Code for circuit board design of switching power supply

In any switching power supply design, the physICal design of the pcb circuit board is the last link. If the design method is improper, the circuit board may radiate too much electromagnetic interference, causing the power supply to work unstably. The following is an analysis of the matters needing attention in each step:

1、 Create component parameters from schematic diagram to circuit board design process ->Input schematic netlist ->Design parameter setting ->Manual layout ->Manual routing ->Verify design ->Review ->CAM output.

2、 The spacing between adjacent conductors in parameter setting must meet the electrical safety requirements, and the spacing should be as wide as possible to facilitate operation and production. The minimum spacing shall at least be suitable for the voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased. The signal lines with high and low level differences shall be as short as possible and the spacing shall be increased. Generally, the wiring spacing is set to 8mil. The distance from the edge of the inner hole of the pad to the edge of the printed board shall be greater than 1mm, so as to avoid pad defects during processing. When the wiring connected with the pad is thin, the connection between the pad and the wiring should be designed as a water drop. This has the advantage that the pad is not easy to peel, but the wiring and the pad are not easy to disconnect.

3、 The practice of component layout has proved that even if the circuit schematic diagram is correctly designed and the printed circuit board is improperly designed, the reliability of electronic equipment will be adversely affected. For example, if two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed and the reflected noise will be formed at the end of the transmission line; The interference caused by improper consideration of power supply and ground wire will degrade the performance of the product. Therefore, correct methods should be used when designing printed circuit boards. Each switching power supply has four current circuits:

·                                 (1).  AC circuit of power switch

·                                 (2).  Output rectifier AC circuit

·                                 (3).  Input signal source current loop

·                                 (4).  Output load current circuit

The input circuit charges the input capacitor through an approximate DC current, and the filter capacitor mainly plays a broadband energy storage role; SIMilarly, the output filter capacitor is also used to store high-frequency energy from the output rectifier and eliminate the DC energy of the output load circuit. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current circuits should be connected to the power supply only from the terminals 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 to the environment by the input or output filter capacitor. The AC circuit of the power switch and the AC circuit of the rectifier contain high amplitude trapezoidal currents. The harmonic component of these currents is very high, and its frequency is far greater 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. The transition time is usually about 50ns. These two circuits are most likely to generate electromagnetic interference, so these AC circuits must be laid before wiring other printed lines in the power supply. The three main components of each circuit, filter capacitor, power switch or rectifier, inductor or transformer, should be placed adjacent to each other. Adjust the component position to make the current path between them as short as possible. The best way to establish the switching power supply layout is similar to its electrical design. The best design process is as follows:

·Place the transformer

·Design power switch current circuit

·Design current circuit of output rectifier

·Control circuit connected to AC power circuit

·Design input current source loop and input filter

The design of output load circuit and output filter shall comply with the following principles when arranging all components of the circuit according to the functional unit of the circuit:

(1) First, consider the size of the circuit board. When the size of the circuit board is too large, the printed line is long, the impedance increases, the noise resistance decreases, and the cost increases; If it is too SMAll, the heat dissipation is poor, and adjacent lines are vulnerable to interference. The optimal shape of the circuit board is rectangular, with a length to width ratio of 3:2 or 4:3. The distance between the components located at the edge of the circuit board and the edge of the circuit board is generally not less than 2mm.

(2) When placing the device, consider the future welding, not too intensive

(3) Take the core components of each functional circuit as the center, and arrange around it. The components shall be evenly, orderly and compactly arranged on the circuit board to minimize and shorten the leads and connections between components, and the decoupling capacitor shall be as close to the VCC of the components as possible

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

(5) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal flow and the signal is kept in the same direction as far as possible.

(6) The primary principle of layout is to ensure the distribution rate of wiring, pay attention to the connection of flying wires when moving components, and put the components with wiring relationship together.

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