Transient over voltage protection circuit for electrical power converters

DETAILED DESCRIPTION Broadly stated, the present invention comprises an overvoltage detection circuit that partially opens a normally closed switch connected between a rectifier circuit and a DC-to-DC converter before the voltage on the input terminals of the DC-to-DC converter exceeds the converter's operational limits, and thereafter enables the switch to return to its normally closed state when the level of voltage on the input terminals of the o converter drops below a predetermined lower voltage level.
A capacitor connected across the input terminals of the converter is charged up when the switch is closed and functions to supply power to the converter when the switch is in this partially open or current limit state.
More specifically, in an electrical power converter having a rectifier means for converting an input AC voltage to an unregulated DC intermediate voltage, said intermediate voltage appearing on first and second output terminals of said rectifier means, and a DC-to-DC converter for converting said unregulated DC intermediate voltage into one or more regulated output DC voltages, including first and second input terminals, a transient voltage protection circuit according to the present invention comprises: a first switch connected in series between one of said first and second output terminals of said rectifier means and a corresponding one of said first and second input terminals of said converter, said first switch being in a normally closed state so as to create a conductive path between said respective output and input terminals; means for electrically coupling the other of said first and second output terminals to the other of said first and second input terminals: a capacitor connected between said first and second input terminals of said converter; means for detecting when the voltage across said capacitor exceeds a first predetermined value and for causing said first switch to switch to a current limit state in response thereto; and means for maintaining said first switch in said current limit state until the voltage across said capacitor drops below a second predetermined value