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Seven Steps of LEDpcb Board Design

Shenzhen Inno Circuit Co.,Ltd | Updated: Jun 08, 2018

Seven Steps of LEDpcb Board Design

 In the LED pcb board design, if the PCB board is improperly designed, it will radiate too much electromagnetic interference. The stable PCB design of the power supply is summarized in the following seven steps: Through the analysis of the items that need attention in each step, the board design can be easily completed in steps!


First, from the schematic to the PCB design process

Create component parameters -> input principle netlist -> design parameter settings -> manual layout -> manual routing -> verification design -> review -> CAM output.

Second, the parameter settings

The distance between adjacent PCBs must meet the electrical safety requirements, and for ease of operation and PCB production, the spacing should be as wide as possible. The minimum PCB pitch should be at least suitable for the withstand voltage. When the PCB density is low, the spacing of the signal lines can be appropriately increased. The signal lines with high and low levels should be as short as possible and increase the spacing. In the case of spacing

The distance between the edge of the inner hole of the pad and the edge of the printed board should be greater than 1mm, so as to avoid the defect of the pad during processing. When the traces connected to the pads are thin, the connection between the pads and the traces is designed to be drop-shaped. This has the advantage that the pads are not easily stripped, but the traces are not easily disconnected from the pads.

Third, the layout of components

Practice has proved that even if the circuit schematic design is correct, improper design of the printed circuit board will also have an adverse effect on the reliability of electronic equipment. For example, if the two fine parallel lines of the printed board are in close proximity, the delay of the signal waveform will be formed, and the reflection noise will be formed at the end of the transmission line; the interference caused by the inconsiderate consideration of the power source and the ground line will make the product The performance is degraded. Therefore, when designing a printed circuit board, care should be taken to use the correct method. Each switching power supply has four current loops:

(1) Power switch AC circuit

(2) Output rectifier circuit

(3) Input signal source current loop

(4) Output load current loop The input loop charges an input capacitor through an approximately DC current. The filter capacitor mainly functions as a broadband energy storage. Similarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier. At the same time, the DC energy of the output load circuit is eliminated. Therefore, the input and output filter capacitor terminals are very important, the input and output current loops should only be connected to the power supply from the filter capacitor terminals respectively; if the connection between the input/output circuit and the power switch/rectifier circuit cannot be connected to the capacitor. The terminals are directly connected and the AC energy will be radiated into the environment by the input or output filter capacitors. The AC circuit of the power switch and the AC circuit of the rectifier contains high-amplitude trapezoidal currents. The harmonic components of these currents are very high, their frequency is much greater than the switching fundamental frequency, and the peak amplitude can be up to 5 times the continuous input/output DC current amplitude. The transition time is usually About 50ns. These two loops are the most susceptible to electromagnetic interference. Therefore, these AC loops must be routed before the other traces in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, and inductors. Or the transformers should be placed next to each other, and the position of the components should be adjusted so that the current path between them is as short as possible.

The best way to establish a switching power supply layout is similar to its electrical design. The best LED PCB board design flow is as follows:

◆Place transformer

◆ Design power switch current loop

◆ Design output rectifier current loop

◆ Control circuit connected to AC power circuit

◆ Design input current source circuit and input filter design Output load circuit and output filter According to the functional unit of the circuit, when the layout of all components of the circuit, it is necessary to comply with the following principles:

(1) First consider the PCB size. When the LEDpcb board is too large, the printed lines are long, the impedance is increased, the anti-noise ability is decreased, and the cost is also increased; if the LEDpcb board is too large, the heat dissipation is not good, and the adjacent lines are susceptible to interference. The best shape of the circuit board is rectangular, and the aspect ratio is 3:2 or 4:3. The components on the edge of the circuit board are generally not less than the edge of the circuit board.

(2) Soldering should be considered when placing the device. Do not use too much density.

(3) Center around the core components of each functional circuit and lay out around it. The components should be arranged evenly, neatly and compactly on the PCB to minimize and shorten the leads and connections between the components. The decoupling capacitors should be as close as possible to the device.

(4) Circuits operating at high frequencies should consider the distribution parameters between the components. General circuits should arrange parallel components as much as possible. In this way, it is not only beautiful, but also easy to install and easy to mass produce.

(5) Arrange the positions of the functional circuit units according to the flow of the circuit so that the layout facilitates the signal flow and makes the signals as consistent as possible.

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

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

Fourth, PCB wiring switching power supply contains high-frequency signals

Any printed wire on the PCB can act as an antenna. The length and width of the printed wire will affect its impedance and inductive reactance, thus affecting the frequency response. Even a printed wire passing through a DC signal may couple from the adjacent printed wire to the RF signal and cause circuit problems (even re-radiate an interference signal). Therefore, all printed lines with alternating current should be designed to be as short and wide as possible, which means that all components connected to the printed lines and connected to other power lines must be placed in close proximity. The length of the trace is directly proportional to the amount of inductance and impedance it exhibits, and the width is inversely proportional to the inductance and impedance of the trace. The length reflects the wavelength of the response of the printed line. The longer the length, the lower the frequency at which the printed line can send and receive electromagnetic waves, it can radiate more RF energy. According to the size of the printed circuit board current, try to increase the width of the power cord to reduce loop resistance. At the same time, the direction of the power line and the ground line and the direction of the current are the same, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current loops of the switching power supply and serves as a common reference point for the circuit. It is an important method of controlling interference. Therefore, careful consideration should be given to the placement of the grounding wire in the layout. Mixing various groundings will cause unstable power supply operation.

