In any switching power supply design, the physical design of the PCB is the last link. If the design method is not correct, the PCB may radiate excessive electromagnetic interference, causing the power supply to be unstable. The following points are required for each step analysis:
1. From the schematic diagram to the PCB design process to build component parameters -> input principle netlist -> design parameter settings -> manual layout -> manual wiring -> verification design -> review -> CAM output.
2. The parameters set adjacent conductor spacing must meet the electrical safety requirements, and in order to facilitate operation and production, the spacing should be as wide as possible. 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 1mm, 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 a drop-shaped shape. This has the advantage that the pads are not easily peeled off, but the traces are not easily disconnected from the pads.
3. The component layout practice proves that even if the circuit schematic design is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the electronic equipment. For example, if the two thin parallel lines of the printed board are close together, a delay of the signal waveform will be formed, and 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 broad band 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 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 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 into the environment by the input or output filter capacitors. The AC circuit of the AC circuit of the power switch and the rectifier contain high-profile 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 50ns. 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 adjacently 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:
Designing the power switch current loop
Design output rectifier current loop
Control circuit connected to the 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 following principles should be met when laying out all the components of the circuit:
(1) First consider the PCB size.
When the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise ability is lowered, 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 2mm from the edge of the board.
(2) Consider the future soldering when placing the device, not too dense.
(3) Center around the core components of each functional circuit and arrange it around 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 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 routing 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.
4. The wiring switching power supply contains high-frequency signals. Any printed circuit on the PCB can function as an antenna. The length and width of the printed circuit will affect its impedance and inductance, thus affecting the frequency response. Even a printed line that passes a DC signal can couple to an RF signal from an adjacent printed line 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 underlying 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, careful consideration should be given to the placement of the grounding wire in the layout. Mixing various groundings will cause unstable power supply operation. Pay attention to the following points in the ground line design:
5. 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 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 ground) 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.
6. If the grounding wire is as thick as possible, the grounding potential changes with the change of the current, which causes 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 as short and wide as possible of the printed line, try to widen the power supply, ground line width, preferably 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. 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 followed:
(1), wiring direction: From the welding surface, the arrangement orientation of the components is kept as consistent as possible with 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. 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 “winding”. That is, a lead is "drilled" from the gap under other resistors, capacitors, and triodes, or "wounded" 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 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.
(4) Input ground and output ground The DC-DC of the switching power supply is low voltage. To feedback the output voltage back to the primary of the transformer, the circuits on both sides should have a common reference ground. Therefore, after the copper wires on both sides are respectively paved with copper, Also connected together to form a common ground.
(5) After the inspection 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 wire, wire and component welding. Whether the distance between the disk, 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' outline 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.
7. According to the "PCB checklist", the content includes design rules, layer definition, line width, spacing, pad, via settings, but also focus on reviewing the rationality of device layout, power, ground network routing, high speed Trace and shield of the clock network, placement and connection of decoupling capacitors, etc.
8. Precautions for designing output illuminating files:
(1) The layer to be output has a wiring layer (bottom layer) , silk screen layer ( including top screen printing, bottom screen printing), solder mask ( bottom solder mask) , drill layer (bottom layer), and also generate drilling files (NCDrill) .
(2) When setting the layer of the silkscreen layer, do not select PartType, select the top layer (bottom layer) and the silkscreen layer of Outline, Text, Linec. When setting the Layer of each layer, select Board Outline, set the layer of the silkscreen layer, do not select Part Type, select the top (bottom) and silkscreen layers of Outline, Text, Line.d. When creating a drill file, use the default settings of Power PCB and do not make any changes.
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