In the design of switching power supply, the physical design of PCB board is the last link. If the design method is improper, PCB may radiate too much electromagnetic interference, resulting in unstable operation of power supply. The matters needing attention in each step are analyzed as follows:
1、 From schematic diagram to PCB design process, establish component parameters – > input principle net list – > design parameter setting – > manual layout – > manual wiring – > verify design – > recheck – > cam output.
2、 The distance between adjacent conductors in parameter setting must meet the electrical safety requirements, and the distance should be as wide as possible in order to facilitate operation and production. The minimum spacing shall at least be suitable for the withstand voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased. For signal lines with great difference between high and low levels, the spacing shall be as short as possible and increased. Generally, the routing spacing is set to 8mil.
The distance from the edge of the inner hole of the pad to the edge of the printed board should be greater than 1mm, which can avoid the defect of the pad during processing. When the wiring connected with the pad is thin, the connection between the pad and the wiring should be designed into a water drop shape. This has the advantage that the pad is not easy to peel, but the wiring and the pad are not easy to be disconnected.
3、 The practice of component layout has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, it will have an adverse impact on the reliability of electronic equipment. For example, if two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed and the reflected noise will be formed at the terminal of the transmission line; The interference caused by the thoughtless consideration of power supply and ground wire will degrade the performance of the product. Therefore, when designing printed circuit board, we should pay attention to the correct method.
Each switching power supply has four current circuits:
(1) AC circuit of power switch
(2) Output rectifier AC circuit
(3) Input signal source current loop
(4) The output load current circuit and the input circuit charge the input capacitor through an approximate DC current, and the filter capacitor mainly plays a role of broadband energy storage; Similarly, the output filter capacitor is also used to store the high-frequency energy from the output rectifier and eliminate the DC energy of the output load circuit. The input and output terminals of capacitor and filter circuit are very important, so the input and output terminals of capacitor and filter circuit are connected respectively; If the connection between the input / output circuit and the power switch / rectifier circuit cannot be directly connected with the terminal of the capacitor, 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 in these currents are very high, and their frequency is much higher 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, and the transition time is usually about 50ns. These two circuits are most prone to electromagnetic interference. Therefore, these AC circuits must be laid before the wiring of other printed wires in the power supply. The three main components of each circuit, filter capacitor, power switch or rectifier, inductance or transformer, should be placed adjacent to each other, and the position of components should be adjusted to make the current path between them as short as possible.
The best way to establish the layout of switching power supply is similar to its electrical design. The best design process is as follows:
Design switching current circuit of power supply
Design output rectifier current loop
Control circuit connected to AC power circuit
When designing the input current source circuit and input filter and the output load circuit and output filter, all components of the circuit shall be arranged according to the functional unit of the circuit, in accordance with the following principles:
(1) First consider the PCB size. When the PCB size is too large, the printed line is long, the impedance increases, the anti noise ability decreases and the cost increases; If it is too small, the heat dissipation is poor, and the adjacent lines are easy to be disturbed. The best shape of the circuit board is rectangular, with an aspect ratio of 3:2 or 4:3. The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.
(2) When placing devices, future welding should be considered and not too dense;
(3) Take the core components of each functional circuit as the center and layout around it. The components shall be evenly, neatly and compactly arranged on the PCB, the leads and connections between components shall be reduced and shortened as far as possible, and the decoupling capacitance shall be close to the VCC of the component as far as possible.
(4) For the circuit working at high frequency, the distribution parameters between components should be considered. For general circuits, components shall be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to assemble and weld and easy to mass produce.
(5) Arrange the position of each functional circuit unit according to the circuit flow, make the layout convenient for signal flow, and keep the signal in the same direction as far as possible.
(6) The primary principle of the layout is to ensure the routing rate of the wiring. Pay attention to the connection of flying wires when moving devices, and put the devices with connection relationship together.
(7) Reduce the loop area as much as possible to suppress the radiation interference of switching power supply.
4、 The wiring 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 inductive reactance, thus affecting the frequency response. Even printed lines passing through DC signals will be coupled to RF signals from adjacent printed lines and cause circuit problems (or even radiate interference signals again). Therefore, all printed lines passing through AC current should be designed as short and wide as possible, which means that all components connected to printed lines and other power lines must be placed close to each other. The length of the printed line is directly proportional to its inductance and impedance, while the width is inversely proportional to its inductance and impedance. The longer the length of the printed line, the more the electromagnetic radiation can be reflected. According to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, make the direction of power line and ground wire consistent with the direction of current, which helps to enhance the anti noise ability. Grounding is the bottom branch of the four current circuits of switching power supply. As the common reference point of the circuit, it plays a very important role. It is an important method to control interference. Therefore, the placement of grounding wires should be carefully considered in the layout. Mixing various grounding wires will cause instability of power supply.