Switching power supply PCB design specifications

In any switching power supply design, physical design of PCB is the last link, if the design method is undeserved, PCB may be too much radiation, electromagnetic interference caused by unstable power supply work, aiming at the matters needed attention in the steps below are analyzed: 

One, from the schematic diagram to the PCB design process - > element parameters input principle of network design parameters table - > Settings - > manual layout - > the manual wiring - design - > > validation review - > CAM output. 

2, parameter setting distance between adjacent wires must be able to meet the requirements of electrical safety, and to facilitate the operation and production, also should try to wide spacing. Minimum distance can be suitable to withstand voltage, at least in wiring density is lower, the line spacing can be appropriately increased, the high and low level disparity of signal lines should be as much as possible short and increasing spacing, line spacing will generally take set to 8 mil. 

Welding disc hole edge to the distance from the edge of the PCB is greater than 1 mm, so that you can avoid solder defects due to process. As with solder connection line is fine, to connect between the pads and go line design into water droplets, such are the benefits of bonding pad is not easy to peel, but go line and welding plate is not easy to disconnect. 

Three, components layout practice has proved that even if the principle diagram of the circuit design is correct, improper printed circuit board design, also can produce adverse effect on the reliability of electronic equipment. For example, if the two parallel lines in close proximity, PCB will form the signal delay, reflection noise in transmission line terminal formation;Due to power supply, the ground wire is not considerate, can make the product performance degradation, therefore, in the design of printed circuit boards, should pay attention to the right way. Each switch power supply with four current loop: 

(1). The power switch ac circuits 

(2). The output rectifier ac circuits 

(3). The input signal source current loop 

Output (4).loadCurrent loop input circuit through an approximation of dc current to the input capacitance charging, filter capacitance is mainly a broadband storage effect;Similarly, the output filter capacitor is used to store from high frequency energy output of the rectifier, eliminate the load circuit of the dc power output at the same time. So, the input and output filter capacitor terminal is very important, input and output current loop should be only from the terminal filter capacitor is connected to the power supply;If the input/output circuit and the connection between the power switch/rectifier circuit cannot be attached directly to the terminal of capacitor, energy will be composed of input or output filter capacitor and radiation to the environment. The power switch communication circuit and the communication of the rectifier circuit contains high of trapezoidal current, the current in the harmonic components is very high, its frequency is greater than the switch base frequency, the peak amplitude of up to 5 times for the amplitude of the input/output dc current, transition time is usually about 50 ns. The two circuits are most likely to produce electromagnetic interference, so must other printed wiring in power supply good cloth before the ac circuits, each circuit of three main components of the filter capacitor, power switch, or rectifier, inductor or transformer shall be placed adjacent to each other, adjust the current path between the element position make them as short as possible. The best way to build switching power supply layout and its electrical design, optimal design process is as follows: 

Place the transformer, 

The power switch circuit, design 

Design output rectifier current circuit 

Connected to the ac power circuit of the control circuit 

Design input current source circuit and inputfilter Design output load circuit and output filter according to the function of the circuit unit, all components of circuit layout, to comply with the following principles: 

Size. (1) the first thing to consider PCB PCB size is too big, the printing lines long, impedance increases, the ability to resist noise down, cost also increase;Is too small, heat dissipation is not good, are susceptible to interference and adjacent lines. Circuit board the best shape of rectangular, aspect ratio is 3:2 or 4:3, located in the edge of the circuit board components, circuit board edge is generally not less than 2 mm. 

(2) to consider when placing device after welding, don't be too dense. 

(3) with the core of each functional circuit element as the center, around it for layout. Compact components should be even and neat, is arranged on the PCB, as far as possible to reduce and shorten the lead and the connection between the components, VCC decoupling capacitor close to the device. 

(4) the work under high frequency circuit, want to consider the distribution parameters of components between. General circuit should be as far as possible to make components in parallel. In this way, not only beautiful, and easy fabrication, easy to mass production. 

(5) according to the process of the circuit to arrange the location of each functional circuit unit, to facilitate signal flow, layout and make the signal as much as possible to maintain consistent direction.

(6) the layout of the first principle is to ensure that the wiring of the completion rate, attention to fly line connection when mobile device, the device together with attachment relation. 

(7) as much as possible to reduce the loop area, and to inhibit the radiated interference of switch power supply. 

