EMC Design of Switching Power Supply

Abstract: This paper discusses the EMC design principles and precautions for the electromagnetic compatibility design of switching power supplies from the aspects of filtering, shielding, PCB design, grounding, etc., and gives a switching power supply design example and test results comparison. Only by fully considering EMC design during design and taking measures from all aspects can we avoid the trouble of remedying anti-interference after design.
Keywords: switching power supply; electromagnetic compatibility (EMC); printed circuit board (PCB); radar0

Introduction
Now, in the design of electronic products, power consumption has become a very important indicator, and switching power supplies have been increasingly widely used due to their high efficiency and small size. Therefore, it has become inevitable to replace linear power supplies with switching power supplies. However, the power devices of switching power supplies work in a switching state, and the switching frequency is generally between tens of kilohertz and hundreds of kilohertz. The switching power supply is a very strong broadband electromagnetic emission source. The greater the power, the more serious the emission. This trend has led to more serious electromagnetic compatibility problems in switching power supplies. Electromagnetic compatibility (EMC) refers to "the coexistence state in which devices can perform their respective functions together in a common electromagnetic environment, that is, the device will not be subject to or suffer unacceptable degradation due to electromagnetic emissions from other devices in the same electromagnetic environment, nor will it cause or suffer unacceptable degradation of other devices in the same electromagnetic environment due to its electromagnetic emissions".
The EMC design of the switching power supply should be considered comprehensively from the aspects of circuit selection, component selection, filtering, shielding, overlap, interconnection, grounding, layout, filter, high-frequency transformer and printed circuit board (PCB). This article mainly proposes the design principles and precautions of the switching power supply EMC from the aspects of filtering, shielding, PCB design, grounding, etc., and takes the design of a radar 24V switching power supply as an example to illustrate the issues that need to be paid attention to in the design.

1 Switching power supply EMC design
1.1 Filtering
The filter is a frequency-selective two-port network composed of inductors, capacitors, resistors or ferrite devices. Setting a suitable line filter on the input and output lines of the power supply can effectively suppress the conducted emission and improve the conducted sensitivity. Reflective and absorptive filters are commonly used. Reflective filters are composed of inductors and capacitors, which provide high series impedance and low parallel impedance in the filter stopband, making it completely mismatched with the impedance of the noise source and the impedance of the load, thereby reflecting unwanted frequencies back to the noise source. Absorption filters are composed of energy-consuming devices, which absorb noise energy in the stopband and convert it into heat loss, thereby playing a filtering role.
When selecting and installing filters, the following principles should be followed:
(1) The filter must be well shielded, and the shielding body should be well overlapped with the power supply.
(2) The input filter should be installed at the input port, and the output filter should be installed at the output port, away from transformers, inductors, power switches, etc. with strong internal electromagnetic emissions. If conditions permit, it should be connected to the power supply as an independent component as much as possible.
(3) The input and output lines of the filter cannot cross, and shielded lines should be used or shielding layers should be set between them.
(4) The components inside the filter should be well electromagnetically shielded and grounded to prevent short-circuit current flowing through the filter grounding wire from causing harmful electromagnetic radiation.
(5) The iron core of the filter inductor should preferably be can-shaped or ring-shaped. If other shapes are used, short-circuit rings or magnetic shielding can be added. The coil is wound in a single layer or segmented manner. When the current is small, a multi-layer coil wound in a honeycomb manner can be used. The conjugate inductor cannot be wound in parallel with two wires. It should be two symmetrical independent coils. The filter capacitor should be a capacitor with good high-frequency characteristics.
(6) When necessary, an active low-pass filter can be used. It has the characteristics of high power, large effective suppression bandwidth and small size.
1.2 Shielding
Shielding technology can suppress the propagation of electromagnetic noise along the space, that is, cut off the propagation path of radiated electromagnetic noise. It is an effective measure to reduce electromagnetic radiation and induction. It is divided into three types: electric shielding, magnetic shielding and electromagnetic shielding. Shielding is to isolate two spatial regions with metal to control the induction and radiation of electric field, magnetic field and electromagnetic field from one region to another.
Therefore, when selecting shielding materials, the following principles should be followed:
(1) When the frequency of the interfering electromagnetic field is high, the eddy current generated in the metal material with low resistivity is used to form a counteracting effect on the external electromagnetic field, thereby achieving the shielding effect.
(2) When the frequency of the interfering electromagnetic field is low, a material with high magnetic permeability should be used to confine the magnetic lines of force within the shielding body to prevent them from spreading to the shielded space.
(3) In some cases, if good shielding effect is required for both high-frequency and low-frequency electromagnetic fields, it is often necessary to use different metal materials to form a multi-layer shield.
(4) The shield is well grounded, and the grounding resistance is generally less than 2mΩ. In strict cases, it is required to be less than 0.5mΩ. The grounding point of the shield should be close to the low-level grounding point to be shielded. The shield adopts a box shape, with no holes or gaps as much as possible.
(5) The power input and output use shielded plug sockets. The input and output lines use twisted pair cables, and shielding is required when necessary.
(6) Pay full attention to the impact of leakage on shielding effectiveness. Leakage channels include seams, doors, covers, vents, holes, non-uniform surfaces, etc.
(7) Switching power supplies must consider both electromagnetic shielding and heat dissipation. Poor heat dissipation will seriously affect reliability. According to relevant reliability data, for every 10°C increase in internal temperature rise, the MTBF value is reduced by about half. Therefore, shielding and heat dissipation must be considered comprehensively.

