PCB Blog - Requirements for PCB design and switching power supply

In any switching power supply design, the physical design of the PCB board is a link. If the design method is improper, the PCB may emit too much electromagnetic interference, resulting in unstable power supply operation. The following is an analysis of the points that need to be paid attention to in each step:

1. Design process from schematic diagram to PCB

Set up component parameters -> Input principle netlist -> Design parameter Settings -> Manual layout -> Manual wiring -> Verify design -> Review ->CAM output.

2. Parameter setting

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 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.

3. 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 loop

3) Input signal source current loop

4) 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, 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 loop susceptible to 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 method of establishing the switching power supply layout is similar to its electrical design, and the design process is as follows:

1) Place the transformer

2) Design the power switch current loop

3) Design the output rectifier current loop

4) 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 units of the circuit, the layout of all the components of the circuit should comply with the following principles:

1) The PCB size should be considered first. When the PCB size is too large, the printed line is long, the impedance increases, the anti-noise ability decreases, and the cost increases. Too small heat dissipation is not good, and adjacent lines are susceptible to interference. The circuit board is rectangular in shape with a length ratio of 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) place the device to consider the future welding, not too dense.

3) To the components of each functional circuit as the center, around it to carry out the layout. Components should be evenly, neatly and compactly arranged on the PCB, minimizing and shortening the leads and connections between components, and decoupling capacitance should be as close to the VCC of the device as possible.

4) For circuits working at high frequencies, the distributed parameters between components should be considered. In general circuits, components should be arranged in parallel as much as possible. In this way, not only beautiful, and easy to install welding, easy to mass production.

5) Arrange the position of each functional circuit unit according to the circuit process, so that the layout is convenient for signal flow, and keep the signal in the same direction as possible.

6) The first principle of layout is to ensure the distribution rate of wiring, pay attention to the connection of flying lines when moving devices, and put the connected devices together.

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

4. 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.

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

1) Correct selection of single point grounding in general, filter capacitance public side should be the other ground coupled to the junction of large current of interact, the ground should be close to with circuit, and the corresponding circuit power filtering capacitance can also on the level ground, mainly considering the current parts back into the ground the circuit is changing, Interference is introduced because the actual impedance of the circuit will lead to the change of the ground potential of each part of the circuit. In this switch power supply, its inductance smaller effect between wiring and devices, and grounding circuit had a greater influence on the formation of the circulation of interference, thus using one-point earthing, the power switch current loop (in the ground of several devices are connected to the ground on his feet, ground of output rectifier current loop of several devices also received the corresponding filter capacitance grounded on his feet, In this way, the power supply works stably and is not easy to self-excitation. Can not do a single point, in a total of two diodes or a small resistance, in fact, connected in a more concentrated piece of copper foil can be.

2) The bold grounding line as far as possible If the ground wire is very thin, grounding potential changes over electric current, the timing signal level instability of the electronic equipment, anti-noise performance is bad, so be sure to every large current earthing end USES short and wide printing line as far as possible, try to widen the power supply, ground wire width, ground is wider than the power cord, their relationship is: Ground wire > power line > signal wire, if possible, the width of the ground wire should be greater than 3mm, also can be used as a large area of copper layer for ground wire, on the printed board, not used to connect with the ground as ground wire. For global wiring, the following principles should also be followed:

1) wiring direction: from the welding surface, the arrangement of components as far as possible to keep consistent with the schematic diagram, wiring direction and circuit diagram wiring direction consistent, because the production process usually need to be in the welding surface of various parameters detection, so it is easy to do in the production of inspection, debugging and maintenance (note: Refers to meet the circuit performance and machine installation and panel layout requirements under the premise.

2) When designing the wiring diagram, the line should turn as little as possible, the line width on the printing arc should not change, the wire corner should be more than 90 degrees, and strive to make the lines simple and clear.

3) The printed circuit is not allowed to have cross circuit, for the line may cross, can use "drill", "around" two ways to solve. That is, let a lead from other resistors, capacitors, transistors at the foot of the gap "drill" past, or from the intersection of a lead may be "wound" past, in special cases how the circuit is very complex, in order to simplify the design also allow the use of wire bridging, to solve the problem of cross circuit. Because a single panel is used, the in-line components are located on the top surface and the surface mount devices are located on the bottom surface, so the in-line components can overlap with the surface mount devices during layout, but the pads should be avoided. Input and output switching power supply for low-voltage DC-DC, to output voltage feedback back to the primary transformer, the circuit on both sides should have a common reference, so on both sides of the ground wire respectively after laying copper, but also connected together to form a common ground

5. Check the

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 must be re-coated with copper. 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.

Notes for output light drawing files:

a. 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)

b. When setting the Layer of the silk screen Layer, do not select Part Type. Select Outline, Text, and Linec for the top (bottom) and 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, and select Outline, Text and Line of the top (bottom) and silk screen Layer. D. Do not change the default Settings of Power PCB board when generating borehole files.