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Selection of Power Adapters and Factors Affecting Stability

Ⅰ. How to choose the right power adapter?


Different instruments, equipment and machines often use adapters with different voltages and currents. Therefore, when choosing a China power adapter, you need to pay attention to the following aspects:


1. In order to improve the reliability of the system, it is recommended that the switching power supply work at 50%-80% load, that is, if the power used is 20W, a switching power supply with an output power of 25W-40W should be selected.


2. Try to use the manufacturer's standard power supply, including standard size and output voltage. In this way, the delivery will be faster; on the contrary, the special size and output voltage will increase the development time and cost.


3. The power adapter clearly defines the input voltage range. Taking AC input as an example, the commonly used input voltage specifications are 110V and 220V, so there are 110V, 220V AC switching, and universal input voltage (AC: 85V-264V) three specifications. The input voltage specification should be selected according to the region of use.


4. The switching power supply will consume a part of the power during operation and release it in the form of heat, so the user should consider the heat dissipation of the switching power supply when designing the system (especially the closed system).


5. Determine the required output voltage and current according to the use situation; the size, installation method and installation hole position of the power supply; there are several outputs, whether each output needs electrical isolation; the input voltage range; according to the ambient temperature, decide the switching power supply The degree of derating, power supply power; whether certification and safety standards are required; the cooling method of the power supply: natural cooling or forced air cooling; electromagnetic compatibility standards.


Ⅱ. What will affect the stability of the power adapter?


Different electronic products have different requirements for power adapters. For example, monitors have relatively high requirements for ripple, and if the ripple is too high, the phenomenon of screen flicker occurs.


1. Reduce the area surrounded by high-voltage and high-current circuits, especially the primary side of the transformer, the switch tube, and the power supply filter capacitor circuit; reduce the area surrounded by the output rectifier circuit, the freewheeling diode circuit and the DC filter circuit; reduce the leakage of the transformer Inductance, distributed capacitance of filter capacitors; use filter capacitors with high resonant frequency, etc.


2. To reduce the internal interference of the power adapter, we should start from the following aspects: pay attention to single-point grounding of digital circuits and analog circuits, single-point grounding of high-current circuits and small currents, especially current and voltage sampling circuits, to reduce common resistance Interference, reduce the influence of ground loop; pay attention to the spacing between adjacent lines and signal properties when wiring to avoid crosstalk; reduce ground wire impedance. For electrostatic discharge, use TVS tubes and corresponding grounding protection, increase the distance between the large and small signal circuits and the chassis, or select devices with anti-static interference to solve the problem.


3. For external anti-interference ability, such as surge and lightning strike, the lightning protection ability of AC input and DC output ports should be optimized. The lightning strike can be solved by a combination of zinc oxide varistor and gas discharge tube.

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