The following points should be noted in the grounding design:

1, the correct choice of single-point grounding In general, the filter capacitor common terminal should be the only connection point of other grounding points coupled to the large current AC ground, the same level of grounding point of the circuit should be as close as possible, and the current circuit power supply filter capacitor Should be connected to the grounding point of this level, mainly considering that the current flowing back to the ground in each part of the circuit is changed, because the impedance of the actually flowing line will lead to the change of the ground potential of various parts of the circuit and introduce interference. In this switching power supply, its influence on the inductance between the wiring and the device is small, and the circulation current formed by the grounding circuit has a great influence on the interference, so a point grounding is adopted, that is, the power switch current loop (the ground lines of several devices in the power supply Connected to the grounding pin, the ground of several devices of the output rectifier current loop is also connected to the corresponding grounding pin of the filter capacitor, so that the power supply is stable and not easy to self-excite. Connect two diodes or a small resistor, it can be connected to a relatively concentrated piece of copper foil.

2, as far as possible to increase the thick grounding line If the grounding line is very thin, the ground potential changes with the current, resulting in the electronic equipment's timing signal level instability, anti-noise performance deterioration, so to ensure that each high-current grounding Use as short and wide printed lines as possible to maximize the width of the power and ground lines. It is better that the ground line is wider than the power line. Their relationship is: ground line> power line> signal line, if possible, ground line Width should be greater than 3mm, can also be used as a large-area copper layer for the ground, in the printed circuit board is not used where and ground are connected as a ground. When performing global cabling, the following principles must also be followed:

(1) Wiring direction: From the soldering surface, the alignment of the components is kept as much as possible in accordance with the schematic diagram. The wiring direction is preferably the same as the wiring direction of the circuit diagram. Because the production process usually requires various parameters at the soldering surface. The inspection, so doing so to facilitate the production of inspection, commissioning and maintenance (Note: refers to meet the circuit performance and overall installation and panel layout requirements under the premise).

(2) When designing the wiring diagram, make the traces as small as possible. The line width on the printed arc should not be abrupt, and the corners of the wires should be ≥90 degrees. Strive to make the lines simple and clear.

(3) Cross-circuits are not allowed in printed circuits. For lines that may intersect, they can be solved by “drilling” or “winding”. That is, if a certain lead is “drilled” from the gap underneath other resistors, capacitors, and triodes, or “wrapped” from one end of one of the leads that may cross. In a special case, how the circuit is complicated is also allowed to simplify the design. Connect with wires to solve cross-circuit problems. Due to the use of a single panel, the in-line components are located on the top surface, and the surface-mount devices are located on the bottom surface, so in-line devices can overlap with surface-mount devices during layout, but pad overlaps should be avoided.

3. The input and output ground DC-DC is low voltage in the switching power supply. To output the output voltage back to the primary of the transformer, both sides of the circuit should have a common reference ground, so after laying the copper on both sides of the ground, Also connect together to form a common ground.

Fifth, check

After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules established by the designer. It is also necessary to confirm whether the established rules are in accordance with the requirements of the PCB production process. Generally, the inspection line and the line, the line and the component pads, and the line Whether the distance between the through hole, the component pad and the through hole, the through hole and the through hole is reasonable, and whether the production requirement is satisfied. Whether the width of the power line and the ground line is appropriate, and whether there is a place where the ground line can be widened in the PCB. Note: Some errors can be ignored. For example, some of the outlines of the connectors are placed outside the frame and errors occur when checking the spacing. After each modification of the traces and vias, they must be re-coppered once.

Sixth, review according to "LEDpcb board checklist"

The content includes design rules, layer definition, line width, spacing, pad, and via settings, as well as reviewing the rationality of device layout, routing of power and ground networks, and routing and shielding of high-speed clock networks. Place and connect capacitors.

Seven, design output and output highlights file notes

a. The PCB needs to output the layers of the wiring layer (bottom layer), silk screen layer (including the top silk screen, bottom screen printing), solder mask (bottom solder mask), drilling layer (bottom layer), in addition to generate drilling files (NCDrill)

b. When the PCB sets the layer of the silk screen layer, do not select PartType, select the top layer (bottom layer) and the outline of the silk screen layer, Text, Linec. When setting the Layer of each layer, select the Board Outline and set the layer of the silk screen layer. Do not select PartType, select the top layer (bottom layer) and silkscreen layer Outline, Text, Line.d. When creating a drill file, use the default PowerPCB settings and do not change anything.