Detailed explanation of several effective methods for electromagnetic interference suppression of switching power supplies

At present, many universities and research institutes have conducted research on switching power supply EMI (Electromagnetic Interference). Some of them start from the mechanism of EMI generation, and some start from the impact of EMI generation, and all of them have proposed many practical and valuable solutions. Here we analyze and compare several effective solutions, and put forward new reference suggestions for the suppression measures of switching power supply EMI.

The generation mechanism of electromagnetic interference of switching power supply

The interference generated by the switching power supply can be divided into two types according to the type of noise interference source: peak interference and harmonic interference. If it is divided according to the coupling path, it can be divided into two types: conducted interference and radiated interference. Now let's explain them separately according to the noise interference source:

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

When the rectifier diode in the high-frequency rectifier circuit is forward-conducting, a large forward current flows through it. When it is turned off by the reverse bias voltage, there are more carriers accumulated in the PN junction. Therefore, the current will flow in the opposite direction for a period of time before the carriers disappear, causing the reverse recovery current of the disappearance of the carriers to decrease sharply and a large current change (di/dt) occurs.

2. Harmonic interference generated when the switch tube is working

A large pulse current flows through the power switch tube when it is turned on. For example, the input current waveform of the forward, push-pull and bridge converters is approximately a rectangular wave when the load is resistive, which contains a lot of high-order harmonic components. When zero current and zero voltage switching is used, this harmonic interference will be very small. In addition, during the cut-off period of the power switch tube, the current mutation caused by the leakage inductance of the high-frequency transformer winding will also produce spike interference.

3. Interference generated by AC input circuit

The rectifier tube at the input end of the switching power supply without an industrial frequency transformer will cause high-frequency attenuation oscillation and interference during the reverse recovery period.

The peak interference and harmonic interference energy generated by the switching power supply are transmitted through the input and output lines of the switching power supply, which is called conducted interference; and the energy of harmonics and parasitic oscillations will generate electric and magnetic fields in space when they are transmitted through the input and output lines. This interference generated by electromagnetic radiation is called radiated interference.

4. Other reasons

The parasitic parameters of components, the schematic design of the switching power supply is not perfect, the printed circuit board (PCB) routing is usually arranged manually, which is very random, the near-field interference of the PCB is large, and the installation, placement, and orientation of the components on the printed board are unreasonable, which will cause EMI interference.

Characteristics of switching power supply EMI

As an energy conversion device working in the switching state, the voltage and current change rate of the switching power supply is very high, and the interference intensity generated is relatively large; the interference source is mainly concentrated in the power switching period and the heat sink and high-level transformer connected to it, and the location of the interference source is relatively clear compared to the digital circuit; the switching frequency is not high (from tens of kilohertz to several megahertz), and the main forms of interference are conducted interference and near-field interference; and the printed circuit board (PCB) routing is usually manually wired, which has greater randomness, which increases the difficulty of extracting PCB distribution parameters and estimating near-field interference.

EMI Testing Technology

There are currently three methods for diagnosing differential and common mode interference: RF current probe, differential mode suppression network, and noise separation network. Using an RF current probe is the simplest method for measuring differential and common mode interference, but the measurement results must be compared with the standard limits through a more complex conversion. The differential mode suppression network has a simple structure (see Figure 1), and the measurement results can be directly compared with the standard limits, but it can only measure common mode interference. The noise separation network is the most ideal method, but the manufacturing requirements for its key component, the transformer, are very high.

Figure 1 Differential mode suppression network

Several current measures to suppress interference

The three elements that form electromagnetic interference are interference source, propagation path and disturbed equipment. Therefore, electromagnetic interference suppression should also start from these three aspects. First, the interference source should be suppressed to directly eliminate the cause of interference; second, the coupling and radiation between the interference source and the disturbed equipment should be eliminated to cut off the propagation path of electromagnetic interference (see Figure 2); third, the anti-interference ability of the disturbed equipment should be improved to reduce its sensitivity to noise. At present, several measures to suppress interference are basically to cut off the coupling channel between the electromagnetic interference source and the disturbed equipment, which are indeed effective methods. Commonly used methods are shielding, grounding and filtering.

