The design of the PCB board with stable switching power supply is summarized in seven steps: by analyzing the items that need attention in each step, it is easy to do the PCB board design step by step!
First, from the schematic to the PCB design processEstablish component parameters - "Input Principle Netlist -" Design Parameter Settings - "Manual Layout -" Manual Wiring - "Verification Design - "Review -" CAM output.
Second, the parameter settingsAdjacent conductor spacing must meet electrical safety requirements, and spacing should be as wide as possible for ease of operation and production. The minimum spacing should be at least suitable for the voltage to withstand. When the wiring density is low, the spacing of the signal lines can be appropriately increased. The signal lines with high and low levels should be as short as possible and the spacing should be increased. Under normal circumstances Set the trace spacing to the edge of the pad to the edge of the board to be greater than 1mm, which can avoid pad defects during processing. When the traces connected to the pads are thinner, the connection between the pads and the traces is designed to be water droplets. This has the advantage that the pads are not easy to peel, but the traces are not easily disconnected from the pads.
Practice has proved that even if the schematic design of the circuit is correct and the printed Circuit Board is not properly designed, it will adversely affect the reliability of the electronic device. For example, if the two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed, and the reflection noise will be formed at the end of the transmission line; the interference caused by the inconsistency of the power supply and the ground line will cause the product to be Performance is degraded, so when designing a printed circuit board, care should be taken to use the correct method. Each switching power supply has four current loops:
(1), power switch AC circuit
(2) Output rectifier circuit
(3), input signal source current loop
(4) The output load current loop input loop charges the input capacitor through an approximately DC current. The filter capacitor mainly functions as a broadband energy storage; similarly, the output filter capacitor is also used to store the high frequency energy from the output rectifier. At the same time, the DC energy of the output load loop is eliminated. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should be connected only from the terminals of the filter capacitor to the power supply; if the connection between the input/output loop and the power switch/rectifier loop cannot be connected to the capacitor The terminals are directly connected and the AC energy is radiated from the input or output filter capacitors to the environment. The AC circuit of the power switch AC loop and rectifier contains high amplitude trapezoidal currents. The harmonic components of these currents are very high. The frequency is much higher than the fundamental frequency of the switch. The peak amplitude can be up to 5 times the amplitude of the continuous input/output DC current. The transition time is usually About 50 ns. These two loops are most prone to electromagnetic interference, so these AC loops must be placed before other traces 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, adjusting the position of the components so that the current path between them is as short as possible.
The best way to establish a switching power supply layout is similar to its electrical design. The best design flow is as follows:
â—†Place transformer
â—†Design power switch current loop
â—†Design output rectifier current loop
â—†Control circuit connected to the AC power circuit
â—† Design input current source loop and input filter design Output load loop and output filter According to the functional unit of the circuit, the following principles should be met when laying out all the components of the circuit:
(1) First consider the size of the PCB. When the PCB size is too large, the printed lines are long, the impedance is increased, the noise resistance is reduced, and the cost is also increased; if the size is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. The optimal shape of the board is rectangular, with an aspect ratio of 3:2 or 4:3. The components at the edge of the board are generally not less than the edge of the board.
(2) When placing the device, consider the future soldering, not too dense;
(3) Centering on the core components of each functional circuit and surrounding it. Components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between the components. The decoupling capacitors are as close as possible to the device.
(4) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to load and weld, and easy to mass produce.
(5) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.
(6) The first principle of layout is to ensure the wiring rate of the wiring. Pay attention to the connection of the flying line 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 switch power supply contains high frequency signalsAny printed line on the PCB can function as an antenna. The length and width of the printed line affect its impedance and inductance, which affects the frequency response. Even a trace through a DC signal can couple to an RF signal from an adjacent trace and cause a circuit problem (even radiating an interference signal again). Therefore, all traces through alternating current should be designed to be as short and wide as possible, which means that all components connected to the trace and connected to other power lines must be placed in close proximity. The length of the trace is 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 at which the printed line can transmit and receive electromagnetic waves, and it radiates more RF energy. 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, the direction of the power line and the ground line are 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. It is an important method to control interference. Therefore, the placement of the grounding wire should be carefully considered in the layout, and mixing various groundings will cause the power supply to be unstable.
