LED switching power supply PCB board design has seven steps
Summary of content: In the design of switching power supplies, if the PCB board is not designed properly, it will radiate too much electromagnetic interference. The PCB board design with stable power supply operation is summarized in the seven-step trick: by analyzing the matters needing attention in each step, the PCB board design can be easily done step by step!
一 、 Design flow from schematic to PCB
Create component parameters-> input principle netlist-> design parameter settings-> manual layout-> manual wiring-> verify the design-> review-> CAM output.
Second, parameter setting
间距 The distance between adjacent wires must meet the requirements of electrical safety, and in order to facilitate operation and production, the distance should be as wide as possible. The minimum distance must be at least suitable for the voltage to withstand. When the wiring density is low, the distance between signal lines can be appropriately increased. For high and low level signal lines, the distance should be as short as possible and the distance should be increased. Generally, Set the distance between traces to the edge of the inner hole of the pad to the edge of the printed board to be greater than 1mm, so as to avoid pad defects during processing. When the traces connected to the pads are thin, the connection between the pads and the traces should be designed in a water droplet shape. This has the advantage that the pads are not easy to peel, but the traces and the pads are not easy to disconnect.
Practice has proved that even if the circuit schematic design is correct 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 a printed board are close to each other, a signal waveform delay will be formed, and reflection noise will be formed at the end of the transmission line; interference caused by inadequate consideration of the power supply and the ground line will cause the product's The performance is degraded. Therefore, when designing a printed circuit board, care should be taken to use the correct method. Each switching power supply has four current loops:
(1), AC circuit of power switch
(2), output rectified AC circuit
(3), input signal source current loop
(4) Output load current loop The input loop charges the input capacitor with an approximately 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 circuit is eliminated. 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 circuit and the power switch / rectifier circuit cannot be connected to the capacitor, The terminals are directly connected, and the AC energy will be radiated to 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, and their frequency is much higher than the switching fundamental frequency. The peak amplitude can be up to 5 times the continuous input / output DC current amplitude. The transition time is usually About 50ns. These two loops are most likely to generate electromagnetic interference, so these AC loops must be laid before other printed wiring 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. Adjust the components so that the current path between them is as short as possible.
的 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 circuit and input filter. Design the output load circuit and output filter. According to the functional unit of the circuit, the layout of all components of the circuit must comply with the following principles:
(1) First of all, consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise capability is reduced, and the cost is also increased; if the PCB size 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 with an aspect ratio of 3: 2 or 4: 3
(2) When placing the device, consider future soldering, not too dense;
(3) Centering on the core components of each functional circuit, layout around it. Components should be evenly, neatly and compactly arranged on the PCB, minimize and shorten the leads and connections between components, and decoupling capacitors should be as close as possible to the device
(4) For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the components should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to mount and weld, and easy to mass produce.
(5) According to the flow of the circuit, arrange the positions of the functional circuit units, make the layout convenient for signal circulation, and keep the signals in the same direction as possible.
(6) The first principle of the layout is to ensure the distribution rate of the wiring. When moving the device, pay attention to the connection of the flying leads, and put the devices with connection relationship together.
(7) Minimize 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
任何 Any printed wire on the PCB can function as an antenna. The length and width of the printed wire will affect its impedance and inductive reactance, and thus affect the frequency response. Even printed wires that pass a DC signal can couple from nearby printed wires to the RF signal and cause circuit problems (even radiating interference signals again). Therefore, all printed wiring that passes AC current should be designed as short and wide as possible, which means that all components connected to the printed wiring and connected to other power lines must be placed close together. The length of the trace is directly proportional to the inductance and impedance it exhibits, and the width is inversely proportional to the inductance and impedance of the trace. The length reflects the wavelength of the response of the printed line. The longer the length, the lower the frequency that the printed line can send and receive electromagnetic waves, and it can radiate more RF energy. According to the current of the printed circuit board, try to increase the width of the power line to reduce the loop resistance. At the same time, the direction of the power line and the ground line should be consistent with the direction of the current, which will help enhance the ability to resist noise. Grounding is the bottom branch of the four current loops of the switching power supply. It plays a very important role as the common reference point of the circuit. It is an important method to control interference. Therefore, the layout of the ground wire should be carefully considered in the layout. Mixing various grounds will cause the power supply to be unstable.
