PCB design specification for switching power supply

In the design of any switching power supply, the physical design of PCB board is the last link. If the design method is not proper, PCB may radiate excessive electromagnetic interference, resulting in unstable power supply. The following is an analysis of the matters that need attention in each step:

One, from schematic to PCB design process to establish component parameters -> input principle netsheet -> design parameter Settings -> manual layout -> manual wiring -> Verification design -> Review ->CAM output.

The spacing between adjacent wires must meet the electrical safety requirements, and in order to facilitate operation and production, the spacing should also be as wide as possible. When the wiring density is low, the distance between signal lines can be increased appropriately. For signal lines with a wide gap between high and low levels, the distance should be shortened and increased as far as possible. In general, the distance between lines should be set to 8mil.

The distance between the inner hole edge of the pad and the printed board edge should be greater than 1mm, so as to avoid the defects of the pad during processing. When the wire connection with the pad is fine, the connection between the pad and the wire should be designed into a droplet shape. The advantage of this is that the pad is not easy to peel, but the wire and the pad are not easy to disconnect.

Component layout practice has proved that even if the circuit schematic design is correct and the printed circuit board is improperly designed, which will adversely affect the reliability of electronic equipment. For example, if the two thin parallel lines on a printed board are close together, there will be a delay in the signal waveform, resulting in reflected noise at the end of the transmission line; Due to the interference caused by the inconsiderate power supply and ground wire, the performance of the product will decline, therefore, in the design of printed circuit board, should pay attention to the correct method. Each switching power supply has four current circuits: (1). Power switch AC circuit

(2) Output rectified 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, and the filter capacitor mainly plays a role of broadband energy storage; Similarly, output filter capacitors are used to store high-frequency energy from the output rectifier while eliminating DC energy from the output load loop. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should be connected to the power supply only from the terminals of the filter capacitors. If the connection between the input/output loop and the power switch/rectifier loop cannot be directly connected to the terminal of the capacitor, AC energy will be filtered by the input or output capacitor and radiated into the environment. The AC circuits of the power switch AC circuit and the rectifier AC circuit contain high amplitude trapezoidal current. These currents have a high harmonic component and their frequency is much higher than the switching fundamental frequency. The peak amplitude can be up to 5 times that of the continuous input/output DC current amplitude. These two circuits are the most likely to produce electromagnetic interference, so they must be laid before other printed wires in the power supply. The three main components of each circuit, the filter capacitor, the power switch or rectifier, the inductor or the transformer, should be placed next to each other, and the position of the components should be adjusted 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:

· Transformer placement

· Design the power switch current loop

· Design the output rectifier current loop

· Control circuit connected to AC power circuit

· Design input current source loop and input filter Design output load loop and output filter according to the functional unit of the circuit, the layout of all components of the circuit should comply with the following principles:

(1) The size of PCB should be considered first. When 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 not good, and the adjacent lines are susceptible to interference. The best shape of the circuit board is rectangular, the aspect ratio is 3:2 or 4:3, and the components located on the edge of the board are generally not less than 2mm away from the edge of the board.

(2) When placing the device, the future welding should be considered, not too intensive.

(3) Lay out around the core components of each functional circuit. The components should be uniform, neat, and compact on the PCB, minimizing and shortening 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 working at high frequency, distribution parameters between components should be considered. General circuits should be arranged as parallel as possible. In this way, not only beautiful, but also easy to assemble welding, easy to mass production.

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

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

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

Four, wiring switch power supply contains high frequency signals, any printed line on the PCB can play the role of an antenna, printed line length and width will affect its impedance and inductive reactance, thereby affecting the frequency response. Even printed lines that pass through the DC signal can couple from neighboring printed lines to the RF signal and cause circuit problems (or even re-radiate interference signals). Therefore all printed wires that pass through the AC current should  be designed to be as short and wide as possible, which means that all components connected to the printed wires and to other power lines must be placed close together. 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 the more radio-frequency energy it can radiate. According to the size of the printed circuit board current, try to thicken the width of the power line, reduce the loop resistance. At the same time, make the direction of the power line and ground line is consistent with  the direction of the current, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current circuits of the switching power supply. As the common reference point of the circuit, it plays an important role. It is an important method to control interference. Therefore, careful consideration should be given to the placement of ground wires in the layout. Mixing all kinds of ground will cause unstable power supply. The following points should be noted in the design of ground wire:

