Five major suppression strategies for EMI in switching power supplies

Switching power supply is a power electronic product that applies power semiconductor devices and integrates power conversion technology, electromagnetic technology, automatic control technology, etc.

Because of its advantages of low power consumption, high efficiency, small size, light weight, stable operation, safety and reliability, and wide voltage regulation range, it is widely used in computers, communications, electronic instruments, industrial automatic control, national defense and household appliances. However, the transient response of the switching power supply is poor, electromagnetic interference is easy to generate, 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, causing interference to communication equipment and electronic instruments, thus limiting the use of switching power supplies to a certain extent.

Causes of electromagnetic interference in switching power supplies

 

Electromagnetic interference (EMI) is a type of performance impairment caused by unintended electromagnetic disturbances in an electronic system or subsystem. It consists of three basic elements: interference sources, i.e. devices that generate electromagnetic interference energy; the connection pathway, that is, the channel or medium that transmits electromagnetic interference; Sensitive equipment, i.e., devices, subsystems, or systems that are damaged by electromagnetic interference. Based on this, the basic measures to control electromagnetic interference are: suppressing interference sources, cutting off the path of disaster, and reducing the response of sensitive equipment to interference or increasing the electromagnetic sensitivity level.

 

 

According to the working principle of the switching power supply, the switching power supply first rectifies the power frequency AC power into DC power, then inverts it into high-frequency AC current, and finally obtains a stable DC voltage through rectification filter output. In the circuit, the power triode and diode mainly work in the state of the switch on and off and work in the order of microseconds. In the process of opening and closing the transistor and closing the transode, 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 formed due to spikes caused by the leakage of the transformer and the reverse recovery current of the output diode.

 

 

Switching power supplies usually operate at high frequencies with frequencies above 02 kHz, so their distributed capacitance cannot be ignored. On the one hand, the insulating sheet between the heat sink and the collector of the switch tube, due to its large contact area and thin insulation sheet, the distributed capacitance between the two cannot be ignored at high frequency, and the high-frequency current will flow to the heat sink through the distributed capacitance, and then flow to the casing, resulting in common mode disturbance; On the other hand, there is a distributed capacitance between the primary and secondary stages of the pulse transformer, which can directly combine the primary winding voltage 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 switches, diodes, high-frequency transformers and other components, as well as AC transmission and rectification output circuits.

 

 

Measures to suppress electromagnetic interference from switching power supplies

Usually, the EMI control of switching power supplies mainly adopts filtering 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 by three frequency bands: 10kHz to 150kHz, 150kHz to 10MHz, and 10MHz. The range of 10kHz to 150kHz is mainly normal interference, which is generally solved by using a universal LC filter. The range of 150kHz to 10 MHz is mainly common-mode interference, which is usually solved by common-mode rejection filters. Measures for the frequency band above 10MHz are to improve the shape of the filter and take measures to shield electromagnetic devices.

 

 

It adopts an AC input EMI filter

There are usually two ways when interfering current is transmitted on a wire: common mode and differential mode. Common-mode interference is the interference between the carrier and the earth: the magnitude and direction of the interference are the same, and exist between the power supply relative to the earth or the midline to the earth, mainly generated by du/dt, and di/dt also produces a certain common-mode interference. Differential mode interference is the interference between the carrier fluids: the interference is equal in size and in the opposite direction, and exists between the power supply phase line and the middle line and 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 mode. However, the common mode interference current can only interfere with useful signals 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 differential mode interference amplitude is small, the frequency is low, and the interference caused is small. The common mode interference amplitude and frequency are large, and radiation can also be generated through the wire, causing greater interference. If an appropriate EMI filter is used at the AC power transmission end, 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, while the intermediate wire grounding capacitors C3 and C4 are used to short-circuit the common mode interference current. A common mode choke is composed of two coils of equal thickness and wound in the same direction on a magnetic core. If the magnetic connection between the two coils is very tight, then the leakage induction will be small, and the differential mode reactance will become very small in the frequency range of the power line. When the load current flows through the common mode choke, the magnetic field lines generated by the coil in series on the phase wire and the magnetic field lines generated by the coil on the center line in series cancel each other out in the core. Therefore, even with large load currents, the core will not be saturated. For common-mode interference current, the magnetic field generated by the two coils is in the same direction and will show a large inductance, thus attenuating the common-mode interference signal. Here, the common mode choke should use ferrite magnetic materials with high magnetic permeability and good frequency characteristics.

