In the design of any switch power supply, the physical design of PCB board is the last step. If the design method is not appropriate, PCB may radiate excessive electromagnetic interference and cause power supply instability. The following is an analysis of matters needing attention in each step:
1. Establish component parameters from schematic diagram to PCB design process -> input principle network table -> design parameters setting -> manual layout -> manual wiring -> verification design -> review ->CAM output.
2. The spacing between adjacent wires for parameter setting shall meet the electrical safety requirements, and the spacing shall be as wide as possible for the convenience of operation and production.The minimum spacing should at least be able to withstand voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased. For the signal lines with large gap between high and low levels, it should be as short and increase the spacing as possible.
The distance from the inner hole edge of the pad to the edge of the printed plate is greater than 1mm, which can avoid the defect of the pad during processing.When the wire connecting with the pad is thinner, the connection between the pad and the pad should be designed as water drop. The advantage of this is that the pad does not peel easily, but the wire and the pad do not disconnect easily.
The practice of component layout has proved that even if the circuit schematic diagram is designed correctly and the printed circuit board is designed improperly, the reliability of the electronic equipment will be adversely affected.For example, if the two thin parallel lines of the printing plate are close together, the signal waveform delay will be generated and reflected noise will be formed at the end of the transmission line.Due to the disturbance caused by the thoughtless consideration of power supply and ground wire, product performance will decline. Therefore, when designing the printed circuit board, the correct method should be adopted.Each switch power supply has four current loops:
(1). Power switch ac loop
(2) output rectifier ac loop
(3). Input signal source current loop
(4) the output load current circuit input circuit charges the input capacitance through an approximate dc current, and the filter capacitance mainly plays a role of broadband energy storage;Similarly, the output filter capacitor is used to store high frequency energy from the output rectifier while eliminating dc energy from the output load loop.Therefore, the connection terminal of the input and output filter capacitors is very important. The input and output current loop should only connect to the power source from the connection terminal of the filter capacitors.If the connection between the input/output loop and the power switch/rectifier loop cannot be directly connected to the capacitor's terminal, the alternating energy will be filtered by the input/output capacitor and radiated to the environment.The ac circuit of the power switch ac circuit and the rectifier consists of a high-amplitude trapezoidal current, in which the harmonic components are very high, the frequency is much higher than the switching base frequency, the peak amplitude can be as high as 5 times the amplitude of the continuous input/output dc current, and the transition time is usually about 50ns.The two circuits are most likely to produce electromagnetic interference, so must the other printed wiring in power source to cloth before these ac circuits, each loop three main components of the filter capacitor, the 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 establish the switch power layout is similar to its electrical design. The best design process is as follows:
· placement transformer
· design current loop of power supply switch
· design the output rectifier current loop
· control circuit connected to ac power circuit
· design input current source circuit and input filter design output load circuit and output filter layout of all components of the circuit according to the functional unit of the circuit shall conform to the following principles:
(1) the PCB size should be considered first.When the PCB size is too large, the length of the printed line increases, the impedance increases, the anti-noise capability decreases, and the cost increases.Too small is not good heat dissipation, and adjacent lines are vulnerable to interference.The optimum rectangular shape of the circuit board, with a length/width ratio of 3:2 or 4:3, is the component on the edge of the circuit board, which is not less than 2mm from the edge of the circuit board.
(2) when placing devices, consider welding in the future, not too dense.
(3) center on the core elements of each functional circuit and arrange around it.The components should be uniformly, neatly and compactly arranged on the PCB, and the leads and connections between the components should be minimized and shortened. The decoupling capacitors should be as close to the VCC of the components as possible.
(4) the distribution parameters between components should be considered for the circuit operating at high frequency.General circuits should be as close to parallel as possible.In this way, not only beautiful, and easy to install welding, easy to mass production.
(5) arrange the position of each functional circuit unit according to the circuit process, make the layout convenient for signal flow, and keep the signal as consistent as possible.