Surge protection system for a d-c power supply during power-up

Claim 1

A surge protection system for a d-c power supply wherein a-c line voltage, received from an a-c power distribution system, is rectified in an uncontrolled rectifier bridge to develop a d-c voltage which is then filtered and smoothed in a low pass filter having a series-connected filter inductor and a shunt-connected filter capacitor, said surge protection system comprising: a resistance, connected in series with the filter capacitor, for limiting the inrush current flowing to the capacitor and the overshoot voltage produced across the capacitor during the power-up period; and control means for essentially bypassing said resistance at the conclusion of the power-up period and including a sensor for sensing the filter capacitor voltage and producing a scaled-down replica of that voltage, a time delay circuit for delaying the replica voltage, and a triggering circuit for turning on a solid state switch to short out said resistance when the replica voltage reaches a preset level, which level indicates that the filter capacitor is fully charged and that the power-up period is concluded.