PCB Tech - Design process and skills of Power PCB

In any power supply design, the physical design of PCB board is a link. If the design method is improper, PCB may radiate too much electromagnetic interference, resulting in unstable power supply. The following is an analysis of the matters needing attention in each step.

Power PCB

1 Design flow from schematic diagram to PCB

Establish component parameters > input principle netlist > Design Parameter Settings > manual layout > manual wiring > verify design > recheck > cam output.

2 parameter setting

The distance between adjacent conductors must meet the electrical safety requirements, and the spacing should be as wide as possible in order to facilitate operation and production. The small spacing should at least be suitable for the voltage to bear. When the wiring density is low, the spacing of signal lines can be appropriately increased. For the signal lines with great difference between high and low levels, the spacing should be as short as possible and the spacing should be increased. Generally, the spacing of the lines should be set as 8 mil.

The distance between the inner hole edge of pad and PCB edge should be greater than 1 mm, which can avoid pad defects during processing. When the wiring connected to the pad is thin, the connection between the pad and the wire should be designed as a water drop. The advantage of this is that the pad is not easy to peel, but the wiring and pad are not easy to be disconnected.

3. Component layout

Practice has proved that even if the circuit schematic design is correct and the printed circuit board is not designed properly, the reliability of electronic equipment will be adversely affected.

For example, if two thin parallel wires of printed circuit board are close to each other, the delay of signal waveform will be formed, and the reflected noise will be formed at the terminal of transmission line; the interference caused by the thoughtless consideration of power supply and ground wire will degrade the performance of the product. Therefore, in the design of printed circuit board, attention should be paid to the correct method.

The AC circuit of power switch and rectifier contains high amplitude trapezoidal current. The harmonic component of these currents is very high, and its frequency is far greater than the fundamental frequency of the switch. The peak amplitude can be as high as 5 times of the continuous input / output DC current amplitude, and the transition time is usually about 50 ns.

These two circuits are easy to produce electromagnetic interference, so these AC circuits must be laid before other printed wiring in the power supply. The filter capacitor, power switch or rectifier, inductance or transformer of each circuit should be placed adjacent to each other, and the position of the components should be adjusted to make the current path between them as short as possible.

4 wiring

Switching power supply contains high-frequency signal. Any printed wire on PCB can play the role of antenna. The length and width of printed wire will affect its impedance and inductive reactance, thus affecting the frequency response. Even printed wiring through DC signals can be coupled to RF signals from adjacent printed lines and cause circuit problems (or even radiate interference signals again).

Therefore, all printed wiring passing through AC current should be designed as short and wide as possible, which means that all components connected to the printed wiring and to other power lines must be placed close together.

The length of a printed wire is directly proportional to its inductance and impedance, while its width is inversely proportional to its inductance and impedance. The longer the length, the lower the frequency at which the printed circuit can transmit and receive electromagnetic waves, and the more RF energy it radiates.

According to the size of the printed circuit board current, try to rent the width of the power line to reduce the loop resistance. At the same time, make the direction of power line and ground wire consistent with the direction of current, which helps to enhance the anti noise ability.

Grounding is the bottom branch of the four current circuits of switching power supply. It plays an important role as the common reference point of the circuit. It is an important method to control the interference. Therefore, the placement of grounding wires should be carefully considered in the layout. Mixing various grounding wires will cause unstable operation of the power supply.

5 inspection

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer, and at the same time, it is also necessary to confirm whether the established rules meet the requirements of PCB production process. Generally, it is necessary to check whether the distances between line and line, line to component pad, line to through hole, component pad to through hole, and through hole to through hole are reasonable and meet the production requirements.

Whether the width of the power line and ground wire is appropriate, and whether there are places in the PCB that can widen the ground wire. Note: some errors can be ignored. For example, a part of the outline of some connectors is placed outside the board frame, and there will be an error when checking the spacing. In addition, after modifying the wiring and vias, the copper coating should be repeated.

According to the "PCB Checklist", the contents include the design rules, layer definition, line width, spacing, pad and via settings, and the rationality of device layout, the wiring of power supply and ground wire network, the wiring and shielding of high-speed clock network, the placement and connection of decoupling capacitors, etc.

6 design output

Notes for outputting photo files:

The output layers include wiring layer (bottom layer), screen printing layer (including top layer screen printing and bottom layer silk screen printing), solder mask layer (bottom layer welding resistance), drilling layer (bottom layer). In addition, drilling file (NC drill) is generated;

When setting the layer of screen printing layer, do not select part type, select top layer (bottom layer) and outline, text and line of silk screen layer;

When setting the layer of each layer, select board outline. When setting the layer of silk screen layer, do not select part type, select the top layer (bottom layer) and the outline and text of silk screen layer.