1.3 PCB Design
PCB electromagnetic interference can be divided into common mode interference and differential mode interference. Differential mode interference depends on the current characteristics in the closed loop; common mode interference is caused by the interference voltage to the ground. The factors that affect PCB electromagnetic interference are mainly the structure of the PCB. Different PCB structures have different interference effects. In addition, the length of the transmission belt, the loop area, the direction of the ground line, the overall layout, etc. will affect the interference effect. The EMC design of the PCB is crucial for the smooth passage of various EMC tests of the switching power supply. A good EMC design of the PCB can achieve twice the result with half the effort. The following points need to be carefully considered during the design:
a. Reasonable layering of the
PCB First, determine whether to use a single-layer board, a double-sided board, or a multi-layer board based on the comprehensive factors such as the type of power/ground, the density of the signal line, the number of signals required for special wiring, and the cost price. If the cost allows, using a multi-layer board to solve the board-level EMC problem is an effective way.
b. Reasonable layout of
the PCB In the design of the PCB, layout is an important link, and the quality of its results will directly affect the effect of the final routing. In order to reduce the generation of noise and prevent malfunctions caused by noise, the following points should be noted when laying out components:
(1) Determine the size of the PCB. When the PCB size is too large, the printed lines are too long, the impedance increases, and the anti-noise ability decreases; when the size is too small, the heat dissipation is poor and the adjacent lines are easily interfered. The best shape of the printed board is a rectangle with an aspect ratio of 3:2 or 4:3. When the area size of the printed board is larger than 200mmx150mm, the mechanical strength should be considered.
(2) Interrelated components should be placed together as much as possible to avoid interference caused by long printed lines due to the components being too far apart; the buffer circuit should be placed as close to the switch tube and the output rectifier diode
as possible. (3) Components that are susceptible to interference should not be too close to each other, and the input and output components should be kept as far away as possible.
(4) The input and output signals should be placed as close to the lead ports as possible to avoid interference caused by coupling paths.
c. Reasonable PCB wiring
Reasonable wiring can significantly reduce the interference of the PCB board to the outside world and improve the anti-interference of the equipment. An effective wiring principle to reduce radiation and improve the immunity of equipment is to control the loop area of ​​current. Although multi-layer boards have many advantages in EMC, double-sided boards are often still chosen for design due to cost considerations. Double-sided boards are suitable for occasions requiring medium assembly density. When wiring double-sided boards, there is no separate power plane and ground plane. At this time, the most reasonable wiring order is: power/ground first, then clock line, bus and other key signal lines, and finally other signal lines, and according to the size of the current, the width of the power/ground line should be widened as much as possible. In actual wiring, it is necessary to pay attention to: add a ground line between two parallel signal lines, as shown in Figure 1, or surround the signal line with a network ground line, as shown in Figure 2, the power supply and the corresponding ground line should be parallel and adjacent as much as possible, and the wiring of the two-layer printed circuit board should be perpendicular to each other as much as possible to reduce coupling and interference between the board layers, which can play a good shielding role. In addition, the printed circuit board is "fully grounded", as shown in Figure 3. When designing and drawing printed circuit boards, in addition to making the ground printed conductors as thick as possible, all unoccupied areas on the board should be used as ground wires to better ground the devices nearby, which can effectively reduce parasitic inductance. Large-area ground wires can effectively reduce noise radiation.

