PCB Tech - How to avoid PCB electromagnetic interference in switching power supply design

In any switching power supply design, the physical design of the PCB board is the last link. If the design method is improper, the PCB may radiate too much electromagnetic interference and cause the power supply to work unstable. The following are the matters needing attention in each step analyze:

1. Establish component parameters from schematic to PCB design flow -> input principle netlist -> design parameter settings -> manual layout -> manual wiring -> verify design -> review -> CAM output.

2. Parameter setting The distance between adjacent wires must be able to meet the electrical safety requirements, and in order to facilitate operation and production, the distance should be as wide as possible. The minimum spacing must be at least suitable for the voltage tolerated. When the wiring density is low, the spacing of the signal lines can be increased appropriately. For signal lines with a large gap between high and low levels, the spacing should be as short as possible and the spacing should be increased. Generally, Set the trace spacing to 8mil. The distance between the edge of the inner hole of the pad and the edge of the printed board should be greater than 1mm, which can avoid the defects of the pad during processing. When the traces connected to the pads are thin, the connection between the pads and the traces should be designed into a drop shape. The advantage of this is that the pads are not easy to peel, but the traces and the pads are not easily disconnected.

Third, the component layout practice has proved that even if the circuit schematic design is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of electronic equipment. For example, if the two thin parallel lines of the printed board are close together, the signal waveform will be delayed and reflected noise will be formed at the terminal of the transmission line. The performance drops, so when designing the printed circuit board, you should pay attention to adopting the correct method.

Each switching power supply has four current loops:

(1) AC circuit of power switch

(2) Output rectifier AC circuit

(3) Input signal source current loop

(4) Output load current loop The input loop charges the input capacitor through an approximate DC current. The filter capacitor mainly plays a role of broadband energy storage; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier. At the same time, the DC energy of the output load loop is eliminated. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current circuits should only be connected to the power supply from the terminals of the filter capacitor respectively; if the connection between the input/output circuit and the power switch/rectifier circuit cannot be connected to the capacitor The terminal is directly connected, and the AC energy will be radiated into the environment by the input or output filter capacitor. The AC circuit of the power switch and the AC circuit of the rectifier contain high-amplitude trapezoidal currents. The harmonic components of these currents are very high. The frequency is much greater than the fundamental frequency of the switch. The peak amplitude can be as high as 5 times the amplitude of the continuous input/output DC current. The transition time is usually About 50ns.

4. Wiring The switching power supply contains high-frequency signals. Any printed line on the PCB can act as an antenna. The length and width of the printed line will affect its impedance and inductance, thereby affecting the frequency response. Even printed lines that pass DC signals can couple to radio frequency signals from adjacent printed lines and cause circuit problems (and even radiate interfering signals again). Therefore, all printed lines that pass AC current should be designed to be as short and wide as possible, which means that all components connected to the printed lines and other power lines must be placed very close. The length of the printed line is proportional to its inductance and impedance, and the width is inversely proportional to the inductance and impedance of the printed line. The length reflects the wavelength of the printed line's response. The longer the length, the lower the frequency at which the printed line can send and receive electromagnetic waves, and it can radiate more radio frequency energy. According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of the current, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current loops of the switching power supply. It plays an important role as a common reference point for the circuit, and it is an important method to control interference. Therefore, the placement of the grounding wire should be carefully considered in the layout. Mixing various groundings will cause unstable power supply operation. The following points should be paid attention to in the ground wire design:

1. Correctly choose single-point grounding. Generally, the common end of the filter capacitor should be the only connection point for other grounding points to couple to the high-current AC ground. It should be connected to the grounding point of this level. The main consideration is that the current returning to the ground in each part of the circuit is changed. The impedance of the actual flowing line will cause the change of the ground potential of each part of the circuit and introduce interference. In this switching power supply, its wiring and the inductance between the devices have little influence, and the circulating current formed by the grounding circuit has a greater influence on the interference. Connected to the ground pin, the ground wires of several components of the output rectifier current loop are also connected to the ground pins of the corresponding filter capacitors, so that the power supply works more stably and is not easy to self-excite. When a single point is not available, share the ground Connect two diodes or a small resistor, in fact, it can be connected to a relatively concentrated piece of copper foil.

2. Thicken the grounding wire as much as possible. If the grounding wire is very thin, the ground potential will change with the change of the current, which will cause the timing signal level of the electronic equipment to be unstable, and the anti-noise performance will deteriorate. Therefore, ensure that each large current ground terminal Use printed lines as short and as wide as possible, and widen the width of the power and ground lines as much as possible. It is best that the ground line is wider than the power line. Their relationship is: ground line>power line>signal line. If possible, ground line The width should be greater than 3mm, and a large area copper layer can also be used as a ground wire. Connect the unused places on the printed circuit board as a ground wire.

3. Input ground and output ground This switching power supply is a low-voltage DC-DC. To feed the output voltage back to the primary of the transformer, the circuits on both sides should have a common reference ground, so after laying copper on the ground wires on both sides, They must be connected together to form a common ground.

5. After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. At the same time, it is also necessary to confirm whether the rules set meet the requirements of the printed board production process. Generally check the wire and wire, wire and component welding Whether the distance between the disk, the wire and the through hole, the component pad and the through hole, and the distance between the through hole and the through hole is reasonable, and whether it meets the production requirements. Whether the width of the power line and the ground line are appropriate, and whether there is a place to widen the ground line in the PCB. Note: Some errors can be ignored. For example, a part of the outline of some connectors is placed outside the board frame, and there will be errors when checking the spacing; in addition, each time the wiring and vias are modified, the copper must be re-plated.

6. Review According to the "PCB checklist", the content includes design rules, layer definitions, line widths, spacing, pads, and via settings. It should also focus on reviewing the rationality of the device layout, the routing of power and ground networks, and high-speed The routing and shielding of the clock network, the placement and connection of decoupling capacitors, etc.