PCB Design Specifications for LED Switching Power Supplies

In any switch power supply design, the physical design of the PCB board is the last link. If the design method is not appropriate, the PCB may radiate too much electromagnetic interference, causing the power supply to work unstably. The following is an analysis of the matters that need to be paid attention to in each step:

1. The design process from schematic to PCB is to establish component parameters -> input schematic netlist -> set design parameters -> manual layout -> manual wiring -> verify design -> review -> CAM output.

2. Parameter setting The spacing between adjacent wires must meet the electrical safety requirements, and for ease of operation and production, the spacing should be as wide as possible. The minimum spacing must at least be suitable for the voltage to be tolerated. When the wiring density is low, the spacing between signal lines can be appropriately increased. For signal lines with large differences in high and low levels, the spacing should be as short as possible and the spacing should be increased. In general, the wiring spacing is set to 8mil.

The distance from the edge of the inner hole of the pad to the edge of the printed circuit board should be greater than 1mm to avoid pad defects during processing. When the trace connected to the pad is thin, the connection between the pad and the trace should be designed into a water drop shape. The advantage of this is that the pad is not easy to peel, but the trace and the pad are not easy to disconnect.

3. Component Layout Practice has proved that even if the circuit schematic is designed correctly, improper design of the printed circuit board will have an adverse effect on the reliability of the electronic equipment. For example, if two thin parallel lines on the printed circuit board are very close, it will cause a delay in the signal waveform and form reflected noise at the terminal of the transmission line; interference caused by inconsiderate consideration of the power supply and ground line will reduce the performance of the product. Therefore, when designing the printed circuit board, you should pay attention to using the correct method. Each switching power supply has four current loops:

(1) Power switch AC circuit

(2) Output rectifier AC circuit

(3) Input signal source current loop

(4) Output load current loop The input loop charges the input capacitor through a current close to DC , and the filter capacitor mainly plays a role of broadband energy storage; similarly, the output filter capacitor is also used to store 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. The input and output current loops should be connected to the power supply only from the terminals of the filter capacitors; if the connection between the input/output loop and the power switch/rectifier loop cannot be directly connected to the capacitor terminals, 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 currents. The harmonic components in these currents are very high, and their frequencies are much greater than the switching base frequency. The peak amplitude can be as high as 5 times the continuous input/output DC current amplitude, and the transition time is usually about 50ns. These two circuits are most likely to generate electromagnetic interference, so these AC circuits must be laid out before other printed wiring in the power supply. The three main components of each circuit, filter capacitors, power switches or rectifiers, inductors or transformers, should be placed adjacent to each other, and the components should be adjusted to make the current path between them as short as possible. The best way to establish a switching power supply layout is similar to its electrical design. The best design process is as follows:

Placing the Transformer

Designing the power switch current loop

Design the output rectifier current loop

Control circuit connected to AC power circuit

Design the input current source loop and input filter. Design the output load loop and output filter. When laying out all the components of the circuit according to the functional units of the circuit, the following principles must be followed:

(1) First, consider the size of the PCB.

When the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; if it is too small, the heat dissipation is poor, and the adjacent lines are easily interfered. The best shape of the circuit board is a rectangle with an aspect ratio of 3:2 or 4:3. The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.

(2) When placing components, consider future soldering and avoid placing them too densely.

(3) The layout should be centered around the core components of each functional circuit. The components should be arranged evenly, neatly and compactly on the PCB, and the leads and connections between the components should be minimized and shortened as much as possible. The decoupling capacitor should be as close to the VCC of the device as possible.

(4) For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, components should be arranged in parallel as much as possible. This not only looks good, but is also easy to assemble and solder, and is easy to mass produce.

(5) Arrange the positions of various functional circuit units according to the circuit flow so that the layout facilitates signal flow and keeps the signal in the same direction as much as possible.

(6) The primary principle of layout is to ensure the routing rate of the wiring. When moving components, pay attention to the connection of flying wires and place components with connection relationships together.

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

4. Wiring Switching power supplies contain high-frequency signals. Any printed line on the PCB can act as an antenna. The length and width of the printed line will affect its impedance and inductance, thereby affecting the frequency response. Even the printed lines that pass DC signals will couple to the RF signals from the adjacent printed lines and cause circuit problems (even radiate interference signals again). Therefore, all printed lines that pass AC current should be designed to be as short and wide as possible, which means that all components connected to the printed lines and to other power lines must be placed very close. The length of the printed line is proportional to the inductance and impedance it exhibits, while the width is inversely proportional to the inductance and impedance of the printed line. The length reflects the wavelength of the printed line response. The longer the length, the lower the frequency of the electromagnetic wave that the printed line can send and receive, and it can radiate more RF energy. According to the size of the current on the printed circuit board, try to increase the width of the power line and reduce the loop resistance . At the same time, make the direction of the power line and ground line consistent with the direction of the current, which helps to enhance the anti-noise ability. 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. It is an important method to control interference. Therefore, the placement of the grounding wire should be carefully considered in the layout. Mixing various groundings will cause unstable power supply operation. The following points should be noted in the grounding design:

