From PCB schematic to PCB design process
Establish component parameters -> input principle netlist -> design parameter settings -> manual layout -> manual wiring -> verify design -> review -> CAM output.
1. Parameter setting
The spacing between adjacent wires must adhere to electrical safety standards, and to facilitate operation and production, it should be maximized. The minimum spacing should be suitable for the voltage tolerated. In cases of low wiring density, signal line spacing can be increased accordingly. For signal lines with significant differences between high and low levels, spacing should be minimized. Typically, trace spacing is set to 8 mils. The distance from the inner hole edge of the pad to the printed board edge should exceed 1 mm to prevent defects during processing. When traces connected to pads are thin, they should be designed in a drop shape, enhancing pad adhesion while reducing the risk of disconnection between pads and traces.
2. Component layout
Experience has shown that even with a correct circuit schematic, improper PCB design can negatively impact electronic equipment reliability. For instance, if two thin parallel lines on the PCB are placed too closely, it can lead to signal waveform delays and reflected noise at the transmission line’s terminal. This degrades performance, so it’s crucial to adopt the right methods when designing the PCB. 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). Output load current loop
The input loop charges the input capacitor with an approximate DC current. The filter capacitor primarily serves as a broad-spectrum energy storage element; similarly, the output filter capacitor is designed to store high-frequency energy from the output rectifier. Additionally, it mitigates the DC energy present in the output load loop. Therefore, the terminals of the input and output filter capacitors are crucial. The input and output current loops should only connect to the power supply through the terminals of their respective filter capacitors. If the connection between the input/output loop and the power switch/rectifier loop does not connect directly to the capacitor terminals, AC energy can radiate into the environment through 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, which exhibit significant harmonic components. These harmonics have frequencies that greatly exceed the fundamental frequency of the switch, with peak amplitudes reaching up to five times the amplitude of the continuous input/output DC current. The transition time is typically around 50 ns. These two loops are particularly susceptible to electromagnetic interference; thus, these AC loops should be laid out prior to the other printed lines in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, and inductors or transformers. Position them adjacent to one another and optimize their placements to minimize the current path between them. The ideal layout for a switching power supply is aligned with its electrical design, following this optimal design process:
1. Place the transformer
2. Design the power switch current loop
3. Design the output rectifier current loop
4. Connect the control circuit to the AC power circuit
When designing the input current source loop and the input filter, as well as the output load loop and output filter, certain principles must be observed:
(1) First, consider the PCB size. A PCB that is too large leads to longer printed lines, increased impedance, reduced noise immunity, and higher costs; conversely, a PCB that is too small can hinder heat dissipation and make adjacent lines more susceptible to interference. The optimal shape for the circuit board is rectangular, ideally with an aspect ratio of 3:2 or 4:3. Components located at the edge of the circuit board should generally maintain a distance of at least 2 mm from the edge.
(2) When positioning devices, keep subsequent soldering in mind and avoid excessive density.
(3) Position the core components of each functional circuit at the center and arrange others around them. Components should be evenly, neatly, and compactly distributed on the PCB, minimizing and shortening leads and connections between them, with decoupling capacitors placed as close as possible to the VCC of the devices.
(4) For circuits operating at high frequencies, consider the distributed parameters between components. Generally, arrange the circuit in parallel as much as possible, enhancing both aesthetics and ease of installation and soldering, facilitating mass production.
(5) Position each functional circuit unit according to the circuit flow to allow convenient signal circulation, keeping signals moving in the same direction when possible.
(6) The primary layout principle is to ensure wiring efficiency, paying attention to the connection of flying leads when positioning devices, and grouping devices with interconnections together.
(7) Minimize the loop area to suppress the radiation interference from the switching power supply.
The above points outline essential considerations in the PCB design process for switching power supplies. I hope this information proves helpful to everyone.
Establish component parameters -> input principle netlist -> design parameter settings -> manual layout -> manual wiring -> verify design -> review -> CAM output.
1. Parameter setting
The spacing between adjacent wires must adhere to electrical safety standards, and to facilitate operation and production, it should be maximized. The minimum spacing should be suitable for the voltage tolerated. In cases of low wiring density, signal line spacing can be increased accordingly. For signal lines with significant differences between high and low levels, spacing should be minimized. Typically, trace spacing is set to 8 mils. The distance from the inner hole edge of the pad to the printed board edge should exceed 1 mm to prevent defects during processing. When traces connected to pads are thin, they should be designed in a drop shape, enhancing pad adhesion while reducing the risk of disconnection between pads and traces.
2. Component layout
Experience has shown that even with a correct circuit schematic, improper PCB design can negatively impact electronic equipment reliability. For instance, if two thin parallel lines on the PCB are placed too closely, it can lead to signal waveform delays and reflected noise at the transmission line’s terminal. This degrades performance, so it’s crucial to adopt the right methods when designing the PCB. 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). Output load current loop
The input loop charges the input capacitor with an approximate DC current. The filter capacitor primarily serves as a broad-spectrum energy storage element; similarly, the output filter capacitor is designed to store high-frequency energy from the output rectifier. Additionally, it mitigates the DC energy present in the output load loop. Therefore, the terminals of the input and output filter capacitors are crucial. The input and output current loops should only connect to the power supply through the terminals of their respective filter capacitors. If the connection between the input/output loop and the power switch/rectifier loop does not connect directly to the capacitor terminals, AC energy can radiate into the environment through 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, which exhibit significant harmonic components. These harmonics have frequencies that greatly exceed the fundamental frequency of the switch, with peak amplitudes reaching up to five times the amplitude of the continuous input/output DC current. The transition time is typically around 50 ns. These two loops are particularly susceptible to electromagnetic interference; thus, these AC loops should be laid out prior to the other printed lines in the power supply. The three main components of each loop are filter capacitors, power switches or rectifiers, and inductors or transformers. Position them adjacent to one another and optimize their placements to minimize the current path between them. The ideal layout for a switching power supply is aligned with its electrical design, following this optimal design process:
1. Place the transformer
2. Design the power switch current loop
3. Design the output rectifier current loop
4. Connect the control circuit to the AC power circuit
When designing the input current source loop and the input filter, as well as the output load loop and output filter, certain principles must be observed:
(1) First, consider the PCB size. A PCB that is too large leads to longer printed lines, increased impedance, reduced noise immunity, and higher costs; conversely, a PCB that is too small can hinder heat dissipation and make adjacent lines more susceptible to interference. The optimal shape for the circuit board is rectangular, ideally with an aspect ratio of 3:2 or 4:3. Components located at the edge of the circuit board should generally maintain a distance of at least 2 mm from the edge.
(2) When positioning devices, keep subsequent soldering in mind and avoid excessive density.
(3) Position the core components of each functional circuit at the center and arrange others around them. Components should be evenly, neatly, and compactly distributed on the PCB, minimizing and shortening leads and connections between them, with decoupling capacitors placed as close as possible to the VCC of the devices.
(4) For circuits operating at high frequencies, consider the distributed parameters between components. Generally, arrange the circuit in parallel as much as possible, enhancing both aesthetics and ease of installation and soldering, facilitating mass production.
(5) Position each functional circuit unit according to the circuit flow to allow convenient signal circulation, keeping signals moving in the same direction when possible.
(6) The primary layout principle is to ensure wiring efficiency, paying attention to the connection of flying leads when positioning devices, and grouping devices with interconnections together.
(7) Minimize the loop area to suppress the radiation interference from the switching power supply.
The above points outline essential considerations in the PCB design process for switching power supplies. I hope this information proves helpful to everyone.
