In any switching power supply design, the physical design of PCB board is the last link. If the design method is improper, PCB may radiate too much electromagnetic interference, resulting in the instability of the power supply 1、 From schematic diagram to PCB design process, establish component parameters, input schematic netlist, design parameter setting, manual layout, manual wiring, design verification, review and cam output.

 

 

 

2、 The distance between adjacent wires must meet the requirements of electrical safety, and the distance should be as wide as possible in order to facilitate operation and production. The minimum spacing should at least be suitable for the withstanding voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased. For the signal lines with large difference between high and low levels, the spacing should be as short as possible and increased. Generally, the wiring spacing should be set as 8 mil. The distance between the edge of the inner hole of the pad and the edge of the printed circuit board should be greater than 1 mm, which can avoid the defect of the pad during processing. When the wire connecting with the pad is thin, the connection between the pad and the wire should be designed as water drop. The advantage is that the pad is not easy to peel, but the wire and the pad are not easy to be disconnected.

 

 

 

3、 The practice of component layout has proved that the reliability of electronic equipment will be adversely affected even if the circuit schematic design is correct and the printed circuit board is not designed properly. For example, if the two thin parallel lines of the printed circuit board are close to each other, the delay of the signal waveform will be formed, and the reflected noise will be formed at the end of the transmission line; the interference caused by the thoughtless consideration of the power supply and ground wire will degrade the performance of the product. Therefore, the correct method should be adopted when designing the printed circuit board.

 

 

 

Each switching power supply has four current circuits

 

 

 

(1) Power switch AC circuit

 

 

 

(2) Output rectifier AC circuit

 

 

 

(3) Input current source

 

 

 

(4) Output load current loop the input loop charges the input capacitor with a current similar to DC, and the filter capacitor mainly plays a role of broadband energy storage. Similarly, the output filter capacitor is also used to store the high frequency energy from the output rectifier and eliminate the DC energy of the output load loop. Therefore, the terminals of the input and output filter capacitors are very important, and the input and output current circuits should only be connected to the power supply from the 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 terminals of the capacitor, the AC energy will be radiated from the input or output filter capacitor to the environment. The AC circuit of the power switch and the AC circuit of the rectifier contain high amplitude trapezoidal current, in which the harmonic component 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 50ns. These two circuits are most likely to produce electromagnetic interference, so these AC circuits must be laid before the wiring of other printed wires in the power supply. The three main components of each circuit, filter capacitor, power switch or rectifier, inductor or transformer, 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. The best way to establish the layout of switching power supply is similar to its electrical design

 

 

 

1 place the transformer

 

 

 

2. Design switching current circuit of power supply

 

 

 

3. Design the current circuit of output rectifier

 

 

 

4 control circuit connected to AC power supply circuit

 

 

 

5 design the input current source circuit and input filter, design the output load circuit and output filter. According to the functional unit of the circuit, the layout of all components of the circuit should comply with the following principles:

 

 

 

(1) The first thing to consider is PCB size. If the size of PCB 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 the size of PCB is too small, the heat dissipation will not be good, and the adjacent lines will be easily disturbed. The best shape of the circuit board is rectangle, the ratio of length to width is 3:2 or 4:3, and the distance between the components at the edge of the circuit board and the edge of the circuit board is generally not less than 2mm.

 

 

 

(2) When placing the device, the welding in the future should be considered, not too dense.

 

 

 

(3) The core component of each functional circuit is taken as the center, and the layout is carried out around it. The components should be evenly, neatly and compactly arranged on the PCB, the leads and connections between the components should be reduced and shortened as far as possible, and the decoupling capacitance should be close to the VCC of the components as far as possible.

 

 

 

(4) The distribution parameters between components should be considered when the circuit works at high frequency. In general, components should be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass produce.

 

 

 

(5) The position of each functional circuit unit is arranged according to the circuit flow, so that the layout is convenient for signal flow, and the signal keeps the same direction as far as possible.