1. Correctly select single-point grounding. Usually, the common end of the filter capacitor should be the only connection point for other grounding points to couple to the AC ground of large current. The grounding points of the same level circuit should be as close as possible, and the power filter capacitor of the circuit of this level should also be connected to the grounding point of this level. The main consideration is that the current flowing back to the ground from each part of the circuit is changing. The impedance of the line actually flowing through will cause the change of the ground potential of each part of the circuit and introduce interference. In this switching power supply, the inductance between its wiring and devices has little effect, while the loop current formed by the grounding circuit has a greater impact on interference. Therefore, a single-point grounding is adopted, that is, the ground wires of several devices in the power switch current loop are connected to the ground pin, and the ground wires of several devices in the output rectifier current loop are also connected to the ground pin of the corresponding filter capacitor. In this way, the power supply works more stably and is not easy to self-excite. When a single point cannot be achieved, two diodes or a small resistor are connected to the common ground. In fact, it can be connected to a relatively concentrated piece of copper foil.

2. Try to thicken the ground wire. If the ground wire is very thin, the ground potential will change with the change of current, causing the timing signal level of the electronic equipment to be unstable and the anti-noise performance to deteriorate. Therefore, ensure that each large current ground terminal uses a printed line that is as short and wide as possible, and try to widen the power and ground wire widths. It is best that the ground wire is wider than the power line. Their relationship is: ground wire>power line>signal line. If possible, the width of the ground wire should be greater than 3mm. A large area of ​​copper layer can also be used as a ground wire. Connect unused areas on the printed circuit board to the ground as ground wires.

When performing global routing, the following principles must also be followed:

(1) Wiring direction: From the welding surface, the arrangement of components should be kept consistent with the schematic diagram as much as possible, and the wiring direction should be consistent with the wiring direction of the circuit diagram . Since various parameters usually need to be tested on the welding surface during the production process, this is convenient for inspection, debugging and maintenance during production (Note: This refers to the premise of meeting the circuit performance and the requirements of the whole machine installation and panel layout).

(2) When designing a wiring diagram, try to avoid making turns in the wiring as much as possible. The line width on the printed arc should not change suddenly. The wire corner should be ≥ 90 degrees. Strive to make the lines simple and clear.

(3) Crossing circuits are not allowed in printed circuits. For lines that may cross, "drilling" and "winding" can be used to solve the problem. That is, let a lead "drill" through the gap under other resistors, capacitors, and transistors, or "wind" through one end of a lead that may cross. In special cases, if the circuit is very complicated, wire jumper is allowed to simplify the design to solve the problem of crossing circuits. Because a single-sided board is used, the direct-insert components are located on the top surface and the surface-mount components are located on the bottom surface. Therefore, the direct-insert components can overlap with the surface-mount components during layout, but the pads must be avoided from overlapping.

3. Input ground and output ground: This switching power supply is a low-voltage DC-DC. In order to feed the output voltage back to the primary of the transformer, the circuits on both sides should have a common reference ground. Therefore, after laying copper on the ground wires on both sides, they must be connected together to form a common ground.

5. After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules set by the designer. At the same time, it is also necessary to confirm whether the rules set meet the requirements of the printed circuit board production process. Generally, check whether the distance between lines, lines and component pads, lines and through holes, component pads and through holes, and through holes are reasonable and meet production requirements. Check 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. Note: Some errors can be ignored. For example, part of the Outline of some connectors is placed outside the board frame, which will cause errors when checking the spacing; in addition, after each modification of the routing and vias, the copper must be re-coated.

6. Review according to the "PCB Checklist", including design rules, layer definition, line width, spacing, pads, via settings, and also focus on reviewing the rationality of device layout, routing of power and ground networks, routing and shielding of high-speed clock networks, placement and connection of decoupling capacitors, etc.

7. Design output. Notes on outputting photo-painted files:

a. The layers that need to be output are wiring layer (bottom layer), silk screen layer (including top silk screen and bottom silk screen), solder mask layer (bottom solder mask), drilling layer (bottom layer), and drilling file (NCDrill) must also be generated.

b. When setting the Layer of the silkscreen layer, do not select PartType, select the Outline, Text, and Line of the top (bottom) layer and the silkscreen layer. c. When setting the Layer of each layer, select Board Outline. When setting the Layer of the silkscreen layer, do not select PartType, select the Outline, Text, and Line of the top (bottom) layer and the silkscreen layer. d. When generating the drilling file, use the default settings of PowerPCB and do not make any changes.