Five major suppression strategies for switching power supply EMI

Switching power supplies are an applicationPower semiconductorsThe device also integrates power electronic products such as power conversion technology, electronic electromagnetic technology, and automatic control technology.

Because it has the advantages of small power consumption, high efficiency, small size, light weight, stable work, safety and reliability, and wide voltage regulation range, it is widely used in computers,correspondence, electronic instruments, industrial automatic control, national defense and household appliances and other fields. However, the transient response of the switching power supply is poor, electromagnetic interference is easy to occur, and the EMI signal occupies a wide frequency range and has a certain amplitude. These EMI signals pollute the electromagnetic environment by conducting and radiating, causing interference to communication equipment and electronic instruments, thus limiting the use of switching power supplies to a certain extent.

The cause of electromagnetic interference generated by switching power supplies

 

Electromagnetic interference (EMI) is a performance impairment of an electronic system or subsystem caused by an unexpected electromagnetic disturbance. It consists of three basic elements: interference sources, i.e. devices that generate electromagnetic interference energy; Coupling pathway, that is, the pathway or medium that transmits 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 the source of interference, cutting off the disaster pathway, and reducing the response of sensitive equipment to interference or increasing the electromagnetic susceptibility level.

 

 

According to the working principle of switching power supply, it is known that the switching power supply first rectifies the power frequency alternating current into direct current, and then inverts it into high-frequency alternating current, and finally passes the rectification filter output to obtain a stable DC voltage. In the circuit, the power transistor and diode mainly work in the on-off state and work in the order of microseconds; In the process of opening and closing flipping, the current changes greatly during the rise and fall time, and it is easy to generate RF energy and form a source of interference. At the same time, potential electromagnetic interference can also be created due to spikes caused by the leakage inductance of the transformer and the reverse recovery current of the output diode.

 

 

Switching power supplies typically operate at high frequencies above 02 kHz, so their distributed capacitance is not negligible. On the one hand, the insulating sheet between the collector of the heat sink and the switch tube, because of its large contact area and thin insulation sheet, therefore, the distributed capacitance between the two cannot be ignored at high frequencies, and the high-frequency current will flow to the heat sink through the distributed capacitance, and then flow to the casing ground, resulting in a common mode thousand disturbances; On the other hand, there is a distributed capacitance between the primary stages of the pulse transformer, which can directly combine the voltage of the primary winding to the secondary winding, and generate common-mode interference on the two power lines of the secondary winding as DC output.

 

 

Therefore, the interference sources in the switching power supply are mainly concentrated in the voltage and current changes, such as switching tubes, diodes, high-frequency transformers and other components, as well as AC input and rectification output circuit parts.

 

 

Measures to suppress electromagnetic interference in switching power supplies

Generally, switching power supply EMI control mainly adopts filter technology, shielding technology, sealing technology, grounding technology, etc. EMI interference is divided into conducted interference and radiated interference according to the propagation path. The switching power supply is mainly conducted interference, and the frequency range is the widest, about 10kHz to 30MHz. The countermeasures to suppress conducted interference are basically solved in three frequency bands: 10kHz to 150kHz, 150kHz to 10MHz, and 10MHz or more. The range from 10kHz to 150kHz is mainly normal interference, and it is generally usedLC filterto solve. The range from 150kHz to 10 MHz is mainly common-mode interference, which is usually solved by using common-mode suppression filters. The countermeasure for the frequency band above 10MHz is to improve the shape of the filter and take electromagnetic shielding measures.

 

 

An AC input EMI filter is used

Generally, there are two ways to transmit interference current on a wire: common-mode and differential-mode. Common-mode interference is the interference between the carrier current and the earth: the interference is consistent in size and direction, and exists in any relative earth or midline of the power supply, mainly generated by du/dt, and di/dt also produces certain common-mode interference. Differential-mode interference is an interference between the carrier currents: the interference is equal in magnitude and opposite in direction, and exists between the phase line and the middle line of the power supply and between the phase line and the phase line. When the interference current is transmitted on the wire, it can appear in both common-mode mode and differential mode; However, the common-mode interference current can only interfere with the useful signal after it becomes a differential-mode interference current.

 

 

There are the above two types of interference on the AC power transmission line, usually low-band differential-mode interference and high-frequency common-mode interference. Under normal circumstances, the amplitude of differential mode interference is small, the frequency is low, and the interference caused is small; The common-mode interference amplitude is large, the frequency is high, and radiation can also be generated through the wire, causing large interference. If an appropriate EMI filter is used at the input end of the AC power supply, electromagnetic interference can be effectively suppressed. The basic principle of the power line EMI filter is shown in Figure 1, where the differential mode capacitors C1 and C2 are used to short-circuit the differential mode interference current, and the intermediate connection ground capacitors C3 and C4 are used to short-circuit the common-mode interference current. A common-mode choke consists of two coils of equal thickness wound in the same direction on a magnetic core. If the magnetic coupling between the two coils is very tight, the leakage inductance will be small, and the differential mode reactance will become small in the power line frequency range; 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 and the magnetic field lines generated by the coils in series on the center line are in opposite directions, and they cancel each other out in the core. Therefore, even with large load currents, the core will not saturate. For common-mode interference current, the magnetic field generated by the two coils is in the same direction, which will show a large inductance, thereby attenuating the common-mode interference signal. Here, the common-mode choke should use ferrite magnetic materials with high magnetic permeability and better frequency characteristics.

