition, during the cut-off period of the power switch, the current sudden change caused by the leakage inductance of the high-frequency transformer winding will also cause spike interference.

3 Interference caused by AC input circuit

The rectifier at the input end of the switching power supply without a power frequency transformer will cause high-frequency attenuated oscillations and interference during the reverse recovery period.

The spike interference and harmonic interference energy generated by the switching power supply are called conduction interference when they propagate through the input and output lines of the switching power supply. When the energy of harmonics and parasitic oscillations propagate through the input and output lines, they will all be in the space. Generate electric and magnetic fields. This kind of interference produced by electromagnetic radiation is called radiated interference.

4. Other reasons

The parasitic parameters of the components and the schematic design of the switching power supply are not perfect. The printed circuit board (PCB) traces are usually arranged manually, which has great randomness. The near-field interference of the PCB is large, and the installation of the components on the printed circuit board, Unreasonable placement and orientation will cause emi interference.

2. Related theories of electromagnetic interference

1. The main electromagnetic interference source of switching power supply

The electromagnetic interference sources in the switching power supply mainly include switching devices, diodes and non-linear passive components. In the switching power supply, improper printed board wiring is also a major factor causing electromagnetic interference.

1.1 Electromagnetic interference generated by switching circuit

For switching power supplies, the electromagnetic interference generated by the switching circuit is one of its main sources of interference. The switching circuit is the core of the switching power supply, which is mainly composed of a switching tube and a high-rated transformer. The dv/dt he produced has a larger pulse, a wide frequency band and rich harmonics. The main reasons for this kind of pulse interference are:

(1) The switch tube load is the primary coil of the high-frequency transformer, which is an inductive load. At the moment when the switch is turned on, the primary coil generates a large inrush current, and a higher surge peak voltage appears at both ends of the primary coil. At the moment when the switch tube is disconnected, due to the leakage flux of the primary coil, part of the energy is not transmitted from the primary coil to the secondary coil. This part of the energy stored in the inductance will form a spike with the capacitance and resistance in the collector circuit. The attenuated oscillation is superimposed on the turn-off voltage to form a turn-off peak voltage.

This kind of power supply voltage interruption will produce the same magnetization impulse current transient as when the primary coil is turned on. This noise will be conducted to the input and output terminals, forming conduction interference, and it may break the switch tube in severe cases.

(2) The high-frequency switching current loop formed by the primary coil of the pulse transformer, the switch tube and the filter capacitor may generate large space radiation and form radiation interference. If the capacitor's filter capacity is insufficient or the high-frequency characteristics are not good, the high frequency on the capacitor Impedance will cause high-frequency current to be conducted into the AC power source in a differential mode to form conduction interference.

1.1.2 Electromagnetic interference generated by diode rectifier circuit

The 1di/dt1 of the reverse recovery current produced by the rectifier diode in the main circuit is much smaller than the freewheeling diode recovery current Idi/dtl. As an electromagnetic interference source to study, the interference caused by the reverse recovery current of the rectifier diode is large and the frequency bandwidth is wide. The voltage jump produced by the rectifier diode is much smaller than the voltage jump produced when the power switch tube in the power supply is turned on and off. Therefore, regardless of the influence of Idv/dtI and Idi/dtl generated by the rectifier diode, it is also possible to study the rectifier circuit as a part of the electromagnetic interference coupling channel.

2, the coupling channel of switching power supply electromagnetic interference

The switching power supply interferes with itself through the coupling channel. Usually use differential mode and common mode interference for analysis.

"Common mode interference" means that the interference has the same magnitude and direction, which exists in any relatively earth of the power supply, or between the neutral line and the earth. Common mode interference is also called longitudinal mode interference, asymmetric interference or ground interference. It is the interference between the carrier fluid and the earth.

"Differential mode interference" means that the interference is equal in magnitude but opposite in direction, and it exists between the power phase line and the neutral line. Differential mode interference is also called normal mode interference, transverse mode interference or symmetrical interference. · This is the interference between the carrier fluid.