注意 Please pay attention to the following points in the ground wire design:
1. Correctly select single-point grounding Generally, the common end of the filter capacitor should be the only connection point where other ground points are coupled to the high-current AC ground. The ground point of the same level of circuit should be as close as possible, and the power supply filter capacitor of this level of circuit is also Should be connected to the ground point of this level, mainly considering that the current flowing back to ground of each part of the circuit is changed, because the impedance of the actual flowing line will cause the ground potential of each part of the circuit to introduce interference. In this switching power supply, the influence of its wiring and the inductance between the components is small, and the circulating current formed by the ground circuit has a large impact on the interference. Therefore, a little ground is used, that is, the power supply current loops (the ground lines of several devices are all Connected to the ground pin, the ground wires of several devices that output the rectifier current loop are also connected to the ground pins of the corresponding filter capacitors. This way the power supply works more stable and is not easy to self-excited. When a single point cannot be achieved, the common ground Connect two diodes or a small resistor everywhere. In fact, it can be connected to a relatively concentrated piece of copper foil.
2. Make the grounding wire as thick as possible. If the grounding wire is very thin, the grounding potential changes with the change of current, which causes the timing signal level of electronic equipment to be unstable and the anti-noise performance to be deteriorated. Therefore, ensure that each high-current grounding terminal Use as short and wide printed wires as possible to widen the width of the power and ground wires. It is best that the ground wires are wider than the power wires. Their relationship is: ground wire> power wire> signal wire. If possible, the ground wire The width should be greater than 3mm. A large-area copper layer can also be used as the ground wire. All unused places on the printed board are connected to the ground as the ground wire. When performing global wiring, the following principles must also be followed:
(1) Wiring direction: Seen from the soldering surface, the arrangement of 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 soldering surface during the production process. This makes it easy to check, debug and overhaul in production (Note: under the premise of meeting the requirements of circuit performance and the installation and panel layout of the whole machine).
(2). When designing the wiring diagram, try to make as few bends as possible, do not change the line width on the printed arc, and the corners of the wire should be ≥90 degrees. Try to keep the lines simple and clear.
(3) Crossed circuits are not allowed in printed circuits. For lines that may cross, two methods can be used: "drilling" and "winding." That is, a certain lead is "drilled" from the gap under other resistors, capacitors, or triode pins, or "wound" from one end of a lead that may cross. In special cases, how the circuit is complicated, it is also allowed to simplify the design. Use wire crossover to solve the problem of cross circuit. Due to the use of a single panel, the in-line components are located on the top surface, and the surface-mount devices are located on the bottom surface, so the in-line devices can overlap the surface-mount devices during layout, but the pads must be avoided.
3. Input ground and output ground The switching power supply is a low-voltage DC-DC. If the output voltage is to be fed back to the primary of the transformer, the circuits on both sides should have a common reference ground. Therefore, after laying the ground wires on both sides separately, They must be 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. At the same time, it is also necessary to confirm whether the formulated rules meet the requirements of the printed board production process. Generally, check the line-to-line, line-to-component pads, and line. Whether the distance from the through hole, the component pad and the through hole, the through hole and the through hole is reasonable and meets the production requirements. Are the widths of the power and ground wires appropriate, and is there any place in the PCB that can widen the ground wires. Note: Some errors can be ignored. For example, part of the outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing. In addition, after modifying the wiring and vias, you need to re-copper the copper again.
26. Re-examination according to "PCB Checklist"
The content includes design rules, layer definition, line width, pitch, pad, and via settings. It is also important to review the rationality of the device layout, the routing of power and ground networks, the routing and shielding of high-speed clock networks, and decoupling Placement and connection of capacitors.
VII.Precautions for design output and output light drawing file
A. The layers that need to be output include wiring layer (bottom layer), silk screen layer (including top screen printing, bottom screen printing), solder mask layer (bottom solder mask), drilling layer (bottom layer), and a drill file (NCDrill) is also generated
b. When setting the silkscreen layer, do not select PartType, select the top (bottom) and outline, text, and linec of the silkscreen layer. When setting the layer of each layer, select the Board Outline, and when setting the silkscreen layer, do not select Select PartType, select the top layer (bottom layer) and the outline, text, and line.d. of the silk screen layer to generate the drilling file, use the default settings of PowerPCB, do not make any changes.