Pay attention to the following points in the ground line design:
1. Correct selection of single-point grounding Generally, the common side of the filter capacitor should be the only connection point where other grounding points are coupled to the AC ground of the large current. The grounding point of the same-stage circuit should be as close as possible, and the power supply filter capacitor of the current-level circuit is also It should be connected to the grounding point of this stage, mainly considering that the current flowing back to the ground of each part of the circuit changes, because the impedance of the actual flowing line will lead to 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 less influence, and the circulating current formed by the grounding circuit has a great influence on the interference, so a little grounding is used, that is, the power switching current loop (the grounding of several devices in the device) Connected to the grounding pin, the ground of several devices of the output rectifier current loop is also connected to the grounding pin of the corresponding filter capacitor, so that the power supply works stably and is not easy to self-excitation. Connect two diodes or a small resistor, in fact, it can be connected to a relatively concentrated piece of copper foil.
2. If the grounding wire is as thick as possible, the grounding potential will change with the change of current, causing the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, ensure the grounding of each large current. Use the shortest and widest printed lines to maximize the power and ground line width. It is better to ground the ground line than the power line. Their relationship is: ground wire "power cord" signal line, if possible, ground wire The width should be greater than 3mm. It can also be used as a ground wire with a large area of ​​copper. The places that are not used on the printed board are connected to the ground as ground. When performing global routing, the following principles must also be observed:
(1) Wiring direction: From the welding surface, the arrangement orientation of the components is kept as close as possible to the schematic diagram, and the wiring direction is preferably consistent with the wiring direction of the circuit diagram, because various parameters are usually required on the welding surface during the production process. The detection, so it is convenient to check, debug and repair in production (Note: refers to the requirements of the circuit performance and machine installation and panel layout requirements).
(2) When designing the wiring diagram, the trace should be turned as little as possible, and the line width on the printing arc should not be abrupt, and the corner of the wire should be ≥90 degrees, and the line should be simple and clear.
(3) Cross circuit is not allowed in the printed circuit. For the lines that may cross, it can be solved by “drilling†or “windingâ€. That is, let a certain lead be "drilled" from the gap under other resistors, capacitors, and triodes, or "wrap" from one end of a lead that may cross. In a special case, the circuit is complicated, and it is allowed to simplify the design. Use wire to bridge and solve cross circuit problems. Because of the single panel, the in-line component is on the top surface, and the surface mount device is on the bottom surface. Therefore, the in-line device can overlap the surface mount device during layout, but the pad overlap should be avoided.
3. Input ground and output ground The DC-DC is low voltage in the switching power supply. 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 the copper is ground on both sides, Also connected together to form a common ground.
V. InspectionAfter the wiring design is completed, it is necessary to carefully check whether the wiring design meets the rules set by the designer, and also to confirm whether the established rules meet the requirements of the printed board production process, and generally check the line and line, line and component pads, and lines. Whether the distance between the through hole, the element pad and the through hole, the through hole and the through hole is reasonable, and whether the production requirement is satisfied. Is the width of the power and ground wires appropriate? Is there a place in the PCB where the ground wire can be widened? Note: Some errors can be ignored. For example, some of the outlines of some connectors are placed outside the board frame, and errors are detected when checking the spacing. In addition, each time the traces and vias are modified, the copper is re-copied once.
6. Review according to the "PCB checklist"The content includes design rules, layer definition, line width, spacing, pad, and via setting. It is also important to review the rationality of device layout, routing of power and ground networks, routing and shielding of high-speed clock networks, and decoupling. Place and connect capacitors.
Seven, design output output illuminating file notes
a. The layers to be output have a wiring layer (bottom layer), a silk screen layer (including top screen printing, bottom screen printing), a solder resist layer (underlayer solder mask), a drilled layer (bottom layer), and a drill file (NCDrill).
b. When setting the layer of the silkscreen layer, do not select PartType, select the top layer (bottom layer) and the silkscreen layer's Outline, Text, Linec. When setting the Layer of each layer, select the Board Outline and set the layer of the silkscreen layer. Select PartType, select the top (bottom) and silkscreen layers of Outline, Text, Line.d. When generating the drill file, use the default settings of PowerPCB, do not make any changes.
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