PCB Tech - Seven Steps in PCB Board Design for LED Switch Power Supply

In the design of switching power supply, excessive EMI will be radiated if the PCB board is not properly designed. The PCB board design with stable power supply is summarized in seven steps: through the analysis of the matters needing attention in each step, it is easy to do PCB board design well step by step!

1. Design process from schematic diagram to PCB

Establish Component Parameters - > Input Principle Network Table - > Design Parameter Settings - > Manual Layout - > Manual Wiring - > Verify Design - > Review - > CAM Output.

2. Parameter Settings

The spacing between adjacent wires must meet the electrical safety requirements and should be as wide as possible for ease of operation and production. Spacing should be at least suitable for withstanding voltage. When the wiring density is low, the spacing between signal lines can be increased appropriately. Signal lines with large differences between high and low levels should be shortened and enlarged as possible. In general, the spacing between routes should be set to

The distance from the edge of the inner hole of the pad to the edge of the printed board is greater than 1mm, which can avoid the defect of the pad during processing. When the wiring connected to the pad is thin, the connection between the pad and the wire should be designed as droplets. The advantage is that the pad is not easy to peel, but the wire and the pad are not easy to disconnect.

3. Component Layout

Practice has proved that even if the circuit schematic is correctly designed and the printed circuit board is not properly designed, it will adversely affect the reliability of electronic equipment. For example, if the two thin parallel lines of the printed circuit board are close together, a delay in the signal waveform will be formed, and a reflection noise will be formed at the end of the transmission line. Due to the interference caused by improper consideration of power supply and ground wire, the performance of the product will be degraded. Therefore, when designing printed circuit boards, 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

(3) Input signal source current loop

(4) The input circuit of the output load current loop charges the input capacitance with a current approximating DC, and the filter capacitance plays a broadband energy storage role. Similarly, the output filter capacitor is used to store high frequency energy from the output rectifier while eliminating the direct current energy from the output load circuit. Therefore, the wiring ends of 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 ends of the filter capacitors. If the connection between the input/output circuit and the power switch/rectifier circuit cannot be directly connected to the terminal of the capacitor, the AC energy will be radiated from the input or output filter capacitor to the environment. The AC circuits of the power switch and rectifier contain high-amplitude trapezoidal currents, which have a high harmonic component and are much more frequent than the basic frequency of the switch. The peak amplitude can be up to 5 times the amplitude of the continuous input/output DC current, and the transition time is usually about 50ns. These two circuits are prone to electromagnetic interference. These AC circuits must therefore be laid out before other printed wiring in the power supply. The three main components of each circuit, filter capacitance, power switch or rectifier, inductance or transformer, should be placed adjacent to each other to adjust the position of the components so that the current path between them is as short as possible.

The method of setting up the layout of switching power supply is similar to its electrical design, and the design process is as follows:

_Place transformer

_Design power switch current circuit

_Design the current circuit of the output rectifier

_Control circuit connected to AC power circuit

_Design the input current source circuit and input filter Design the output load circuit and output filter Layout all the components of the circuit according to the functional unit of the circuit, in accordance with the following principles:

(1) First consider the size of PCB. When the size of PCB is too large, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases. Too small will not heat well and adjacent lines will be susceptible to interference. A rectangular shape with a aspect ratio of 3:2 or 4:3 for a printed circuit board. Components located at the edge of the printed circuit board are not less than or equal to the edge of the printed circuit board.

(2) Consider future welding when placing the device, not too dense;

(3) Layout around the elements of each functional circuit. Components should be evenly, neatly and compactly arranged on the PCB, minimizing and shortening the lead and connection between components, and decoupling capacitance as close as possible to the device

(4) Circuits operating at high frequencies should take into account the distribution parameters between components. General circuits should arrange the components as parallel as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass production.

(5) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates the signal flow and keeps the signal in the same direction as possible.

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

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

4. Wiring switch power supply contains high frequency signal

Any printed line on a PCB can act as an antenna. The length and width of the printed line affect its impedance and inductance, thereby affecting the frequency response. Even printed lines that pass through the DC signal can be coupled from adjacent printed lines to the RF signal and cause circuit problems (even re-radiating interference signals). Therefore, all printed wires that pass through AC current should be designed as short and wide as possible, which means that all components connected to the printed wires and to other power cords must be placed close together. The length of the printed line is proportional to the inductance and impedance it displays, while the width is inversely proportional to the inductance and impedance of the printed line. 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, the more radio frequency energy it can radiate. According to the current size of printed circuit board, try to increase the rental power cable width and reduce the loop resistance. At the same time, make the direction of the power cord, ground wire and current consistent, which helps to enhance the anti-noise ability. Grounding is the bottom branch of the four current circuits of switching power supply. It plays an important role as the common reference point of the circuit and is an important method to control interference. Therefore, the placement of grounding wires should be carefully considered in the layout. Mixing all kinds of grounding wires will cause power supply instability.

