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Common offline power adapter features and basic requirements

Issue Time:2006-11-30
Introduction
The "direct-off-line" power adapter is named because it is powered directly from the AC adapter, rather than the bulky 50-60Hz low-frequency isolation transformer commonly used with linear power adapters.

Although various switching conversion techniques are quite different in circuit design, after years of development, they have similar basic functional characteristics and have become a generally accepted industry standard.

In order to meet the national or international safety standards, electromagnetic compatibility and power adapter transients, the industry has to use relatively standardized technology, including wiring, component layout, noise filtering design and transient protection technology, cautious design Those who are familiar with these requirements will be familiar with all the requirements of these organizations before designing. Many good designs fail because they cannot meet the relevant standards of the safety standards department.

Regardless of the design strategy and circuitry, many of the requirements outlined here are generally applicable to all power adapters. Although all power adapter functions tend to be the same, the circuit technologies that implement these functions are vastly different. There are multiple ways to meet these requirements, and there are usually best implementations for a particular application.

Designers must consider all the details of the standard before deciding on the design strategy. If some details of the system requirements are not taken into account at the beginning of the design, then this design may be completely ineffective. For example, to get an indication or signal that the power supply is normal, it is required to provide an auxiliary power adapter regardless of the state of the converter. Otherwise, once the operation of the converter is prohibited, this design will be completely ineffective! At the end of design and development It is often very difficult to add some subtle, once neglected features.

The remainder of this chapter gives an overview of the common basic input and output functions that are either required by the user or as specified by national or international standards. They will help to inspect or form the initial technical requirements, all of which should be considered before entering the design phase.

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

The 1EE5871980 standard of the Institute of Electrical and Electronics Engineers gives the results of a survey of this phenomenon in various situations. These occasions are classified into low strength grade A, medium strength grade B, and high strength grade C. Most power adapters are in low- or medium-intensity applications where the voltage can reach 6000V when the current reaches 3000A.

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