LED Switching Power Supply PCB Circuit Board Design(II)
4.The wiring switching power supply contains high frequency signals
Any printed circuit on the PCB can play the role of the antenna,the length and width of the printed line will affect its impedance and inductance, thus affecting the frequency response.Even the printed lines through the DC signal are coupled from the adjacent traces to RF signal and cause circuit problems(even re-emit the interfering signal).Therefore,all the lines passing through the AC current should be designed as short and wide as possible,which means that all components connected to the printed circuit and connected to other power lines must be placed very close.The length of the printed wire is proportional to the inductance and impedance it exhibits, and the width is inversely proportional to the inductance and impedance of the printed wire.The length reflects the wavelength of the printed circuit response,the longer the length,the lower the frequency of the transmission and reception of electromagnetic waves,it can radiate more radio frequency energy.according to the size of the printed circuit board current.as far as possible to add the power line width,reduce loop resistance.at the same time,make the power line,ground direction and current direction,this will help enhance the anti-noise ability.Grounding is the bottom of the four current loop switching power supply circuit,as the circuit’s common reference point plays a very important role,it is an important way to control interference.Therefore, the layout should be carefully considered the placement of the ground wire,a variety of grounding will cause the power supply instability.
In the ground design should pay attention to the following:
a. Proper selection if the single-point grounding, the filter capacitor common should be the other ground point coupled to the exchange of high current point,the same level of circuit grounding should be as close as possible, and the level of the circuit power supply filter capacitor should also be connected in this level of ground,the main part of the circuit to consider the current back to the ground current is changing,because the actual flow of the impedance of the circuit will lead to changes in the various parts of the circuit and the introduction of interference.In this switching power supply,its wiring and the inductance between the devices less affected,and the formation of the ground circuit on the interference of greater impact, and thus a little ground,that is,the power switch current loop( in several devices are ground connected to the ground feet),output rectifier current loop several devices of the ground is also connected to the corresponding filter capacitor ground pin,so the power supply is more stable,easy to self-excited. Can not be a single point, in a total of two diodes or small resistance,in fact,then a relatively concentrated in a copper.
b. As far as possible,if the ground wire is very fine,the ground potential changes with the current changes,resulting in electronic equipment,timing signal level instability,anti-noise performance deterioration,so to ensure that each of the high current ground short and wide printed wire,as far as possible to widen the power supply, ground width,preferably the ground than the power line width,their relationship is :ground>power line>signal line,if possible,the ground wire width should be greater than 3mm,can also use large area copper layer for ground.In the printed circuit board has not been used on the ground are connected with the ground as a ground. For global cabling, follow these guidelines:
(1.) Wiring direction: from the welding surface, the arrangement of components as much as possible consistent with the schematic diagram,the wiring direction is best consistent with the direction of the circuit diagram alignment,the production process is usually required in the welding surface for a variety of parameters,This is done to facilitate the inspection, commissioning and maintenance of the production (Note: refers to meet the circuit performance and machine installation and panel layout requirements under the premise).
(2.)Design wiring diagram as little as possible when the alignment, printing arc on the line width of the sudden change, the wire corner should be ≥ 90 degrees, and strive to simple lines clear.
(3.)The printed circuit is not allowed to cross the circuit, for the cross may be crossed, you can use “drill”, “around” two ways to solve. That is, let a lead from the other resistance, capacitance, the triad pin at the foot of the “drill” in the past, or from the end of a lead may be “around” in the past, in special circumstances how the circuit is very complex, to simplify the design also allows Cross the wire to solve the cross circuit problem. Due to the use of single-sided, the in-line components are located on the top surface, the surface mount device is located on the bottom surface, so the placement device can overlap with the surface mount device, but avoid pad overlap.
c. Input and output ground switch power supply for the low-voltage DC-DC, want to return the output voltage back to the primary transformer, both sides of the circuit should have a common reference, so the two sides of the ground after the copper, but also Connected together to form a common ground.
After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules set by the designer. It also needs to confirm whether the rules are in line with the requirements of the PCB production process. Generally check the lines and lines, And whether the distance between the through hole, the element pad and the through hole, the through hole and the through hole is reasonable, and whether or not the production requirement is satisfied. Whether the width of the power cord and ground wire is appropriate, and whether there is a place where the ground is widened. Note: Some errors can be ignored, for example, some of the connectors Outline part of the board and frame, check the gap will be wrong; the other changes each time after the alignment and vias, have to re-copper once.
6.Review According to the “PCB Checklist”
Including the design rules, layer definition, line width, spacing, pad, vias settings, but also focus on review the rationality of the layout of the device, power, ground network routing, high-speed clock network alignment and shielding, decoupling Capacitor placement and connection.
7.Design the output light to draw the file to note
a.The output layer Including top layer silk screen, bottom screen printing, solder mask layer (bottom solder mask), drilling layer (bottom layer), in addition to the formation of drilling documents (NCDrill), the hole layer (bottom layer).
b.When setting the Layer of the Silkscreen Layer, do not select PartType, select Outline, Text, Lines of the top layer and the silk screen. When setting the Layer for each layer, select Board Outline and set the Layer, Select the top layer (bottom) and silk screen layer Outline, Text, Line.d. Generate a drilling file, the use of PowerPCB default settings, do not make any changes.