TABLE OF CONTENTS As previously mentioned, 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 50 Hz) isolation transformer normally found in linear power supplies.
In the switchmode system, the input-to-output galvanic isolation is provided by a much smaller high-frequency transformer, driven by a semiconductor inverter circuit so as to provide some form of DC-to-DC conversion. To provide a DC input to the converter, it is normal practice to rectify and smooth the 50/60-Hz ac supply, using semiconductor power rectifiers and large electrolytic capacitors. (Exceptions to this would be special low-distortion systems, where input boost regulators are used to improve the power factor. These special systems will not be considered here.)
For dual input voltage operation [nominally 120/240 V ac], it is common practice to use a full-bridge rectifier for the high-input-voltage conditions, and various link arrangements to obtain voltage doubler action for the low-input-voltage conditions. Using this approach, the high-frequency DC-to-DC converter can be designed for a nominal DC input of approximately 320 V for both input voltages.
An important aspect of the system design is the correct sizing of input inductors, rectifier current ratings, input switch ratings, filter component size, and input fuse ratings. To size these components correctly, a full knowledge of the relevant applied stress is required. For example, to size the rectifier diodes, input fuses, and filter inductors correctly, the values of peak and rms input currents will be required,...