Common mode interference shows that the interference is coupled into the circuit by radiation or crosstalk. The differential mode interference shows that the interference originates from the same circuit. Usually these two kinds of interference exist at the same time. Due to the imbalance of the line impedance, the two kinds of interference will also transform into each other during transmission. So the situation is very complicated.

Three, several measures to suppress interference

The three elements that form electromagnetic interference are the source of interference, the path of propagation, and the victim equipment. Therefore, suppression of electromagnetic interference should also start from these three aspects. First of all, the interference source should be suppressed, and the cause of the interference should be directly eliminated. The second is to eliminate the coupling and radiation between the interference source and the victim device, and cut off the propagation path of electromagnetic interference. The third is to improve the interference immunity of the victim device and reduce its impact on noise. Sensitivity. At present, several measures to suppress interference are basically to cut off the coupling channel between the electromagnetic interference source and the disturbed device, and they are indeed effective methods. The commonly used methods are shielding, grounding and filtering.

The use of shielding technology can effectively suppress the electromagnetic radiation interference of the switching power supply. For example, power switch tubes and output diodes usually have a large power loss. In order to dissipate heat, it is often necessary to install a radiator or directly install it on the power baseboard. The insulating sheet with good thermal conductivity is required for insulation during device installation. This causes distributed capacitance between the device and the bottom plate and the heat sink. The bottom plate of the switching power supply is the ground wire of the AC power supply. Therefore, the distributed capacitance between the device and the bottom plate couples electromagnetic interference to the AC input terminal to generate common mode interference. The solution to this problem is to sandwich a shielding sheet between two insulating sheets and connect the shielding sheet to the DC ground, cutting off the path of radio frequency interference to the input power grid. In order to suppress the radiation generated by the switching power supply and the influence of electromagnetic interference on other electronic equipment, the shielding cover can be processed completely in accordance with the method of shielding the magnetic field, and then the entire shielding cover is connected with the system casing and the ground as a whole, and the electromagnetic field can be protected. Perform effective shielding. Some parts of the power supply connected to the ground can play a role in suppressing interference. For example, grounding the electrostatic shielding layer can suppress the interference of changing electric fields.

The conductor used for electromagnetic shielding can be ungrounded in principle, but ungrounded shielding conductors often enhance electrostatic coupling and produce the so-called "negative electrostatic shielding" effect. Therefore, it is still better to ground, so that electromagnetic shielding can play the role of electrostatic shielding at the same time. The common reference point of the circuit is connected to the ground, which can provide a stable reference potential for the signal loop. Therefore, after the safety protection ground wire, shield ground wire and public reference ground wire in the system form a ground bus, they are finally connected to the earth.

should be followed in the circuit system design. According to the principle of “one point grounding”, if multiple points of grounding are formed, a closed ground loop will appear, and magnetic induction noise will be generated when the magnetic lines of force pass through the loop. In fact, it is difficult to achieve “one point grounding”. Therefore, in order to reduce grounding impedance and eliminate The influence of distributed capacitance is flat or multi-point grounding. A conductive plane (the conductive plane layer of the bottom board or multi-layer printed circuit board, etc.) is used as a reference ground. All parts that need to be grounded are connected to the reference ground nearby. For further To reduce the voltage drop of the ground loop, bypass capacitors can be used to reduce the amplitude of the return current. In a circuit system where low-frequency and high-frequency coexist, the ground wires of the low-frequency circuit, high-frequency circuit, and power circuit should be connected separately. Connect to a common reference point.

Filtering is a very good way to suppress conducted interference. For example, connecting a filter at the input end of the power supply can suppress the interference generated by the switching power supply and fed back to the grid. It can also restrain the noise from the power grid from harming the power supply itself. In the filter circuit, many special filter components are also used, such as through-core capacitors, three-terminal capacitors, and ferrite magnetic rings, which can improve the filter characteristics of the circuit. Appropriate design or selection of filters, and correct installation and use of filters are an important part of anti-interference technology.

There are still many factors that produce electromagnetic interference in switching power supplies, and there is still a lot of work to suppress electromagnetic interference. Suppressing various noises of switching power supply will make the switching power supply more widely used.