Figure 1 Basic circuit diagram of a power line filter

 

Switching waveforms are improved using absorption loops

 

During the switch tube or diode turn-on and shutdown process, due to the presence of transformer leakage inductance and line inductance, the diode stores capacitance and distributed capacitance, and it is easy to generate spike voltages at the collector of the switch tube, both ends of the emitter and the diode. Normally, RC/RCD absorption loops are used, and the RCD surge voltage absorption loops are shown in Fig. 2.

 

Fig. 2 RCD surge voltage absorption loop

 

When the voltage on the absorption loop exceeds a certain amplitude, the devices are quickly turned on, thereby releasing the surge energy and limiting the surge voltage to a certain amplitude. Saturable core coils or microcrystalline magnetic beads are connected in series on the positive leads of the switch tube collector and output diode, and the material is generally cobalt (Co), and the core is saturated when passing through normal current, and the inductance is very small. Once the current is to flow in the opposite direction, it will generate a large back potential, which can effectively suppress the reverse inrush current of the diode VD.

 

Utilizes switching frequency modulation technology

 

Frequency control technology is based on switching interference energy mainly concentrated on a specific frequency and has a large spectral peak. If this energy can be distributed over a wide frequency band, the purpose of reducing the peak of the scrambled spectrum can be achieved. There are usually two processing methods: random frequency method and modulated frequency method.

 

The random frequency method is to add a random disturbance component to the circuit switching interval to disperse the switching interference energy in a certain range of frequency bands. The results show that the switching jamming spectrum changes from discrete spike pulse interference to continuously distributed interference, and its peak is greatly reduced.

 

The modulation frequency method is to add a human modulation wave (white noise) to the sawtooth wave, form a side frequency band around the discrete frequency band that generates interference, and expand the discrete frequency band modulation of the interference into a distributed frequency band. In this way, the interfering energy is dispersed over these distributed frequency bands. Without affecting the working characteristics of the converter, this control method can well suppress the interference during switching on and off.

 

Soft switch technology is used

 

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

 

If a small inductor, capacitor and other resonant components are added to the switching circuit, the auxiliary network is formed. Before and after the switching process, the resonance process is induced, so that the voltage is reduced to zero before the switch is turned on, so that the phenomenon of overlapping voltage and current 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, the current is zero before the switch is turned off, so that there will be no loss and interference when the switch is turned off, which is called zero current shutdown; or if the voltage is set to zero before the switch is turned on, then there will be no loss and interference when the switch is turned on, which is called zero voltage activation. In many cases, it is no longer indicated to turn on or off, only to call zero current switch and zero voltage switch, as shown in Figure 3 and Figure 4.

 

Fig.3 Zero-voltage switching resonant circuit

 

Fig.4 Zero current switching resonant circuit

 

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

 

Electromagnetic shielding measures are adopted

Generally, electromagnetic shielding measures can effectively suppress the electromagnetic radiation interference of switching power supplies. The shielding measures of switching power supplies are mainly for switching tubes and high-frequency transformers. When the switch tube works, it generates a lot of heat, and it needs to be equipped with a heat sink, so that the collector of the switch tube and the heat sink produce a large distributed capacitance. 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 machine shell, and the metal layer is connected to the zero potential of the hot end, so as to reduce the coupling capacitance between the collector and the heat sink, so as to reduce the radiation interference generated by the heat sink. For high-frequency transformers, the conductive magnet structure should first be selected according to the shielding properties of the conductive magnet, such as using a can type iron core and an El type iron core, so that the shielding effect of the conductive 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 a multi-layer shield with an air gap is used, the shielding effect obtained will be better. In addition, in high-frequency transformers, it is often necessary to eliminate the distributed capacitance between the primary and secondary coils, and an open circuit belt ring made of copper foil can be placed between the coils along the full length of the coil to reduce the misfortune between them. If conditions permit, the entire switching power supply is equipped with a shield, which will better suppress radiated interference.

 

Conclusion

As switching power supplies become smaller and more power-dense, 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 path of electromagnetic interference, and improve the electromagnetic compatibility of the switching power supply.