PCB News - PCB layout specification for switching power supply

In any 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, which will cause the power supply to work unstable. The following are the matters needing attention in each step. analyze:

1. The design process from schematic to PCB. Establish component parameters -> input principle netlist -> design parameter settings -> manual layout -> manual wiring -> verify design -> review -> CAM output.

2. Parameter setting The distance between adjacent wires must be able to meet the electrical safety requirements, and in order to facilitate operation and production, the distance should also be as wide as possible. The minimum distance must be at least suitable for the withstand voltage. When the wiring density is low, the distance between the signal lines It can be increased appropriately.The signal lines with high and low levels should be as short as possible and the spacing should be increased.Under normal circumstances, the trace spacing is set to 8mil.

The distance between the edge of the inner hole of the pad and the edge of the printed board should be greater than 1mm, which can avoid the defects of the pad during processing. When the trace connected to the pad is thin, the connection between the pad and the trace should be designed In the shape of a drop, the advantage of this is that the pad is not easy to peel, but the trace and the pad are not easily disconnected.

Third, the component layout practice has proved that even if the circuit schematic is designed correctly and the printed circuit board is designed incorrectly, it will have an adverse effect on the reliability of electronic equipment. For example, if the two thin parallel lines of the printed board are very close, It will cause the delay of the signal waveform and the formation of reflected noise at the terminal of the transmission line; the interference caused by the improper consideration of the power supply and the ground line will reduce the performance of the product. Therefore, when designing the printed circuit board, you should pay attention to it. Use the correct method. Every switching power supply has four current loops:

(1). Power switch AC circuit

(2). Output rectifier AC circuit

(3). Input signal source current loop

(4). The output load current loop The input loop charges the input capacitor through an approximate DC current, and 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 loop is eliminated. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current loops should only be connected to the power supply from the terminals of the filter capacitor. The connection between the rectifier circuit cannot be directly connected to the terminal of the capacitor, 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, and these currents contain harmonics. The component is very high, its 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, and the transition time is usually about 50ns. These two loops are the most prone to electromagnetic interference, so they must be other in the power supply Lay out these AC loops before wiring the printed lines. The three main components of each loop: filter capacitors, power switches or rectifiers, inductors or transformers should be placed next to each other, 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 the 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 When designing 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 the PCB. When the PCB size is too large, the printed lines will be longer, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if the PCB size is too small, the heat dissipation will not be good, and the adjacent lines are susceptible to interference. The best shape 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 future soldering, not too dense.

(3) Take the core components of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB to minimize and shorten the leads and connections between the components, decoupling The capacitor should be as close as possible to the VCC of the device.

(4) For circuits that work at high frequencies, the distribution parameters between components should 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 solder, 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 the layout is to ensure the wiring rate, pay attention to the connection of the flying wires 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 switching power supply contains high-frequency signals, and any printed line on the PCB 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 through DC Signal printed lines will also couple from adjacent printed lines to radio frequency signals and cause circuit problems (even radiating interference signals again). Therefore, all printed lines that pass AC current should be designed as short and wide as possible. This means that all components connected to the printed line and other power lines must be placed very close. The length of the printed line is proportional to its inductance and impedance, and the width is proportional to the inductance of the printed line It is inversely proportional to impedance. The length reflects the wavelength of the printed line response. The longer the length, the lower the frequency at which the printed line can send and receive electromagnetic waves, and the more radio frequency energy it can radiate. 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 support of the four current loops of the switching power supply. As the common reference point of the circuit, the circuit plays a very important role. It is an important method to control interference. Therefore, the placement of the ground wire should be carefully considered in the layout. Mixing various grounds will cause the power supply to work unstable. The following points should be paid attention to in line design:

1. Correctly choose single-point grounding. Generally, the common end of the filter capacitor should be the only connection point for other grounding points to couple to the high-current AC ground. 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, and 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 a small effect, and the circulating current formed by the grounding circuit has a greater impact on the interference, so one point grounding is used, that is, the power switch current loop (the ground wires of several devices are 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 stable and is not easy to self-excite. When it is not possible to achieve a single point, connect two diodes or A small resistance can actually be connected to a relatively concentrated piece of copper foil.

2. Make the ground wire as thick as possible. If the ground 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 to deteriorate, so ensure that each large current ground terminal Use printed lines that are as short and wide as possible, and widen the width of the power and ground lines as much as possible. It is best that the ground line is wider than the power line. Their relationship is: ground line> power line> signal line. The width should be greater than 3mm, and a large area of copper layer can also be used as a ground wire. Connect the unused parts on the printed board 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 welding 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 welding surface during the production process. Therefore, this is convenient for inspection, debugging and maintenance in production (Note: It 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, the line width on the printed arc should not be changed suddenly, the corner of the wire should be ≥90 degrees, and the line 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 from the gap under other resistors, capacitors, and triode pins. "Drilling" in the past, or "winding" from one end of a lead that may cross, in special circumstances, how the circuit is very complicated, in order to simplify the design, it is also allowed to use the wire jumper to solve the problem of the cross circuit. Because of the single-sided board, straight plug The component is located on the top surface, and the surface mount device is located on the bottom surface, so the in-line device can overlap with the surface mount device during layout, but the overlap of the pads should be avoided.

3. Input ground and output ground This switching power supply is a low-voltage DC-DC. If you want 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 laying copper on the ground wires on both sides, you must Connected together to form a common ground.

5. After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. At the same time, it is also necessary to confirm whether the rules set meet the requirements of the printed board production process. Generally check the wire and wire, wire and component welding Whether the distance between the disk, line and through-hole, component pad and through-hole, through-hole and through-hole is reasonable, and whether it meets the production requirements. Whether the width of the power line and the ground line is appropriate, and whether there is any ground in the PCB board The place where the line is widened. Note: Some errors can be ignored, for example, a part of the outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing; in addition, every time the wiring and vias are modified, they must be rewritten. Copper once.

6. Review According to the "PCB checklist", the content includes design rules, layer definitions, line widths, spacing, pads, and via settings, and also focus on reviewing the rationality of the device layout, the wiring of the power supply and the ground network, and high-speed The routing and shielding of the clock network, the placement and connection of decoupling capacitors, etc.

Seven, design output matters needing attention when outputting Gerber files:

a. The layers that need to be output are wiring layer (bottom layer), silk screen layer (including top screen printing, bottom screen printing), solder mask (bottom solder mask), drilling layer (bottom layer), and a drilling file (NC Drill) )

b. When setting the Layer of the silk screen layer, do not select Part Type, select the top layer (bottom layer) and the Outline, Text, and Line of the silk screen layer. When setting the Layer of each layer, select the Board Outline. When setting the Layer of the silk screen layer, Don't choose Part Type, choose Outline, Text, Line of the top layer (bottom layer) and silk screen layer.d.When generating the drilling file, use the default settings of PowerPCB and do not make any changes.

The above is the introduction of switching power supply PCB Layout specification, Ipcb also provides PCB manufacturers and PCB manufacturing technology