In any switching power supply design, the physical design of the PCB is the final step. An improper design method can lead to excessive electromagnetic interference, causing instability in power supply operation. Let’s analyze the process:
1. Begin with component parameters from the schematic to PCB design flow -> input the principal netlist -> set design parameters -> manual layout -> manual routing -> verify design -> review -> CAM output.
2. For parameter settings, the distance between adjacent traces must meet electrical safety requirements, and for ease of handling and production, this distance should be maximized. The minimum spacing must at least be suitable for the voltage withstand. In cases of low wiring density, the spacing for signal lines can be increased accordingly. Set trace spacing to 8 mils.
The distance between the inner edge of the pad hole and the edge of the printed board should exceed 1 mm to prevent pad defects during processing. When traces connected to pads are narrow, the junction between pads and traces should be designed in a drop shape. This design minimizes the risk of pad peeling while ensuring that traces and pads remain securely connected.
Third, practical experience with component layout has demonstrated that even if the PCB schematic design is correct, improper printed circuit board design can adversely affect the reliability of electronic equipment. For instance, if two closely spaced thin parallel lines on the printed board are too near, this may lead to signal waveform delays and reflected noise at the transmission line’s terminals. Additionally, interference due to inadequate consideration of power supply and ground can result in decreased product performance. Therefore, it is essential to follow correct methods when designing printed circuit boards. Each switching power supply contains four current loops:
(1). Power switch AC circuit
(2). Output rectifier AC circuit
(3). Input signal source current loop
(4). Input circuit of the output load current loop, which charges the input capacitor with an approximate DC current. The filter capacitor primarily serves as broadband energy storage; similarly, the output filter capacitor stores high-frequency energy from the output rectifier while eliminating DC energy from the output load loop. Thus, the terminals of the input and output filter capacitors are crucial. The input and output current loops should connect only to the power supply from the terminals of the filter capacitors; any connection between the input/output loop and the power switch/rectifier loop cannot directly link to the capacitor’s terminal, as AC energy could radiate into the environment from the input or output filter capacitor. The power switch and rectifier AC circuits handle high-amplitude trapezoidal currents, with harmonic components at frequencies significantly higher than the fundamental frequency of the switch. The peak amplitude can reach up to five times that of the continuous input/output DC current, with a transition time usually around 50 ns. These two loops are particularly susceptible to electromagnetic interference, making it essential to lay out these AC loops before other printed lines in the power supply. Each loop consists of three main components: filter capacitors, power switches or rectifiers, and inductors or transformers. Position them closely together and minimize the current path between them. The optimal layout for a switching power supply mirrors its electrical design, following this ideal sequence:
Place the transformer
Design the power switch current loop
Design the output rectifier current loop
Connect the control circuit to the AC power circuit
Design the input current source loop and filter
Design the output load loop and filter based on the circuit’s functional unit. While laying out the components, the following principles should be observed:
(1) First, consider the PCB size. A board that is too large results in long printed lines, increased impedance, reduced noise immunity, and higher costs. Conversely, a board that is too small can hinder heat dissipation and invite interference from adjacent lines. The ideal shape is rectangular, with an aspect ratio of 3:2 or 4:3. Components located at the edges should be at least 2 mm away from the board’s edge.
(2) When placing devices, account for future soldering, avoiding overly dense arrangements.
(3) Use the core component of each functional circuit as a central point for layout, ensuring components are evenly, neatly, and compactly arranged on the PCB. Minimize the length of leads and connections, and position decoupling capacitors as close as possible to the VCC of the device.
(4) For circuits operating at high frequencies, consider the distributed parameters between components. Arrange the circuit in parallel as much as possible to enhance aesthetics, facilitate installation and soldering, and ease mass production.
(5) Position each functional circuit unit according to the circuit flow, allowing for convenient signal circulation while maintaining consistent signal direction.
(6) The foremost principle of PCB layout is to ensure routing efficiency. Be mindful of flying line connections when relocating devices, and group interconnected devices together.
