Analysis on EMI control technology of switching power supply
In this paper, the mechanism of EMI in switching power supply is analyzed in detail, and a series of EMI suppression strategies are put forward, thus effectively improving the electromagnetic compatibility of switching power supply.
Switching power supply is a kind of power electronic product which uses power semiconductor devices and integrates power conversion technology, electronic electromagnetic technology and automatic control technology. Because of its advantages of low power consumption, high efficiency, small volume, light weight, stable work, safety and reliability, and wide voltage stabilizing range, it is widely used in the fields of computers, communications, electronic instruments, industrial automatic control, national defense and household appliances. However, the switching power supply has poor transient response and is prone to electromagnetic interference (EMD), and the EMI signal occupies a wide frequency range and has a certain amplitude. These EMI signals pollute the electromagnetic environment through conduction and radiation, and cause interference to communication equipment and electronic instruments, thus limiting the use of switching power supply to some extent.
1 switching power supply causes electromagnetic interference
Electromagneticlnterference (EMI) is a kind of performance damage of electronic system or subsystem caused by unexpected electromagnetic disturbance. It consists of three basic elements: interference source, that is, equipment that generates electromagnetic interference energy; Coupling channel, that is, the channel or medium for transmitting electromagnetic interference; Sensitive equipment, that is, devices, equipment, subsystems or systems damaged by electromagnetic interference. Based on this, the basic measures to control electromagnetic interference are: suppressing interference sources, cutting off the path of disaster, reducing the response of sensitive equipment to interference or increasing the electromagnetic sensitivity level.
According to the working principle of switching power supply, it is known that switching power supply first rectifies power frequency alternating current into direct current, then inverts it into high frequency alternating current, and finally outputs it through rectification and filtering to obtain stable direct current voltage. In the circuit, the power triode and diode mainly work in the switching state, and work in the microsecond order; When triode and diode are turned on and off, the current changes greatly during the rising and falling time, which is easy to generate radio frequency energy and form interference sources. At the same time, the leakage inductance of transformer and the peak caused by the reverse recovery current of output diode will also form potential electromagnetic interference.
Switching power supply usually works at high frequency, and the frequency is above 02 kHz, so its distributed capacitance can not be ignored. On the one hand, the insulation sheet between the heat sink and the collector of the switch tube has a large contact area and a thin insulation sheet, so the distributed capacitance between them can not be ignored at high frequency, and high-frequency current will flow to the heat sink through the distributed capacitance and then to the chassis ground, resulting in common-mode interference; On the other hand, there is a distributed capacitance between the primary stages of the pulse transformer, which can directly fuse the voltage of the primary winding to the secondary winding and produce common-mode interference on the two power lines with DC output of the secondary winding.
Therefore, the interference sources in switching power supply are mainly concentrated in the components such as switching tubes, diodes and high-frequency transformers, as well as AC input and rectification output circuits.
2 Measures to suppress electromagnetic interference of switching power supply
Usually, EMI control of switching power supply mainly adopts filtering technology, shielding technology, sealing technology and grounding technology. EMI interference can be divided into conduction interference and radiation interference according to the transmission route. Switching power supply mainly conducts interference, and its frequency range is the widest, about 10kHz-30MHz. The countermeasures to suppress conducted interference are basically solved in three frequency bands: 10kHz-150kHz, 150kHz-10MHz and above. Normal interference is mainly in the range of 10kHz to 150kHz, which is generally solved by general LC filter. Common-mode interference is mainly in the range of 150kHz-10 MHz, which is usually solved by common-mode rejection filter. The countermeasures for the frequency band above 10MHz are to improve the shape of the filter and take electromagnetic shielding measures.
2.1 using AC input EMI filter.
Usually, there are two ways to transmit interference current on the conductor: common mode and differential mode. Common-mode interference is the interference between the carrier fluid and the earth: the interference has the same magnitude and direction, and exists between any relative earth of the power supply or between the neutral line and the earth. It is mainly produced by du/dt, and di/dt also produces certain common-mode interference. The differential mode interference is the interference between carrier fluids: the interference is equal in magnitude and opposite in direction, and exists between the phase line and the neutral line of the power supply and the phase line and the phase line. When the interference current is transmitted on the conductor, it can appear in both common mode and differential mode. However, common-mode interference current can only interfere with useful signals after it becomes differential-mode interference current.
There are the above two kinds of interference in AC power transmission line, usually low-frequency differential mode interference and high-frequency common mode interference. In general, the amplitude of differential mode interference is small, the frequency is low, and the interference caused is small; Common-mode interference has large amplitude and high frequency, and it can also produce radiation through wires, which causes great interference. If an appropriate EMI filter is used at the input end of AC power supply, electromagnetic interference can be effectively suppressed. The basic principle of the power line EMI filter is shown in Figure 1, in which the differential mode capacitors C1 and C2 are used to short-circuit the differential mode interference current, while the intermediate line grounding capacitors C3 and C4 are used to short-circuit the common mode interference current. Common-mode choke coil is composed of two coils with equal thickness and wound on a magnetic core in the same direction. If the magnetic coupling between the two coils is very close, the leakage inductance will be very small, which is poor in the frequency range of the power line.
The mode reactance will become very small; When the load current flows through the common-mode choke, the magnetic field lines generated by the coils connected in series on the phase line are opposite to those generated by the coils connected in series on the neutral line, and they cancel each other in the magnetic core. Therefore, even in the case of large load current, the magnetic core will not be saturated. For the common-mode interference current, the magnetic fields generated by the two coils are in the same direction, which will present a large inductance, thus playing a role in attenuating the common-mode interference signal. Here, the common mode choke coil should be made of ferrite magnetic material with high permeability and good frequency characteristics.