Figure 2 Electromagnetic interference source and coupling path model

The use of shielding technology can effectively suppress the electromagnetic radiation interference of the switching power supply. For example, the power switch tube and the output diode usually have a large power loss. In order to dissipate heat, it is often necessary to install a heat sink or directly install it on the power supply base. When installing the device, an insulating sheet with good thermal conductivity is required for insulation, which creates distributed capacitance between the device and the base plate and the heat sink. The base plate of the switching power supply is the ground wire of the AC power supply. Therefore, the distributed capacitance between the device and the base plate couples the electromagnetic interference to the AC input end to generate common mode interference. The solution to this problem is to use a shielding sheet sandwiched between two layers of insulating sheets, and connect the shielding sheet to the DC ground, cutting off the path for the radio frequency interference to propagate to the input power grid. In order to suppress the radiation generated by the switching power supply and the impact of electromagnetic interference on other electronic equipment, the shielding cover can be processed completely according to the method of magnetic field shielding, and then the entire shielding cover can be connected to the system casing and the ground as a whole, which can effectively shield the electromagnetic field. Connecting some parts of the power supply to the ground can play a role in suppressing interference. For example, grounding the electrostatic shielding layer can suppress the interference of the changing electric field; the conductor used for electromagnetic shielding can be ungrounded in principle, but the ungrounded shielding conductor often enhances the electrostatic coupling and produces the so-called "negative electrostatic shielding" effect, so it is still better to ground it, so that the electromagnetic shielding can also play the role of electrostatic shielding. The common reference point of the circuit is connected to the earth, which can provide a stable reference potential for the signal loop. Therefore, the safety protection ground wire, shielding ground wire and common reference ground wire in the system each form a ground bus, and finally they are connected to the earth.

The principle of "one-point grounding" should be followed in circuit system design. If multiple points of grounding are formed, a closed grounding loop will appear. When the magnetic lines of force pass through the loop, magnetic induction noise will be generated. In fact, it is difficult to achieve "one-point grounding". Therefore, in order to reduce the grounding impedance and eliminate the influence of distributed capacitance, planar or multi-point grounding is adopted. A conductive plane (the conductive plane layer of the bottom plate or multi-layer printed circuit board circuit, etc.) is used as the reference ground, and the parts that need to be grounded are connected to the reference ground nearby. In order to further reduce the voltage drop of the grounding 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, and then connected to the common reference point.

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

EMI filtering technology is an effective measure to suppress spike interference, and can filter out conducted interference caused by various reasons. Figure 3 is an EMI filter composed of capacitors and inductors, connected to the input end of the switching power supply. In the circuit, C1 and C5 are high-frequency bypass capacitors, used to filter out the differential mode interference between the two input power lines; L1 and C2, C4; L2 and C3, C4 form a common mode interference filtering link, used to filter out the asymmetric common mode interference between the power line and the ground; L3 and L4 have equal primary and secondary turns and opposite polarities, and the magnetic flux generated by the alternating current in the magnetic core is opposite, so it can effectively suppress common mode interference. Tests have shown that as long as the parameters of the components are properly selected, the conducted interference generated by the switching power supply can be better suppressed.

The shortcomings of current switching power supply EMI suppression measures

Most of the existing suppression measures are based on eliminating the coupling and radiation between the interference source and the interfered equipment, and cutting off the propagation path of electromagnetic interference. This is indeed an effective way to suppress interference, but few people involve directly controlling the interference source, eliminating interference, or improving the anti-interference ability of the interfered equipment. Little do people know that the latter still has a lot of room for development.

Suggestions for improvement

At present, the interference suppression based on the propagation path of electromagnetic interference has gradually matured. Our viewpoint should return to the switching power supply device itself. From many years of work practice, we should pay attention to the following points in the circuit:

(1) When laying out the printed circuit board, the analog circuit area and the digital circuit area should be reasonably separated, the power supply and ground wires should be led out separately, and the power supply should be gathered at one point; when wiring the PCB, high-frequency digital signal lines should be short, and the main signal lines should be concentrated in the center of the PCB board. At the same time, the power line should be as far away from the high-frequency digital signal line as possible or separated by a ground wire. Secondly, the wiring can be done according to the coupling coefficient to minimize interference coupling. (See Table 1)

(2) The power and ground lines of the printed circuit board should be as wide as possible to reduce line impedance, thereby reducing interference noise caused by common impedance.

(3) Surface mount components are often used and the pin length of the components is shortened as much as possible to reduce the impact of the distributed inductance of the components.

(4) Connect filter capacitors as close to the device as possible at the Vdd and Vcc power supply terminals to shorten the flow path of the switching current, such as connecting 10μF aluminum electrolytic and 0.1μF capacitors in parallel to the power supply pins. For the power supply terminal of high-speed digital ICs, tantalum electrolytic capacitors can be used instead of aluminum electrolytic capacitors because the impedance of tantalum electrolytic capacitors to ground is much smaller than that of aluminum electrolytic capacitors.

in conclusion

There are many factors that generate electromagnetic interference in switching power supplies , and there is still a lot of work to be done to suppress electromagnetic interference. Comprehensive suppression of various noises in switching power supplies will enable switching power supplies to be more widely used.