(7) Minimize loop area to suppress radiation interference from the switching power supply.
1. Begin with component parameters from the schematic to PCB design flow -> input the principal netlist -> set design parameters -> manual layout -> manual routing -> verify design -> review -> CAM output.
2. For parameter settings, the distance between adjacent traces must meet electrical safety requirements, and for ease of handling and production, this distance should be maximized. The minimum spacing must at least be suitable for the voltage withstand. In cases of low wiring density, the spacing for signal lines can be increased accordingly. Set trace spacing to 8 mils.
The distance between the inner edge of the pad hole and the edge of the printed board should exceed 1 mm to prevent pad defects during processing. When traces connected to pads are narrow, the junction between pads and traces should be designed in a drop shape. This design minimizes the risk of pad peeling while ensuring that traces and pads remain securely connected.
Third, practical experience with component layout has demonstrated that even if the PCB schematic design is correct, improper printed circuit board design can adversely affect the reliability of electronic equipment. For instance, if two closely spaced thin parallel lines on the printed board are too near, this may lead to signal waveform delays and reflected noise at the transmission line’s terminals. Additionally, interference due to inadequate consideration of power supply and ground can result in decreased product performance. Therefore, it is essential to follow correct methods when designing printed circuit boards. Each switching power supply contains four current loops:
(1). Power switch AC circuit
(2). Output rectifier AC circuit
(3). Input signal source current loop
(4). Input circuit of the output load current loop, which charges the input capacitor with an approximate DC current. The filter capacitor primarily serves as broadband energy storage; similarly, the output filter capacitor stores high-frequency energy from the output rectifier while eliminating DC energy from the output load loop. Thus, the terminals of the input and output filter capacitors are crucial. The input and output current loops should connect only to the power supply from the terminals of the filter capacitors; any connection between the input/output loop and the power switch/rectifier loop cannot directly link to the capacitor’s terminal, as AC energy could radiate into the environment from the input or output filter capacitor. The power switch and rectifier AC circuits handle high-amplitude trapezoidal currents, with harmonic components at frequencies significantly higher than the fundamental frequency of the switch. The peak amplitude can reach up to five times that of the continuous input/output DC current, with a transition time usually around 50 ns. These two loops are particularly susceptible to electromagnetic interference, making it essential to lay out these AC loops before other printed lines in the power supply. Each loop consists of three main components: filter capacitors, power switches or rectifiers, and inductors or transformers. Position them closely together and minimize the current path between them. The optimal layout for a switching power supply mirrors its electrical design, following this ideal sequence:
Place the transformer
Design the power switch current loop
Design the output rectifier current loop
Connect the control circuit to the AC power circuit
Design the input current source loop and filter
Design the output load loop and filter based on the circuit’s functional unit. While laying out the components, the following principles should be observed:
(1) First, consider the PCB size. A board that is too large results in long printed lines, increased impedance, reduced noise immunity, and higher costs. Conversely, a board that is too small can hinder heat dissipation and invite interference from adjacent lines. The ideal shape is rectangular, with an aspect ratio of 3:2 or 4:3. Components located at the edges should be at least 2 mm away from the board’s edge.
(2) When placing devices, account for future soldering, avoiding overly dense arrangements.
(3) Use the core component of each functional circuit as a central point for layout, ensuring components are evenly, neatly, and compactly arranged on the PCB. Minimize the length of leads and connections, and position decoupling capacitors as close as possible to the VCC of the device.
(4) For circuits operating at high frequencies, consider the distributed parameters between components. Arrange the circuit in parallel as much as possible to enhance aesthetics, facilitate installation and soldering, and ease mass production.
(5) Position each functional circuit unit according to the circuit flow, allowing for convenient signal circulation while maintaining consistent signal direction.
(6) The foremost principle of PCB layout is to ensure routing efficiency. Be mindful of flying line connections when relocating devices, and group interconnected devices together.
(7) Minimize loop area to suppress radiation interference from the switching power supply.