 

 

 

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

 

 

 

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

 

 

 

4、 There are high frequency signals in the wiring switching power supply. Any printed wire on PCB can act as an antenna. The length and width of printed wire will affect its impedance and inductive reactance, thus affecting the frequency response. Even the printed line through DC signal will couple to RF signal from adjacent printed line and cause circuit problems (even radiate interference signal again). Therefore, all printed wires passing AC current should be designed as short and wide as possible, which means that all components connected to printed wires and other power lines must be placed close to each other. The length of the printed wire is directly proportional to its inductance and impedance, while the 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 can radiate. According to the current of PCB, the width of power line should be increased to reduce the loop resistance. At the same time, make the direction of power line and ground line consistent with the direction of current, which helps to enhance the ability of anti noise. Grounding is the bottom branch of the four current circuits of switching power supply. As the common reference point of the circuit, it plays a very important role. It is an important method to control interference. Therefore, the placement of grounding wires should be carefully considered in the layout, and mixing all kinds of grounding wires will cause instability of power supply. The following points should be noted in the design of ground wires:

 

 

 

1. In general, the common terminal of filter capacitor should be the only connection point of other grounding points coupled to high current AC ground. The grounding point of the same level circuit should be as close as possible, and the power filter capacitor of the current level circuit should also be connected to this level grounding point. The main consideration is that the current of each part of the circuit returning to the ground is variable, because of the impedance of the actual line It will lead to the change of ground potential of each part of the circuit and introduce interference. In this switching power supply, its wiring and inductance between devices have little influence, while the circulating current formed by the grounding circuit has great influence on the interference, so one point grounding is adopted, that is, the switching current circuit of the power supply In this way, the output current of several self-excited devices can not be connected to the ground wire of the rectifier. If a single point is not possible, connect two diodes or a small resistor to the common ground. In fact, connect them to a concentrated 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 current, resulting in the instability of timing signal level of electronic equipment and the deterioration of anti noise performance. Therefore, it is necessary to ensure that the short and wide printed wire is used for each high current ground terminal, and the width of power supply and ground wire is widened as far as possible. It is better that the line width of ground wire is wider than that of power supply. The relationship between them is: ground wire > power line > signal line, if possible The width of the grounding wire should be greater than 3mm. Large area copper layer can also be used as the grounding wire. On the printed circuit board, the unused parts are connected with the ground as the grounding wire. When carrying out global wiring, the following principles should also be followed:

 

 

 

(1) Wiring direction: from the perspective of welding surface, the arrangement orientation of components should be consistent with the schematic diagram as far as possible, and the wiring direction should be consistent with the wiring direction of circuit diagram. Because it is usually necessary to detect various parameters on the welding surface in the production process, it is convenient for inspection, debugging and maintenance in production (Note: before meeting the requirements of circuit performance, whole machine installation and panel layout) Put it down).

 

 

 

(2) When designing the wiring diagram, the routing should try to avoid turning, the line width on the printing arc should not change suddenly, and the wire corner should be more than 90 degrees, so as to make the lines simple and clear.

 

 

 

(3) Cross circuit is not allowed in printed circuit. For lines that may cross, we can solve them by "drilling" and "winding". That is to say, let a lead "drill" through the gap at the foot of other resistors, capacitors and triodes, or "wind" through one end of a lead that may cross. Under special circumstances, the circuit is very complex. In order to simplify the design, it is also allowed to use the lead to cross to solve the problem of cross circuit. Because of the single panel, the in-line device is located on the top side and the surface mount device is located on the bottom side, so in the layout, the in-line device can overlap the surface mount device, but the pad overlap should be avoided.

 

 

 

3. The input ground and output ground are low-voltage DC-DC in the switching power supply. To feed back the output voltage 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, they should be connected together to form a common ground.

 

 

 

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 also confirm whether the rules formulated conform to the requirements of PCB production process. Generally, it is necessary to check whether the distances between lines, line and component pad, line and through hole, component pad and through hole, and through hole and through hole are reasonable and meet the production requirements. Whether the width of power line and ground wire is appropriate, and whether there is any place in PCB that can widen the ground wire. Note: some errors can be ignored. For example, some connector outline parts are placed outside the board frame, so there will be errors when checking the spacing. In addition, after modifying the wiring and vias, the copper should be coated again.

 

 

 

6、 According to the "PCB Checklist", the review includes the design rules, layer definition, line width, spacing, pad and via settings, as well as the rationality of device layout, the routing of power supply and ground network, the routing and shielding of high-speed clock network, and the placement and connection of decoupling capacitors.

 

 

 

7、 Notes for designing output photo file:

 

 

 

a. The output layers include wiring layer (bottom layer), silk screen layer (including top layer silk screen and bottom layer silk screen), solder mask layer (bottom layer solder mask), drilling layer (bottom layer), and drilling file (NC drill).

 

 

 

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

 

 

 

c. When setting the layer of each layer, select board outline. When setting the layer of silk screen layer, do not select part type, but select the outline, text and line of 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.