Understand the skills of switching power supply and wiring rules

Switching power supply is one of the most common modern electronic power supply products in daily power supply applications. Unlike linear power supplies, the conversion transistors used in switching power supplies mostly switch between fully open mode (saturation zone) and fully closed mode (cutoff zone). Both methods have the characteristic of low loss. The conversion between conversions will have high losses, but the time is not long, thus saving energy and generating less waste heat. Ideally, the switching power supply itself does not consume electrical energy. Voltage stabilization is achieved by adjusting the transistor conduction and short-circuit time. On the contrary, during the process of generating output voltage, the transistor operates in the amplification region and consumes electrical energy itself. The high conversion efficiency of switching power supplies is a great advantage. Due to the high output power of switching power supplies, ultra small and lightweight transformers can be used. Therefore, the size of switching power supplies will be lower than that of linear power supplies, and the net weight will be relatively light.

If considering the high efficiency, volume, and weight of the power supply, switching power supply is superior to linear power supply. However, switching power supplies are more complex, and internal transistors will frequently switch. If the conversion current is not resolved, noise and interference signals may affect other devices. If the switching power supply is not specially designed, its power factor may not be high.

When handling the wiring of switching power supplies, we still need to pay attention to the following items:

1. Basic parameters

The distance between adjacent wires must comply with electrical safety regulations, and the spacing should be as wide as possible to facilitate operation and production. The minimum interval should be at least suitable for voltage. When the wiring density is low, the spacing between signal lines can be appropriately increased. For power lines with significant differences in high and low levels, the spacing should be as short as possible and increased. In general, the wiring spacing should be set to 8mil. The distance between the edge of the inner hole in the welding layer and the edge of the PCB board should be greater than 1mm to prevent defects in the welding layer during processing. When the wire connected to the welding layer is thin, the connection between the welding layer and the wire should be designed as water droplets. The advantage of doing so is that the welding layer is not easily detached, and the wiring and welding layer are not easily broken.

2. Component layout

Practice has proven that even if the circuit design drawings are correctly designed, improper PCB circuit board design can have adverse effects on the stability of electronic devices. For example, if two thin lines on a PCB circuit board are close to each other, it will cause a delay in the signal waveform and generate reflected noise at the end of the transmission line; Because regardless of the impact of the power supply and grounding wire, it will reduce the performance of the product. Therefore, when planning PCB circuit boards, the correct method should be chosen.

Each switching power supply has four current circuits (1), power switch AC circuit (2), output rectifier AC circuit (3), input signal source current circuit (4), and output load current circuit

The input circuit charges the input capacitor based on a similar DC current, and the filter capacitor plays a crucial role in broadband energy storage; Similarly, the export filter capacitor is also used to store the high-frequency energy of the export rectifier and clear the DC energy circuit of the output load. Therefore, the terminals of the input and output filter capacitors are very important. The input and output current circuit needs to be connected from the terminals of the filter capacitor to the power supply; If the connection between the input/output circuit and the power switch/rectifier circuit cannot be connected to the terminals of the capacitor, the AC kinetic energy of the input or output filter capacitor will be radiated into the environment.

The AC circuit of the power switch and rectifier contains high amplitude trapezoidal current. The harmonic component of these currents is very high, and their frequency is far greater than the fundamental frequency of the switch. The peak amplitude can reach up to 5 times the amplitude of the continuous input/output DC current, and the transition time is usually around 50ns. Both circuits are prone to generating interference signals. Therefore, these AC circuits must be laid in front of other printed circuits in the power supply. The filter capacitors, power switches or rectifiers, inductors or transformers of each circuit should be placed nearby, and the position of the equipment should be adjusted to make the current path between them as short as possible.

3. The wiring switch power supply contains high-frequency signals. The printed wire on any printed circuit board can serve as an antenna. The length and total width of the printed line will affect the impedance and inductance reactance, thereby affecting the frequency response. Even printed lines based on DC signals can be connected to the RF signals of adjacent printed lines, leading to circuit problems (or even re radiating electromagnetic interference). Therefore, all printed circuits designed based on AC current should be as short and wide as possible, which means that all components connected to the printed circuit and other power lines must be placed together. The length of the printed line is positively correlated with inductance and impedance, while the total width is inversely proportional to inductance and impedance.

The longer the time it takes for printed circuits to send and receive electromagnetic waves, the more RF energy radiated. According to the current of the printed circuit board, try to reduce the width of the power line as much as possible to reduce the circuit resistance. At the same time, aligning the direction of the power and ground wires with the current direction is beneficial for improving noise resistance. Grounding is the bottom support of the four current circuits of a switching power supply. It plays an important role as a positioning point for the power supply. This is the main way to manipulate influence. Therefore, careful consideration should be given to the layout of the grounding wire during layout. The mixing of various grounding wires can lead to unstable operation of the power supply.

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