PCB Tech - Physical Design Analysis of PCB Board in Switching Power Supply Design

In the 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, causing the power supply to work unstable. The following is an analysis of the matters needing attention in each step:

One. Design flow from schematic to PCB. Establish component parameters-"Input principle netlist-"Design parameter setting-"Manual layout -" Manual wiring-"Verify design-"Recheck -" CAM output.

2. Parameter setting The distance between adjacent conductors must be able to meet 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 appropriately increased. 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, the printed circuit board is not designed properly, it will have an adverse effect on 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). Power switch AC circuit

(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 serves as a broadband energy storage function; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier while eliminating the output load. DC energy of the loop. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should only be connected to the power supply from the terminals of the filter capacitor respectively; if the connection between the input/output loop and the power switch/rectifier loop 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. These two loops are the most prone to electromagnetic interference, so these AC loops must be laid out before the other printed lines in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, inductors or transformers. Place them next to each other and adjust the position of the components to make the current path between them as short as possible. The best way to establish a switching power supply layout is similar to its electrical design. The best design process is as follows:

place the transformer

design power switch current loop

design output rectifier current loop

Control circuit connected to AC power circuit

Design the input current source loop and input filter When designing the output load loop and output filter according to the functional unit of the circuit, all the components of the circuit should be laid out in accordance with the following principles:

(1) First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed. The best shape of the circuit board is rectangular, and the aspect ratio is 3:2 or 4:3. The components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. (2) When placing the device, consider the subsequent soldering, not too dense.

(3) Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB, minimize and shorten the leads and connections between the components, and the decoupling capacitor should be as close as possible to the VCC of the device.

(4) For circuits operating at high frequencies, the distributed parameters between components must be considered. Generally, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld, and easy to mass produce.

(5) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.

(6) The first principle of layout is to ensure the wiring rate, pay attention to the connection of the flying leads when moving the device, and put the devices with the connection relationship together.

(7) Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply.

Fourth, the wiring switching power supply contains high-frequency signals. Any printed line on the PCB can function 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 AC ground of high current. 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 ground wire as much as possible. If the ground wire is very thin, the ground potential will change with the change of current, resulting in unstable timing signal level of electronic equipment and deterioration of anti-noise performance