PCB design

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, resulting in the unstable work of the power supply. The following is an analysis of the matters needed to pay attention to in each step.

From schematic diagram to PCB design process

Set up component parameters – > Input principle netlist – > Design parameter setting – > Manual layout – > Manual cabling – > Validate design – > Review – & gt; CAM output.

Parameter Settings

The spacing between adjacent wires must meet the requirements of electrical safety, and for the convenience of operation and production, the spacing should be as wide as possible. The minimum spacing should be suitable for the voltage at least. When the wiring density is low, the spacing of signal lines can be appropriately increased. For the signal lines with high and low level disparity, the spacing should be as short as possible and the spacing should be increased.

The distance between the edge of the inner hole of the pad and the edge of the printed board should be greater than 1mm to avoid defects of the pad during machining. When the wire connected with the pad is relatively thin, the connection between the pad and the wire is designed into a droplet shape. The advantage is that the pad is not easy to peel, but the wire and the pad is not easy to disconnect.

Component layout

Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, the reliability of electronic equipment will be adversely affected.

For example, if two thin parallel lines of a printed board are close together, there will be a delay in the signal waveform, resulting in reflected noise at the end of the transmission line. The interference caused by the power supply and grounding wire will degrade the performance of the product. Therefore, when designing the printed circuit board, attention should be paid to the correct method.

Each switching power supply has four current loops:

① Ac circuit of power switch

② Output rectifier AC circuit

Input signal source current loop

④ Output load current loop Input loop

By charging the input capacitor with an approximate dc current, the filter capacitor mainly plays a role of broadband energy storage. Similarly, output filter capacitors are used to store high frequency energy from the output rectifier while eliminating dc energy from the output load loop.

Therefore, the wiring terminals of the input and output filter capacitors are very important. The input and output current loops should be connected to the power supply only from the wiring terminals of the filter capacitor respectively. If the connection between the input/output circuit and the power switch/rectifier circuit cannot be directly connected to the capacitor’s terminal, ac energy will pass through the input or output filter capacitor and radiate into the environment.

The ac circuits of the power supply switch and the rectifier contain high-amplitude trapezoidal currents, which have a high harmonic component and a frequency much higher than the fundamental frequency of the switch. The peak amplitude can be up to 5 times that of the continuous input/output dc current. The transition time is usually about 50ns.

The two circuits are most likely to produce electromagnetic interference, so must the other printed wiring in power source to cloth before these ac circuits, each loop three main components of the filter capacitor, the power switch or rectifier, inductor or transformer shall be placed adjacent to each other, adjust the current path between the element position make them as short as possible.

The best way to establish the switching power supply layout is similar to its electrical design, the best design process is as follows:

① Place transformer

② Design the power switch current loop

③ Design the output rectifier current loop

④ The control circuit connected to the AC power supply circuit

wiring

The switching power supply contains a high frequency signal, and any printed line on the PCB can act as an antenna. The length and width of printed line will affect its impedance and inductive reactance, thus affecting the frequency response. Even printed lines that pass through dc signals can be coupled to rf signals from adjacent printed lines and cause circuit problems (or even re-radiate interference signals).

All printed lines running through ac current should therefore be designed to be as short and wide as possible, which means that all components connected to printed lines and to other power lines must be placed close together.

The length of the printed line is directly proportional to its inductance and impedance, and 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 of the printed line can send and receive electromagnetic waves, and the more rf energy it can radiate.

According to the size of the printed circuit board current, as far as possible to increase the width of the power line, reduce the resistance of the loop. At the same time, make the power line, ground line and current direction consistent, which helps enhance the anti-noise ability.

Grounding is the bottom branch of four current circuits of switching power supply, which plays a very important role as the common reference point of the circuit, and it is an important method to control interference. Therefore, carefully consider the grounding cables in the layout. Mixing grounding cables may cause unstable power supply.

check

Wiring design is completed, it is necessary to carefully check the wiring design by the designers is in line with the rules, rules at the same time also need to confirm whether accord with the demand of the PCB production process, general inspection line to line, line and element bonding pad, the line and communicating pores, element bonding pad and communicating pores, through hole and the distance between the through hole is reasonable, whether to meet the production requirements.

Whether the width of the power cord and ground wire is appropriate, and whether there is room for the ground wire to be widened in the PCB. Note: Some errors can be ignored, for example, part of Outline of some connectors is placed outside the board frame, so it will be wrong to check the spacing; In addition, after each modification of the wiring and hole, it is necessary to re-coat copper once.

Review according to the “PCB checklist”, including design rules, layer definition, line width, spacing, pads, hole Settings, but also focus on the review of the rationality of device layout, power supply, grounding network wiring, high-speed clock network wiring and shielding, decoupling capacitor placement and connection.

Design output

Notes for output light drawing files:

(1) Need to output layer wiring layer (bottom), screen printing layer (including top screen printing, bottom screen printing), welding layer (bottom welding), drilling layer (bottom), in addition to generate drilling file (NC Drill)

② When setting the Layer of the screen printing Layer, do not select Part Type, select the Outline, Text and Line of the top (bottom) and screen printing Layer

③ When setting the Layer of each Layer, select Board Outline. When setting the Layer of screen printing Layer, do not select Part Type, and select the Outline and Text of the top (bottom) and screen printing Layer.