1. Correct selection of single point grounding. Usually, the filter capacitor common terminal should be the only connection point of other ground points coupled to the AC ground of large current. The ground point of the same level circuit should be as close as possible, and the power filter capacitor of this level circuit should also be connected to the ground point of this level, mainly considering that the current of each part of the circuit back to the ground is changed. Interference is introduced due to the impedance of the actual flowing line, which causes changes in the ground potential of various parts of the circuit. In this switching power supply, the inductance between wiring and components has little influence, while the circulation formed by grounding circuit has great influence on interference. Therefore, one-point grounding is adopted, that is, ground wires of several devices in the power switching current loop () are connected to the ground pin, and ground wires of several devices in the output rectifier current loop are also connected to the ground pin of the corresponding filter capacitor. This power supply work more stable, not easy to self-excitation. When you can't do a single point, you can connect two diodes or a small resistor at a common location. In fact, you can connect it to a concentrated piece of copper foil.

2. If the ground wire is very thin, the ground potential will change with the change of the current, resulting in the timing signal level of electronic equipment is unstable, anti-noise performance deteriorated, so to ensure that the grounding terminal of each large current uses as short and wide as possible printed wire, as far as possible to widen the width of the power supply, ground wire, it is best to ground wire wider than the power line, their relationship is: Ground > power cable > signal cable. If possible, the width of the ground wire should be greater than 3mm. A large area of copper layer can also be used as ground wire. When conducting global wiring, the following principles must be followed:

(1). Wiring direction: from the perspective of welding surface, the arrangement orientation of components should be consistent with the schematic diagram as far as possible, and the wiring direction should be consistent with the wiring direction of the circuit diagram. Because various parameters are usually detected in the welding surface during the production process, it is convenient to do so for inspection, debugging and maintenance in the production (Note: On the premise of meeting the requirements of circuit performance, overall installation and panel layout).

(2). When designing the wiring diagram, tthe wiring is routed as little as possible, the line width on the printed arc should not be abrupt, and the corner of the wire should be ≥ 90 degrees, and the line should be simple and clear.

(3) The printed circuit is not allowed to have a cross circuit, for the possible cross lines, you can use "drill", "wind" two ways to solve. That is, let a lead from other resistance, capacitance, the gap at the foot of the audion "drill" in the past, or from the possible cross of a lead end "wound" in the past, in special cases how to circuit is very complex, in order to simplify the design also allows the use of wire straddle, to solve the problem of cross circuit. Because of the single panel, the in-line component is located on the top surface and the surface paste device is located on the bottom surface, so in the layout of the in-line component and the surface paste device overlap, but to avoid the overlap of the pad. 3. Input ground and output ground in the switching power supply for low-voltage DC-DC, to output voltage feedback back to the primary transformer, both sides of the circuit should have a common reference, so after the ground wire on both sides of the copper, but also connected together, to form a common ground.

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, and also confirm whether the rules formulated meet the requirements of the printed board production process. Generally, it is necessary to check whether the distance between wire and wire, wire and component pad, the wire and through hole, component pad and through hole, through hole and through hole is reasonable, and whether it meets the production requirements. Whether the width of the power cord and ground wire is appropriate, and whether there is still a place in the PCB that can widen the ground wire Note: Some errors can be ignored. For example, some connectors have part of their Outline lying outside the frame, and errors occur when checking spacing. In addition, after each modification of the line and hole, it is necessary to re-cover the copper once.

Vi. Review According to the "PCB checklist", the content includes the design rules, layer definition, wire width, spacing, pad, hole setting, but also focus on the review of the rationality of the device layout, power supply, ground network wiring, high-speed clock network wiring and shielding, the placement and connection of the coupling capacitor.

Seven, the design output Considerations for outputting light drawing files:

a. Need to output the layer of wiring layer (bottom), screen printing layer (including top screen printing, bottom screen printing), welding resistance layer (bottom welding resistance), drilling layer (bottom), in addition to the generation of drilling file (NC Drill)

b. When setting the Layer of the screen printing layer, do not select Part Type. Select the Outline, Text, and Linec of the top (bottom) layer and the screen printing layer. When setting the Layer of each Layer, select the Board Outline. When setting the layer of the screen printing layer, do not select Part Type. Select the Outline, Text, and Line of the top layer (bottom layer) and the screen printing layer. d. Use the default Settings of the PowerPCB when generating the drilling file.