Four, wiring contains a high frequency signal in the switching power supply, PCB, any printed line can have the effect of antenna, length and width of printed line will affect the impedance and the inductance, which affects the frequency response. Even through the dc signal to print line from neighboring coupled to the rf signal and the circuit problem (even the radiation interference signal again). Therefore all by alternating current printing lines should be designed to be as short and wide as possible, which means that all must be connected to the printing line and connected to other components placed very close to the power cord. The length of the printed line and is directly proportional to the inductance and resistance of the show, and the width is inversely proportional to the inductance and resistance of printed line. The length of the reflecting the printed line response wavelength, the longer the length, the printing line can send and receive electromagnetic wave frequency is lower, it can radiate more radio frequency energy. According to the size of the printed circuit board current, rent increases the power cord width, as far as possible to reduce the loop resistance. At the same time, make the power cord, ground wire and the direction of the current in the same direction, it helps to enhance the ability to resist noise. Grounding is the ground floor of the four current switch power supply circuit branch, as a common reference point of the circuit plays a very important role, it is an important way to control interference. Therefore, should be carefully considered in the layout of the placement of ground wire, mix all kinds of grounding will cause unstable power supply work. On the ground should be paid attention to in the design of the following: 

1. Correct selection of single point grounding in general, filter capacitance public side should be the other ground coupled to the only join points, large current of interact with the primary circuit of the ground should be close to, and the corresponding circuit power filtering capacitance can also on the level ground, mainly considering the current parts back into the ground the circuit is change, due to the actual flow of the impedance of the line can lead to circuit ground potential changes in each part introduce the interference. In this switch power supply, its wiring and devices between the inductance of a smaller effect, and the grounding circuit had a greater influence on the formation of the circulation of interference, thus using one-point earthing, the power switch current loop (a ground of several devices are connected to the ground on his feet, ground of output rectifier current loop of several devices also received the corresponding filter capacitance grounded on his feet, the power supply work more stable, not easy self-excited. When making less than a single point, in is located in twodiodeOr a small resistance, actually in the relatively concentration can be a piece of copper foil. 

2. The bold grounding line as far as possible If the ground wire is very thin, grounding potential changes over electric current, the timing signal level instability of the electronic equipment, anti-noise performance is bad, so be sure to every large current earthing end USES short and wide printing line as far as possible, try to widen the power supply, ground wire width, it is best to ground wide than the power cord, their relationship is: ground > > power line signal lines, if possible, the width of the ground wire should be greater than 3 mm, also can use copper layer on the ground a large area, all the places are not used on the PCB as earth wire connected to ground. For global wiring, still must follow the following principles: 

: (1). Wiring direction from the surface of welding, the arrangement of the components bearing as far as possible to keep consistent with schematic diagram, wiring should take line direction is consistent with the circuit diagram, because of the production process often need to test the various parameters on surface of the welding, so do check in for production, debug and repair (note: refers to the circuit performance and under the premise of the machine installation and panel layout requirements). 

(2). Design go line turn less as far as possible when wiring diagram, printed on the arc line width don't mutation, wire corner should be 90 degrees or higher, line is straightforward. 

(3). In printed circuit does not allow to have the cross circuit, that may cross the line, you can use the "diamond", "round" two ways to solve. Is to let a lead from another resistor, capacitor, transistor at the foot of the space "diamond" in the past, or crossing from one of the lead at one end of the "around" in the past, under special circumstances how circuit is complex, to simplify the design also allows for wire jumper, solve the problem of cross circuit. Because of using single panel, upright components located on the top surface, table device located on the bottom surface, so when the layout of the upright device can is it stick to device overlap with the table, but avoid overlap welding plate. 

3. The input fields and output for the low voltage DC - DC switching power supply, to the output voltage feedback back to transformer primary, on both sides of the circuit should have a common reference, so after the shop on the ground on both sides of the copper, even together, form a common ground. 

5, check the wiring design is completed, it is necessary to carefully check the wiring design by the designers is in line with the rules, rules at the same time also need to confirm whether accord with the demand of the PCB production process, general inspection line to line, line and element bonding pad, the line and communicating pores, element bonding pad and the communicating pores, through hole and the distance between the through hole is reasonable, whether to meet the production requirements. The power cord, and the width of the ground is appropriate, whether in the PCB and make ground widened. Note: some mistakes can be ignored, for example, some part of the connector of the Outline on the plate and frame, check the spacing make mistakes;And every time after modified line and hole, copper clad again once. 

Six, review according to "PCB inspection table", the content including design rules, layer definition, line width, spacing, welding plate, via setting, but also focus on review the rationality of the device layout, power supply, ground network line, high speed clock network with line and shielding, decoupling capacitor placement and connection, etc. 

Seven of output light painting, design output documents notice: 

A. need to output layer has a wiring layer (bottom), screen printing (including the top screen printing, screen printing) bottom, resistance welding layer (the bottom resistance welding), drilling layer (bottom), also generate drilling files (NC Drill) 

B. set printing Layer Layer, don't choose Part Type, select the top (bottom) and the Outline, Text, screen printing Linec. At each Layer of the Layer is set, the Board Outline selected, set the screen printing Layer Layer, don't choose Part Type, select the top (bottom) and the Outline, Text, screen printing Line. D. generated drilling file, use the default of PowerPCB, don't make any change.