TABLE OF CONTENTS Soft-start action is quite different from the inrush limiting discussed in Sec. 7.1, although the two functions are complementary. Both actions reduce the inrush current to the supply during the initial switch-on period. However, whereas inrush limiting directly limits the current into the input capacitors, soft start acts upon the converter control circuit to give a progressively increasing pulse width. This progressive start not only reduces the inrush current stress on the output capacitors and converter components, it also reduces the problems of transformer flux doubling in push-pull and bridge topologies. (See Part 3, Chap. 7.)
It is normal practice with switchmode supplies to take the line input directly to the rectifier and a large storage and/or filter capacitor via a low-impedance noise filter. To prevent large inrush currents on initial switch-on, inrush-control circuitry is normally provided. In large power systems, the inrush limiting often consists of a series resistor which is shorted out by a triac, SCR, or relay when the input capacitors are fully charged. (Part 1, Chap. 7 shows typical inrush-control circuits.)
To allow the input capacitors to fully charge during start-up, it is necessary to delay the start-up of the power converter so that it does not draw current from the input capacitors until these are fully charged. If the capacitors have not been fully charged, there will be a current surge when the inrush-control SCR or triac operates to bypass the inrush-limiting series resistor. Furthermore, if the converter was allowed to start up...
