TABLE OF CONTENTS The direct-off-line switchmode supply is so called because it takes its power input directly from the ac power lines, without using the rather large low-frequency (60 to 50 Hz) isolation transformer normally found in linear power supplies.
Although the various switchmode conversion techniques are often very different in terms of circuit design, they have, over many years, developed very similar basic functional characteristics which have become generally accepted industry standards.
Further, the need to satisfy various national and international safety, electromagnetic compatibility, and line transient requirements has forced the adoption of relatively standard techniques for track and component spacing, noise filter design, and transient protection. The prudent designer will be familiar with all these agency needs before proceeding with a design. Many otherwise sound designs have failed as a result of their inability to satisfy safety agency standards.
Many of the requirements outlined in this section will be common to all switching supplies, irrespective of the design strategy or circuit. Although the functions tend to remain the same for all units, the circuit techniques used to obtain them may be quite different. There are many ways of meeting these needs, and there will usually be a best approach for a particular application.
The designer must also consider all the minor facets of the specification before deciding on a design strategy. Failure to consider at an early stage some very minor system requirement could completely negate a design approach for example, power good and power failure indicators and signals, which...
