In the design of switching power supply, if the PCB board is not designed properly, it will radiate too much electromagnetic interference. The PCB board design with stable power supply work now summarizes the seven tricks: through the analysis of the matters needing attention in each step, the PCB board design can be easily done step by step!
1. The design process from schematic to PCB
Establish component parameters -> 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 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 be greater than 1mm from the edge of the inner hole of the pad to the edge of the printed board, so as to 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.
3. Component layout
Practice has proved that even if the circuit schematic is designed correctly and the printed circuit board is not designed properly, it will adversely affect the reliability of electronic equipment. For example, if two thin parallel lines of the printed board are close together, it will cause signal waveform delay and reflection noise at the end of the transmission line; interference caused by improper consideration of power and ground will cause the product to suffer Performance drops, therefore, when designing printed circuit boards, attention should be paid to the correct method. Each switching power supply has four current loops:
(1) AC circuit of power switch
(2) Output rectifier AC circuit
(3) Current loop of input signal source
(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; 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 circuit is eliminated. 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 loop of the power switch and the AC loop of the rectifier contain high-amplitude trapezoidal currents. These currents have high harmonic components and their frequency is much greater than the fundamental frequency of the switch. The peak amplitude can be as high as 5 times the continuous input/output DC current amplitude. The transition time is usually About 50ns. These two loops are 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, and inductors. Or the transformers should be placed next to each other, and the component positions 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:
◆Place the transformer
◆Design power switch current loop
◆Design 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, when laying out all the components of the circuit, the following principles should be met:
(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 at the edge of the circuit board are generally not less than the edge of the circuit board
(2) When placing the device, consider future 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 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 the layout is to ensure the wiring rate, pay attention to the connection of the flying wires 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.
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 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 interference 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 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 a very important role as a common reference point for the circuit. 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:
A. 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. The grounding points of the same level circuit should be as close as possible, and the power supply filter capacitor of this level circuit should also be Should be connected to the grounding point of this level, mainly considering that the current returning to the ground in each part of the circuit is changed, and 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, so one point grounding is used, that is, the power switch current loop (the ground wires of several devices are all Connected to the grounding pin, the ground wires of several components of the output rectifier current loop are also connected to the grounding pins of the corresponding filter capacitors, so that the power supply is stable and 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.
B. Thicken the grounding wire as much as possible. If the grounding wire is very thin, the ground potential will change with the change of 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 better 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. When performing global wiring, the following principles must also be followed:
(1) Wiring direction: From the perspective of the welding surface, the arrangement of the components should be as consistent as possible with the schematic diagram. The wiring direction should be consistent with the wiring direction of the circuit diagram, because various parameters are usually required on the welding surface during the production process. Therefore, it is convenient for inspection, debugging and maintenance in production (Note: It refers to the premise of meeting the circuit performance and the requirements of the whole machine installation and panel layout).
(2) When designing the wiring diagram, the wiring should not bend as much as possible, the line width on the printed arc should not be changed suddenly, the corner of the wire should be ≥90 degrees, and the lines should be simple and clear.
(3) Cross circuits are not allowed in the printed circuit. For the lines that may cross, you can use “drilling” and “winding” to solve them. That is, let a lead “drill” through the gap under other resistors, capacitors, and triode pins, or “wind” from one end of a lead that may cross. In special circumstances, how complex the circuit is, it is also allowed to simplify the design. Use wires to bridge to solve the cross circuit problem. Because the single-sided board is adopted, the in-line components are located on the top surface and the surface-mount devices are located on the bottom surface. Therefore, the in-line devices can overlap with the surface-mount devices during layout, but overlap of the pads should be avoided.
C. Input ground and output ground This switching power supply is a low-voltage DC-DC. If you want to feedback 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. Check
After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules set by the designer, and at the same time, it is also necessary to confirm whether the established rules meet the requirements of the printed board production process. Generally check the line and line, line and component pad, line Whether the distances from through holes, component pads and through holes, through holes and through holes are reasonable, and whether they meet 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 errors will occur when checking the spacing; in addition, each time the wiring and vias are modified, the copper must be re-coated.
6. Re-check according to “PCB Checklist”
The content includes design rules, layer definitions, line widths, spacing, pads, and via settings. It is also important to review the rationality of the device layout, the wiring of power and ground networks, the wiring and shielding of high-speed clock networks, and decoupling Placement and connection of capacitors, etc.
7. the matters needing attention in designing and outputting Gerber files
a. The layers that need to be output include wiring layer (bottom layer), silk screen layer (including top silk screen, bottom silk screen), solder mask (bottom solder mask), drilling layer (bottom layer), and a drilling file (NCDrill)
b. When setting the Silk screen layer, do not select PartType, select the top layer (bottom layer) and Outline, Text, Linec of the silk screen layer. When setting the Layer of each layer, select Board Outline. When setting the silk screen layer, do not Select PartType, select Outline, Text, Line.d of the top layer (bottom layer) and silk screen layer. When generating drilling files, use the default settings of PowerPCB and do not make any changes.