How to do the pcb design of the switching power supply? Get these knowledge points to easily complete the layout
In the switching power supply design, the physical design of the PCB board is the last link. If the design method is improper, the PCB may radiate too much electromagnetic interference, causing the power supply to work unstable. The following points are analyzed for the matters needing attention in each step:
Component parameters from schematic to PCB design flow - "Input Principle Netlist -" Design Parameter Settings - "Manual Layout -" Manual Wiring - "Verification Design - "Review -" CAM Output.
Component layout 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 AC circuit (3) input signal source current loop (4) output load current loop input loop charges the input capacitor through an approximate DC current, the filter capacitor mainly plays a broadband storage Similarly, the output filter capacitor is also used to store the high frequency energy from the output rectifier while eliminating the DC energy of 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 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 circuit and the rectifier contains high amplitude trapezoidal currents. The harmonic components of these currents are very high, and the frequency is much larger 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 It is about 50 ns.
These two loops are the most susceptible 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, inductors or transformers. Place them adjacent to each other and adjust the component position 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:
Place transformer design power switch current loop design output rectifier current loop connection to AC power circuit control circuit design input current source loop and input filter design output load loop and output filter according to the functional unit of the circuit, all components of the circuit When laying out the layout, the following principles must be met:
(1) First consider the size of the PC B. When the size of PC B 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. Components located at the edge of the board are generally no less than 2mm from the edge of the board.
(2) Consider the future soldering when placing the device, not too dense.
(3) Centering around the core components of each functional circuit, it is laid out around it. Components should be evenly, neatly and compactly placed on PC B to minimize and shorten leads and connections between components. The decoupling capacitors should be as close as possible to the VCC of 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 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 radiated interference from the switching power supply.
Parameter settings Adjacent 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. Set the trace spacing to 8 mils.
The distance from the edge of the pad to the edge of the board is greater than 1m, which avoids 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 drop-shaped. This has the advantage that the pads are not easy to peel, but the traces are not easily disconnected from the pads.
Wiring The switching power supply contains high-frequency signals. Any printed circuit on PC B can function as an antenna. The length and width of the printed circuit 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 capability. 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-level 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 line actually flowing will cause the change of the ground potential of each part of the circuit to 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 greater 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 wire 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-excite. When a single point is not achieved, 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, which will cause the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, ensure the grounding of each large current. Use as short and wide as possible of the printed circuit, try to widen the power supply, ground line width, preferably the ground line is wider than the power line, their relationship is: Ground wire "power line" signal line, if possible, ground wire The width should be greater than 3m, and a large area of ​​copper can be used as the ground line. The unused areas 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 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. Inspection, so it is easy to check, debug and repair in production (Note: refers to the premise of meeting the circuit performance and machine installation and panel layout requirements).
(2) When designing the wiring diagram, the wiring should be bent as little as possible. The line width on the printing arc should not be abrupt. The corner of the conductor 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 to solve cross circuit problems. Because of the single panel, the in-line component is on the to p side, and the surface mount device is on the bottom side, so the in-line device can overlap the surface mount device during layout, but avoid pad overlap.
3. Input ground and output ground The switching power supply is a low voltage DC-DC. To feed back 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.
After checking the wiring design, 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, generally check the line and line, line and component pads, Whether the distance between the wire and the through hole, the component 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 connectors' outlines 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-applied once.
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.
Considerations for designing output output illuminating files:
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 (NCD rill). )
b. When setting the layer of the silkscreen layer, do not select Part Type, select the top (bottom) and silkscreen layers of Outline, Text, Line.
c. When setting the Layer for each layer, select the Board Outline. When setting the Layer of the silkscreen layer, do not select Part Type, select the top (bottom) and silkscreen layers of Outline, Text, Line.
d. When generating the drilling file, use the default settings of Power PCB, do not make any changes.
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