Figure 1 Basic circuit diagram of a power line filter

 

Use an absorption loop to improve the switching waveform

 

During the turn-on and turn-off process of the switch or diode, due to the transformer leakage inductance and line inductance, the diode storage capacitance and the distributed capacitance, it is easy to generate voltage spikes on the collector, emitter and diode of the switch tube. Typically, an RC/RCD absorption loop is used, and the RCD surge voltage absorption loop is shown in Figure 2.

 

Figure 2 RCD surge voltage absorption loop

 

When the voltage on the absorption loop exceeds a certain amplitude, the devices are quickly turned on, thereby venting the surge energy and limiting the surge voltage to a certain amplitude. Connecting in series on the positive lead of the switch collector and the output diode Saturated core coil or microcrystalline bead, generally made of cobalt (Co), when passing through normal current, the core is saturated, and the inductance is small. Once the current is to flow in the opposite direction, it will generate a large back EMF, which can effectively suppress the reverse inrush current of the diode VD.

 

Utilizes switching frequency modulation techniques

 

Frequency control technology is based on switching interference energy is mainly concentrated on specific frequencies and has large spectral peaks. If this energy can be dispersed over a wide frequency band, the purpose of reducing the peak of the disturbance spectrum can be achieved. There are generally two treatment methods: random frequency method and modulation frequency method.

 

The random frequency method is to add a random disturbance component to the circuit switching interval so that the switching interference energy is dispersed in a certain frequency band. The results show that the switching interference spectrum changes from discrete spike interference to continuous distribution interference, and its peak value decreases greatly.

 

The modulation frequency method is to add a modulation wave (white noise) to the sawtooth wave, forming a side band around the discrete frequency band that generates interference, and spreading the discrete frequency band modulation of the interference into a distributed frequency band. In this way, the interference energy is dispersed over these distribution bands. Without affecting the operating characteristics of the converter, this control method can well suppress the interference during turn-on and turn-off.

 

Soft switching technology is adopted

 

One of the disturbances of the switching power supply is from the du/dt when the power switch is on/off, so reducing the du/dt of the power switch on / off is an important measure to suppress the interference of the switching power supply. Soft switching technology can reduce the du/dt of switching transistors on/off.

 

If a small resonant element such as inductor and capacitor is added to the switching circuit, an auxiliary network is formed. Before and after the switching process, the resonance process is introduced, so that the voltage before the switch is turned on is first reduced to zero, so that the phenomenon of voltage and current overlap in the opening process can be eliminated, and the switching loss and interference can be reduced or even eliminated, which is called a soft switching circuit.

 

According to the above principle, two methods can be adopted, that is, before the switch is turned off, the current is zero, then there will be no loss and interference when the switch is turned off, this shutdown method is called zero current shutdown; Or make the voltage zero before the switch is turned on, then there will be no loss and interference when the switch is turned on, this opening method is called zero voltage on. In many cases, no longer indicate turn-on or off, only zero-current switches and zero-voltage switches, and the basic circuitry is shown in Figure 3 and Figure 4.

 

Figure 3 Zero-voltage switching resonant circuit

 

Figure 4 Zero-current switching resonant circuit

 

Usually the use of soft switching circuit control technology, combined with reasonable component layout and printed circuit board layout, grounding technology, has a certain effect on the EMI interference of switching power supply.

 

Electromagnetic shielding measures are adopted

Generally, the use of electromagnetic shielding measures can effectively suppress the electromagnetic radiation interference of switching power supply. The shielding measures of switching power supplies are mainly for switching transistors and high-frequency transformers. When the switch tube works, a large amount of heat is generated, and it needs to be equipped with a heat sink, so that a large distribution capacitance is generated between the collector of the switch tube and the heat sink. Therefore, an insulating shielding metal layer is placed between the collector and the heat sink of the switch tube, and the heat sink is connected to the ground of the casing, and the metal layer is connected to the zero potential of the hot end, reducing the coupling capacitance between the collector and the heat sink, thereby reducing the radiation interference generated by the heat sink. For high-frequency transformers, the magnet structure should first be selected according to the shielding properties of the magnet, such as using a tank core and an El-type core, the shielding effect of the magnet is very good. When the transformer is shielded, the shielding box should not be close to the outside of the transformer, and there should be a certain air gap. If an air gap multilayer shield is used, the resulting shielding effect will be better. In addition, in high-frequency transformers, it is often necessary to eliminate the distribution capacitance between the primary and secondary coils, and open circuit band rings made of copper foil can be placed along the full length of the coils to reduce the calamity between them, this open circuit band ring is connected with the iron core of the transformer and connected with the ground of the power supply, playing an electrostatic shielding role. If conditions permit, add a shielding cover to the entire switching power supply, which will better suppress radiated interference.

 

Conclusion

As switching power supplies become smaller and denser, EMI control issues become a key factor in the stability of switching power supplies. From the above analysis, it can be seen that the use of EMI filtering technology, shielding technology, sealing technology and grounding technology can effectively suppress and eliminate the disaster and radiation between the interference source and the disturbed equipment, cut off the propagation route of electromagnetic interference, and improve the electromagnetic compatibility of the switching power supply.