The following points should be noted in line design:

1. Selecting a single-point grounding correctly usually means that the common end of the filter capacitance should be the connection point of other connecting points coupled to the AC ground with large current, the grounding point of the same circuit should be as close as possible, and the power filter capacitance of the circuit should also be connected to the grounding point of this level, mainly considering that the current returned to the ground by each part of the circuit is changing. Interference is introduced because the impedance of the line that is actually flowing can cause changes in the potential of parts of the circuit. In this switching power supply, its wiring and inductance between devices have little influence, while the circulation formed by the grounding circuit has a greater influence on the interference. Therefore, a point grounding is used, that is, the switching current loop of the power supply. (The ground wires of several of the devices are connected to the ground foot, and the ground wires of several devices of the output rectifier current circuit are also connected to the ground foot of the corresponding filter capacitor, so that the power supply works more stably and is not easy to self-excite. If you can't do a single point, you can connect two diodes or a small resistance together, in fact, in a relatively concentrated piece of copper foil.

2. Make the grounding wire as thick as possible If the grounding wire is very thin, the grounding potential will change with the change of current, resulting in unstable timer signal level and deteriorated noise resistance of electronic equipment. Therefore, to ensure that each large current grounding end uses printed lines as short and wide as possible, and to widen the width of power supply and ground wire as wide as possible, it is that the ground wire is wider than the power line. Their relationship is: ground wire>power line>signal Number wire, if possible, should be wider than 3mm, or can be used as ground wire with a large area of copper. Connect unused places to the ground on the printed circuit board as ground wire. The following principles must also be followed for global wiring:

(1) Wiring direction: From the welded surface, elements should be arranged in the same direction as the schematic diagram as possible, and wiring direction should be in accordance with the circuit diagram. Because various parameters need to be detected on the welded surface during the production process, it is easy to inspect, debug and repair in production. (Note: refers to the premise of meeting the requirements of circuit performance, whole set installation and panel layout).

(2) When designing the wiring plan, make as few turns as possible in the route, do not abruptly change the line width on the printed arc, and make the corners of the traverse more than 90 degrees, so as to make the lines simple and clear.

(3) No crossing circuit is allowed in the printed circuit. For possible crossing lines, it can be solved by "drilling" or "winding". That is, let a lead "drill" past the gap under other resistors, capacitors, triodes or "winding" from one end of a lead that may cross. In the past, how to make the circuit complex in special cases, it was also allowed to use wires to straddle in order to simplify the design, which solved the cross circuit problem. Because of single panel, direct insertion element on top surface, and surface-mounted device on bottom surface, direct insertion device can overlap surface-mounted device in layout, but avoid overlap of pad.

3. In order to feedback the output voltage back to the primary transformer, the two sides of the circuit should have a common reference, so after laying copper on the two sides of the ground wire, they should also 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 meets the rules laid down by the designer. At the same time, it is also necessary to confirm whether the rules laid down meet the requirements of the production process of printed circuit boards. Generally, it is necessary to check whether the distances between lines and wires, between wires and component pads, between wires and through holes, between component pads and through holes, between through holes and through holes are reasonable, and whether they meet the production requirements. Whether the widths of the lines and ground lines are appropriate, and whether there is room for widening the ground lines in the PCB. Note: Some errors can be ignored, such as some plug-in Outline parts being placed outside the board frame, errors will occur when checking the spacing, and copper will be re-coated every time the lines and holes have been modified.

6. Review according to "PCB Checklist"

The contents include the design rules, layer definition, line width, spacing, pad, through hole settings, but also focus on reviewing the rationality of device layout, the wiring of power supply and ground network, the wiring and shielding of high-speed clock network, the placement and connection of decoupling capacitors, etc.

7. Notes for designing output light-drawing documents

A. The layers to be output are wiring layer (bottom layer), screen printing layer (including top layer, bottom layer), resistance layer (bottom layer), drilling layer (bottom layer), and drilling file (NCDrill).

B. Do not select PartType when setting the Layer of the screen printing layer, select Outline, Text, Linec of the top (bottom) and screen printing layer. When setting the Layer of each layer, select PCB board Outline. When setting the Layer of the screen printing layer, do not select PartType, select the top (bottom) When generating drilling files with Outline, Text, Line.d. of the screen printing layer, use the default settings of PowerPCB and do not make any changes.