In the design of switching power supply, 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 and cause the power supply to work unstable. The following are the matters needing attention in each step analyze:

The design flow from schematic to PCB

Establishing component parameters-“input principle netlist-“design parameter settings -” manual layout-“manual wiring-“verification design -” review-“CAM output.

Component layout

Practice has proved 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 electronic equipment. For example, if the two thin parallel lines of the printed board are close together, it will cause the delay of the signal waveform and the reflection noise at the end of the transmission line; the interference caused by the improper consideration of the power supply and the ground line will cause the product to be damaged. The performance is reduced, so when designing the printed circuit board, attention should be paid to adopting 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

The input capacitor is charged by an approximate DC current. The filter capacitor mainly acts as a broadband energy storage; similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier and eliminate 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 circuits should only be connected to the power supply from the terminals of the filter capacitor respectively; if the connection between the input/output circuit and the power switch/rectifier circuit 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 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. The frequency is much greater than the fundamental frequency of the switch. The peak amplitude can be as high as 5 times the amplitude of the continuous input/output DC current. The transition time is usually Approximately 50 ns.

These two loops are the 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, inductors or transformers. Place them next to each other and adjust the position of the components 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 the input current source loop and input filter. Design the 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 must be met:

(1) First, consider the size of PC B. When the PC B 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 PC B 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, the aspect ratio is 3: 2 or 4: 3, and the components located on the edge of the circuit board are generally not less than 2mm from the edge of the circuit board.

(2) When placing the device, consider the subsequent 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 PC B, minimize and shorten the leads and connections between the components, and the decoupling capacitor should be as close as possible to the VCC of 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 layout is to ensure the wiring rate, pay attention to the connection of flying leads when moving the device, and put the connected devices together.

(7) Reduce the loop area as much as possible to suppress the radiation interference of the switching power supply.

parameter settings

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 tolerable. 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. Set the trace spacing to 8mil.

The distance from the edge of the inner hole of the pad to the edge of the printed board should be greater than 1mm, 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.

Wiring

The switching power supply contains high-frequency signals. Any printed line on PC B can function 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 interfering 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 the inductance and impedance it exhibits, while the width is inversely proportional to the inductance and impedance of the printed line. The length reflects the wavelength of the printed line’s 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 current of the printed circuit board, 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 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. Mixing various groundings will cause unstable power supply.

The following points should be paid attention to in the ground wire design:

1. Correctly choose single-point grounding. Generally, the common terminal of the filter capacitor should be the only connection point for coupling other grounding points to the AC ground of high current. It should be connected to the grounding point of this level, mainly considering that the current flowing back to the ground in each part of the circuit is changed. 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. Connected to the ground pin, the ground wires of several components of the output rectifier current loop are also connected to the ground pins of the corresponding filter capacitors, so that the power supply works more stably and is not easy to self-excite. Connect two diodes or a small resistor, in fact, it can be connected to a relatively concentrated piece of copper foil.

2. Thicken the grounding wire as much as possible. If the grounding wire is very thin, the ground potential will change with the change of the current, which will cause the timing signal level of the electronic equipment to be unstable, and the anti-noise performance will deteriorate. Therefore, it is necessary to ensure that each large current grounding terminal Use printed wires as short and as wide as possible, and widen the width of power and ground wires as much as possible. It is best to make the ground wires wider than the power wires. Their relationship is: ground wire “power wire” signal wire. The width should be greater than 3mm, and a large area of ​​copper layer can also be used as a ground wire, and the unused places on the printed circuit board are connected to the ground as a ground wire. When performing global wiring, the following principles must also be followed:

(1) Wiring direction: From the perspective of the soldering surface, the arrangement of the components should be as consistent as possible with the schematic diagram. The wiring direction is best to be consistent with the wiring direction of the circuit diagram, because various parameters are usually required on the soldering surface during the production process. Inspection, so this is convenient for inspection, debugging and overhaul in production (Note: 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, and the line width on the printed arc should not change 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 the problem. That is, let a certain lead “drill” through the gap under other resistors, capacitors, and triode pins, or “wind” through the end of a certain 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. Due to the single-sided board, the in-line components are located on the to p 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 overlapping of the pads should be avoided.

3. Input ground and output ground This switching power supply is a low-voltage DC-DC. To feed 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.

an examination

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 necessary to confirm whether the established rules meet the requirements of the printed board production process. Generally, check the lines and lines, the lines and the component pads, and the lines. 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, when a part of the outline of some connectors is placed outside the board frame, errors will occur when checking the spacing; in addition, each time the wiring and vias are modified, the copper must be re-coated.