Switching power supply PCB design specifications

Switching power supply PCB design specifications

I. Design flow from schematic to PCB

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

Second, the parameter settings The distance between adjacent wires must meet the requirements of electrical safety, and in order to facilitate operation and production, the distance should also be as wide as possible. The minimum distance must be at least suitable for the voltage to withstand. When the wiring density is low, the distance between signal lines can be appropriately increased. For high and low level signal lines, the distance should be as short as possible and the distance should be increased. Generally, Set the trace pitch to 8mil. 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 pad defects during processing. When the traces connected to the pads are thin, the connection between the pads and the traces should be designed in a water droplet shape. This has the advantage that the pads are not easy to peel, but the traces and the pads are not easy to disconnect.

3. The practice of component layout proves that even if the circuit schematic design is correct and the printed circuit board is not designed properly, it will adversely affect the reliability of electronic equipment. For example, if two thin parallel lines of a printed board are close to each other, a signal waveform delay will be formed, and reflection noise will be formed at the end of the transmission line; interference caused by inadequate consideration of power and ground leads to 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). AC circuit of power switch

(2). Output rectified AC circuit

(3). Input signal source current loop

(4). Output load current loop

The best way to establish a switching power supply layout is similar to its electrical design. The best design process is as follows:

● Place the transformer

● Design power switch current loop

● Design output rectifier current loop

● Control circuit connected to AC power circuit

● Design the input current source loop and input filter

When designing the output load circuit and output filter according to the functional units of the circuit, the layout of all components of the circuit must meet the following principles:

(1) First consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise ability is reduced, and the cost is also increased; if the PCB size is too small, the heat dissipation is not good, and the adjacent lines are easily affected. The optimal shape of the circuit board is rectangular, with an aspect ratio of 3: 2 or 4: 3. Components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board.

(2) When placing the device, consider future soldering, not too dense.

(3) Centering on the core components of each functional circuit, layout around it. Components should be evenly, neatly and compactly arranged on the PCB, minimize and shorten the leads and connections between components, and decoupling capacitors should be as close as possible to the device's VCC

(4) For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the components should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to mount and weld, and easy to mass produce.

(5) (5) Arrange the position of each functional circuit unit according to the flow of the circuit, make the layout convenient for signal circulation, and keep the signal in the same direction as possible.

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

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

Fourth, the wiring switching power supply contains high-frequency signals, any printed wire on the PCB can function as an antenna, the length and width of the printed wire will affect its impedance and inductive reactance, thereby affecting the frequency response. Even printed wires that pass a DC signal can couple from nearby printed wires to the RF signal and cause circuit problems (even radiating interference signals again). Therefore, all printed wiring that passes AC current should be designed as short and wide as possible, which means that all components connected to the printed wiring and connected to other power lines must be placed close together. The length of the trace is directly proportional to the 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 that the printed line can send and receive electromagnetic waves, and it can radiate more RF energy. According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, the direction of the power line and the ground line should be consistent with the direction of the current, which will help enhance the ability to resist noise. Grounding is the bottom branch of the four current loops of the switching power supply. It plays a very important role as the common reference point of the circuit. It is an important method to control interference. Therefore, the layout of the ground wire should be carefully considered in the layout. Mixing various grounds will cause the power supply to be unstable.

    The following points should be noted in the ground wire design

1. Correctly select single-point ground. Generally, the common end of the filter capacitor should be the only connection point where other ground points are coupled to the high-current AC ground. Should be connected to the ground point of this level, mainly considering that the current flowing back to ground of each part of the circuit is changed, because the impedance of the actual flowing line will cause the ground potential of each part of the circuit to introduce interference. In this switching power supply, the influence of its wiring and the inductance between the components is small, and the circulating current formed by the ground circuit has a large impact on the interference. Therefore, a little ground is used, that is, the power supply current loops (the ground lines of several devices are all Connected to the ground pin, the ground wires of several devices that output the rectifier current loop are also connected to the ground pins of the corresponding filter capacitors. This way the power supply works more stable and is not easy to self-excited. When a single point cannot be achieved, the common ground Connect two diodes or a small resistor everywhere. In fact, it can be connected to a relatively concentrated piece of copper foil.

2. Make the ground wire as thick as possible. If the ground wire is very thin, the ground potential changes with the change of current, which causes the timing signal level of electronic equipment to be unstable and the anti-noise performance deteriorated. Therefore, ensure that each high-current ground terminal Use as short and wide printed wires as possible to widen the width of the power and ground wires. It is best that the ground wires are wider than the power wires. Their relationship is: ground wire> power wire> signal wire. If possible, the ground wire The width should be greater than 3mm. A large-area copper layer can also be used as the ground wire. All unused places on the printed board are connected to the ground as the ground wire.

    When performing global wiring, the following principles must also be followed

(1). Wiring direction: Seen from the soldering surface, the arrangement orientation of the components should be consistent with the schematic diagram as much as possible. The wiring direction should be consistent with the wiring direction of the circuit diagram, because various parameters are usually required on the soldering surface during the production process. This makes it easy to check, debug and overhaul in production (Note: under the premise of meeting the requirements of circuit performance and the installation and panel layout of the whole machine).

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

(3). Cross circuits are not allowed in printed circuits. For lines that may cross, two methods can be used: "drilling" and "winding". That is, a certain lead is "drilled" from the gap under other resistors, capacitors, or triode pins, or "wound" from one end of a lead that may cross. In special cases, how the circuit is complicated, it is allowed to simplify the design Use wire crossover to solve the problem of cross circuit. Due to the use of a single panel, the DIP components are located on the top surface, and the surface-mount devices are located on the bottom surface. Therefore, the DIP devices can overlap the surface-mount devices during layout, but the pads must be avoided.

3. Input ground and output ground The switching power supply is a low-voltage DC-DC. If the output voltage is to be fed back to the primary of the transformer, the circuits on both sides should have a common reference ground, so after laying the ground wires on both sides separately, But also connected together to form a common ground.

Five, check

    After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer. At the same time, it is also necessary to confirm whether the formulated rules meet the requirements of the printed board production process. Generally, check the wire-to-wire, wire and component pads, and wires Whether the distance from the through hole, the component pad and the through hole, the through hole and the through hole is reasonable and meets the production requirements. Are the widths of the power and ground wires appropriate, and is there any place in the PCB that can widen the ground wires. Note: Some errors can be ignored. For example, part of the outline of some connectors is placed outside the board frame, and errors will occur when checking the spacing. In addition, after modifying the wiring and vias, you need to re-copper the copper again.

6. Review According to the "PCB Checklist", the content includes design rules, layer definitions, line width, pitch, pads, and via settings. It is also important to review the rationality of device layout, power and ground network routing, and high speed. Clock network routing and shielding, placement and connection of decoupling capacitors, etc.

7. Design output Notes on outputting light drawing files:

a. The layers that need to be output are wiring layer (bottom layer), silk screen layer (including top silk screen, bottom silk screen), solder mask layer (bottom solder mask), drilling layer (bottom layer), and drilling files (NC Drill) )

b. When setting the silk screen layer, do not select Part Type, select the top (bottom) and outline, text, and linec of the silk screen layer. When setting the layer for each layer, select the Board Outline, and when setting the silk screen layer, Do not select Part Type. Select Outline, Text, and Line for the top (bottom) and silkscreen layers. d. When generating the drilling file, use the default settings of PowerPCB. Do not make any changes.