In power supply design and manufacturing, PCB design and fabrication are crucial. In any switching power supply design, the physical design of the PCB is the final step; improper design methods can lead to numerous problems. Based on years of PCB design experience, especially in power supply design and fabrication, the author analyzes the key considerations for each step below.
Design process
The design process from schematic to PCB is as follows: establish component parameters → input schematic netlist → design parameter settings → manual placement → manual routing → design verification → review → CAM output.
Electrical safety requirements
The spacing of conductors must meet electrical safety requirements. The minimum spacing must be sufficient to withstand the required voltage, and the spacing should be as wide as possible for ease of operation and production. When wiring density is low, the spacing of signal lines can be appropriately increased. For signal lines with significant differences in high and low voltage levels, the spacing should be increased as much as possible, typically 8 mil. The distance from the edge of the pad's inner hole to the edge of the printed circuit board should be greater than 1 mm to avoid pad damage during processing. When the trace connected to the pad is thin, the connection between the pad and the trace should be designed in a teardrop shape. This prevents the pad from peeling and makes it less likely for the trace to break off from the pad.
Component layout
Layout practice has shown that even if the circuit schematic is designed correctly, improper printed circuit board (PCB) design can adversely affect the reliability of electronic equipment. For example, if two thin parallel lines on the PCB are too close together, it will cause signal waveform delay and create reflected noise at the end of the transmission line. Therefore, proper methods should be used when designing PCBs.
Every switching power supply has four current loops: the power switch AC loop, the output rectifier AC loop, the input signal source current loop, and the output load current loop. The input loop charges the input capacitor with a near-DC current, and the filter capacitor primarily functions as a broadband energy storage device. Similarly, the output filter capacitor stores high-frequency energy from the output rectifier while eliminating DC energy from the output load loop. Therefore, the input and output current loops should only be connected to the power supply through the filter capacitor terminals. If the connection between the input/output loop and the power switch/rectifier loop cannot be directly connected to the capacitor terminals, AC energy will radiate into the environment via the input or output filter capacitor. The power switch AC loop and the rectifier AC loop contain high-amplitude trapezoidal currents with high harmonic content. These currents have frequencies much higher than the switching fundamental frequency, with peak amplitudes up to five times the continuous input/output DC current amplitude, and transition times typically around 50ns. These two loops are most prone to electromagnetic interference, therefore, these AC loops must be routed before other printed circuit boards in the power supply. The filter capacitors, power switches or rectifiers, inductors or transformers in each circuit should be placed adjacent to each other to make the current path between them as short as possible.
When laying out all the components of a circuit, the following principles should be followed:
● When the PCB size is too large, the printed lines are long, the impedance increases, the noise immunity decreases, and the cost also increases; if it is too small, the heat dissipation is poor, and adjacent lines are easily interfered with. The optimal shape of the circuit board is rectangular, with an aspect ratio of 3 : 2 or 4 : 3. Components located at the edge of the circuit board should generally be no less than 2mm away from the edge.
● When placing components, consider future soldering; avoid placing them too close together.
● The layout should be centered around the core component of each functional circuit. Components should be arranged evenly, neatly, and compactly on the PCB, minimizing and shortening the leads and connections between components. Decoupling capacitors should be placed as close as possible to the VCC of the components.
● For circuits operating at high frequencies, the distributed parameters between components must be considered. Generally, components should be arranged in parallel as much as possible. This not only improves aesthetics but also facilitates assembly, soldering, and mass production.
● Arrange the positions of each functional circuit unit according to the circuit flow to facilitate signal flow and keep the signals in the same transmission direction as much as possible.
●The primary principle of layout is to ensure the routing continuity. When moving components, pay attention to the connection of flying wires and place components that are connected together.
● Minimize the loop area as much as possible to suppress radiated interference from the switching power supply.
High frequency processing
The length and width of traces affect their impedance and inductive reactance, thus impacting frequency response. Even traces carrying DC signals can couple to RF signals from neighboring traces, causing circuit problems (and even radiating interference signals again). Therefore, all traces carrying AC current should be designed to be as short and wide as possible, meaning all components connecting to traces and other power lines must be placed close together. The width of power lines should be increased as much as possible according to the current on the printed circuit board to reduce loop resistance. Simultaneously, the routing of power and ground lines should align with the direction of current, which helps enhance noise immunity. Grounding is the lowest branch of the four current loops in a switching power supply, playing a crucial role as a common reference point for the circuit and a key factor in controlling interference. Therefore, the placement of grounding wires should be carefully considered in the layout; mixing various grounding methods can cause power supply instability. The following points should be considered in grounding design.
1. Correctly select single-point grounding
Typically, the common terminal of the capacitor should be the only connection point for coupling other grounding points to the high-current AC ground. Grounding points of the same stage of the circuit should be as close as possible, and the power supply filter capacitor of this stage should also be connected to this stage's grounding point. Single-point grounding can be used, where the ground wires of several components in the power switch current loop are connected to the grounding pin, and the ground wires of several components in the output rectifier current loop are connected to the grounding pin of the corresponding filter capacitor. This makes the power supply more stable and less prone to self-oscillation. If single-point grounding is not feasible, two diodes or a small resistor can be connected at the common ground point, or the connection can be made to a relatively concentrated area of copper foil.
2. Use thicker grounding wires as much as possible.
The ground wire is very thin, and the ground potential changes with the current, causing the timing signal level of electronic equipment to be unstable and the noise immunity to deteriorate. Therefore, it is necessary to ensure that each high current grounding terminal uses the shortest and widest possible printed line, and to widen the power and ground wires as much as possible. Ideally, the ground wire should be wider than the power wire. Their relationship is: ground wire > power wire > signal wire. If possible, the width of the ground wire should be greater than 3mm. A large area of copper layer can also be used as a ground wire . On the printed circuit board, all unused areas are connected to the ground as ground wires.
Global cabling considerations
In terms of interface, the arrangement of components should be kept as consistent as possible with the schematic diagram, and the wiring direction should ideally be consistent with the routing direction of the circuit diagram.
When creating a wiring diagram, minimize the number of bends in the traces, avoid abrupt changes in line width on printed arcs, and ensure that conductor corners are ≥90°. Strive for simple and clear lines.
Intersecting circuits are not allowed. For potentially intersecting lines, two methods can be used: "drilling" or "winding." This means allowing a lead to "drill" through the gaps between the pins of other resistors, capacitors, or transistors, or to "wind" around one end of a potentially intersecting lead. If the circuit is very complex, bridging wires can be used to solve the intersecting circuit problem to simplify the design.
Inspection and re-examination
After the design is completed, it is necessary to carefully check whether the routing design conforms to the rules set by the designer, and also to confirm whether the set rules meet the requirements of the printed circuit board manufacturing process. Generally, check whether the distances between lines, between lines and component pads, between lines and vias, between component pads and vias, and between vias are reasonable and meet production requirements. Also check whether the widths of power and ground lines are appropriate, and whether there are any areas on the PCB where the ground line could be widened.
According to the "PCB Checklist", the contents include design rules, layer definitions, trace widths, spacing, pads, via settings, and a focus on reviewing the rationality of component layout, power and ground network routing, high-speed clock network routing and shielding, and the placement and connection of decoupling capacitors, etc.
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