1.4 Grounding
Grounding refers to the design of a conductive path between a certain selected point and a grounding surface. There are three purposes of grounding:
(1) Grounding enables all unit circuits in the entire circuit system to have a common reference zero potential to ensure that the circuit system can work stably.
(2) Prevent interference from external electromagnetic fields. The grounding of the chassis can allow the large amount of charge accumulated on the chassis due to electrostatic induction to be discharged through the earth. Otherwise, the high voltage formed by these charges may cause spark discharge inside the equipment and cause interference. In addition, for the shielding body of the circuit, if a suitable grounding is selected, a good shielding effect can also be obtained.
(3) Ensure work safety. When direct electromagnetic induction of lightning occurs, the destruction of electronic equipment can be avoided; when the input voltage of the industrial frequency AC power supply is directly connected to the chassis due to poor insulation or other reasons, the electric shock accident of the operator can be avoided.
Therefore, grounding is the main method to suppress noise and prevent interference. The type of grounding to be used depends on the actual circuit, but the principle is to minimize the interference voltage generated on the common grounding impedance of multiple circuits and avoid the formation of unnecessary ground loops. The selection of grounding points is also very important. Because the potentials of two grounding points are rarely the same, when multiple grounding points are used, the potential difference between the grounding points will be reflected in the circuit, and noise will be generated. Generally speaking, the choice of grounding points should minimize the interference to the low-level circuit, and the grounding wire should be as short as possible. In practical applications, the combination of grounding with filtering, shielding and other technologies can achieve twice the result with half the effort in suppressing interference.

2 Design Example
The following is a 48V/16A power supply designed for a tracked radar. Its main electrical technical indicators are: AC input voltage: 220V±10%, frequency: 50Hz±5%; DC output voltage: 48V, 16A. According to the above design principles. The author designed this power supply. The input and output ends of the power supply use shielded plug sockets, and the input end is equipped with a military filter. The filter is installed at the input port and connected to the shell to ensure large-area grounding. The entire circuit board is shielded, and the output end of the input filter is connected to the input end of the circuit board.

3 Test results
The results obtained when testing the Army Ground CE102 and RE102 in GJB151A for the first time are shown in Figures 4 and 5. Both indicators do not meet the requirements. After taking corresponding measures, the test results of the second test are shown in Figures 6 and 7. From the test results, within all the frequency bands tested, the noise spectrum obtained basically meets the overall technical indicators and meets the requirements of the Army Ground CE102 and RE102 in GJB151A.

4 Conclusion
EMC design is a systematic and holistic concept that runs through the entire process of switching power supply design, including the initial mechanical structure design, circuit principle design, module and component selection, PCB board design, and even the final power supply installation. This article mainly discusses the principles and rules of switching power supply EMC design from the aspects of filtering, shielding, PCB design, grounding, etc., and uses a switching power supply design example as a demonstration to illustrate, hoping that it will be helpful to beginners.