Commonly used offline power adapter functions and basic requirements

　　"Offline" (direct-off-line) power adapter is named because it is directly powered by AC power adapter instead of the bulky 50-60Hz low-frequency isolation transformer commonly used in linear power adapters.

　　Although various switch conversion technologies have great differences in circuit design, after years of development, they have similar basic functional characteristics and have become generally accepted industrial standards.

　　In order to meet the requirements of various national or international safety standards, electromagnetic compatibility and power adapter transients, the industry has to use relatively standardized technologies, including wiring, component layout, noise filtering design and transient protection technologies, and careful design Before proceeding with the design, the designer will first get familiar with all the requirements of these institutions. Many good designs fail because they cannot meet the relevant standards of the safety standards department.

　　Regardless of the design strategy and circuit, many of the requirements outlined here are basically applicable to all power adapters. Although all power adapter functions tend to be the same, the circuit technology to achieve these functions is extremely different. There are many ways to meet these requirements, and there is usually the best way to achieve it for a particular application.

　　The designer must consider all the details of the standard before determining the design strategy. If some details of the system requirements are not considered in the early design stage, then this design scheme may be completely invalid. For example, to obtain an indication or signal of whether the power supply is normal or not, it is required to provide an auxiliary power adapter regardless of the status of the converter, otherwise, once the operation of the converter is prohibited, this design scheme will be completely invalid! It is often very difficult to add some subtle, once neglected functions at the end of design and development.

　　The remainder of this chapter will give an overview of commonly used basic input and output functions. These functions are either required by users or prescribed by national or international standards. They will help to check or form the initial technical requirements, all of which should be considered before entering the design phase.

　　Input transient voltage protection

　　When the conditions are met, man-made and natural discharge phenomena will occasionally cause large transient voltages on the power line.

https://www.nxbtx.com/ 　　The 1EE5871980 standard of the Institute of Electrical and Electronics Engineers gives the results of investigations on this phenomenon in various occasions. These occasions are classified as low-intensity level A, medium-intensity level B, and high-intensity level C. Most power adapters are in low-strength or medium-strength occasions. In these occasions, when the current reaches 3000A, the voltage can reach 6000V.

　　Under this stress, the power adapter is usually required to protect itself and the terminal equipment, for which special protective devices are used.

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