PCB Blog - PCB board design specification in switching regulator production

In the design and production of power supply, the design and production of PCB board is very important. In any switching power supply design, the physical design of the PCB board is a link. If the design method is not appropriate, the PCB board may cause many problems. Based on years of experience in PCB board design, especially the experience in power supply design and production, the author analyzes the matters needing attention in each step.

Design Flow

The design process from the drawing to the PCB board is: establishment of component parameters → input principle netlist → design parameter setting → manual layout → manual routing → verification design → review → CAM output.

Electrical Safety Requirements

The spacing of the wires must meet the electrical safety requirements, the spacing must be at least suitable the withstand voltage, and for ease of operation and production, the spacing should be as wide as possible. When the wiring density is low, the pitch of the signal lines can be appropriately increased. For the signal lines with high and low-level differences, the spacing should be increased as much as possible, generally 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 pad defect during processing. When the traces connected to the pads are thin, the connection between the pads and the traces should be designed into a water drop shape, which has the advantage that the pads are not easy to peel, and the traces and the pads are not easily disconnected.

Component layout

Component layout practice has proved that even if the circuit schematic design is correct, the printed circuit board is not properly designed, which will adversely affect the reliability of electronic equipment. For example, if two thin parallel lines on the printed board are close together, it will cause a delay in the signal waveform, resulting in reflection noise at the end of the transmission line. Therefore, when designing the printed circuit board, attention should be paid to the correct method. The switching power supply has four current loops: the power switch AC loop, the output rectifier AC loop, the input signal source current loop, and the output load current loop. The input loop charges the input capacitor with an approximate DC current, and the filter capacitor mainly acts as a broadband energy 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 input and output current circuits should only be connected to the power supply from the filter capacitor terminals; if the connection between the input/output circuit and the power switch/rectifier circuit cannot be directly connected to the capacitor terminals, the AC energy will pass through the input or output. filter capacitor and radiate to the environment. The AC loop of the power switch and the AC loop of the rectifier contains high-amplitude trapezoidal currents. These currents have high harmonic content, and their frequencies are much greater than the switching fundamental frequency. The peak amplitude can be as high as 5 times the amplitude of the continuous input/output DC current. The transition time is usually about 50ns. These two loops are prone to electromagnetic interference, so these AC loops must be routed before other traces in the power supply. Filter capacitors, power switches or rectifiers, inductors, or transformers for each loop should be placed next to each other so that the current path between them is as short as possible.

When laying out all the components of the circuit, the following principles should be followed:

1) When the size of the PCB board is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if it is too small, the heat dissipation will be poor, and the adjacent lines will have easily interfered. The 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) Layout with the components of each functional circuit as the center. Components should be arranged evenly, neatly, and compactly on the PCB, minimize and shorten the leads and connections between components, and the decoupling capacitors 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. In general circuits, the components 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 positions of each functional circuit unit according to the circuit process, so that the layout is convenient for signal circulation, and the signal transmission direction is kept as consistent as possible.

6) The first principle of layout is to ensure the routing rate of wiring, pay attention to the connection of flying wires when moving devices, and put devices with a connection relationship together.

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

High-frequency processing

The length and width of the wire affect its impedance and inductance, which in turn affects the frequency response. Even traces passing through DC signals can couple to RF signals from adjacent traces and cause circuit problems (or even radiate interfering signals again). All traces that carry AC current should therefore be designed to be as short and wide as possible, which means that all components connected to traces and other power lines must be placed close together. According to the size of the printed circuit board current, 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 will help 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 the common reference point of the circuit and is an important factor in controlling interference. Therefore, the placement of the ground wire should be carefully considered in the layout. Mixing various grounds will cause unstable power supply operations. The following points should be paid attention to in the design of the ground wire.

1. Correctly choose single-point grounding

Usually, the common terminal of the capacitor should be the connection point where other ground points are coupled to the high-current AC ground, the ground point of the same circuit should be as close as possible, and the power filter capacitor of the circuit of this stage should also be connected to the ground point of this stage. One-point grounding can be used, that is, the ground wires of several devices in the current loop of the power switch are connected to the ground pins, and the ground wires of several devices in the output rectifier current loop are connected to the ground pins of the corresponding filter capacitors, so that the power supply works More stable, not easy to self-excite. When a single point cannot be achieved, connect two diodes or a small resistor at the common ground, or connect it to a relatively concentrated piece of copper foil.

2. Try to thicken the ground wire

The ground wire is very thin, and the ground potential changes with the current, which makes the timing signal level of the electronic equipment unstable and the anti-noise performance deteriorates. Therefore, it is necessary to ensure that each high-current ground terminal is printed as short and wide as possible Wire, try to widen the width of the power and ground wires. The ground wire is wider than the power wire. The relationship between them is ground wire > power wire > signal wire. If possible, the width of the ground wire should be greater than 3mm. The layer is used as a ground wire, and the unused places on the printed board are connected to the ground as a ground wire.

Global Routing Considerations

From the interface, the arrangement of components should be as consistent as possible with the schematic diagram, and the wiring direction should be consistent with the wiring direction of the circuit diagram. In the wiring diagram, the traces should be turned as little as possible, the line width on the printing arc should not be abruptly changed, the corners of the wires should be ≥90°, and the lines should be simple and clear. Cross circuits are not allowed in the circuit. For lines that may cross, you can use two methods of "drilling" and "winding". That is, let a lead "drill" from the gap under other resistors, capacitors, and triode pins, or "wrap" from one end of a lead that may cross. If the circuit is very complex, it is also allowed to use a wire jumper to simplify the design to solve the problem of cross-circuit.

Inspection and Review

After the design is completed, it is necessary to carefully check whether the wiring design conforms to the rules made by the designer, and at the same time, it is necessary to confirm whether the rules made meet the requirements of the printed board production process. Generally check whether the distance between wire and wire, wire and component pad, wire and through the hole, component pad and through the hole, through the 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 place in the PCB that can widen the ground line. According to the "PCB Board Checklist", the contents include design rules, layer definition, line width, spacing, pads, and via settings. It is also necessary to review the rationality of device layout, power supply and ground network routing, and high-speed clock network. The wiring and shielding, placement, and connection of decoupling capacitors on PCB board.