Transient over voltage protection circuit for electrical power converters

In an electrical power converter having a rectifier means for converting an input AC voltage to a 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 overvoltage protection circuit comprising:

• 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;
• a second switch being in a normally opened state and being responsive to said means for detecting, said second switch being caused to close upon the detection of a voltage across said capacitor which exceeds said first predetermined value, the closure of said second switch causing said first switch to switch to a current limit state wherein said first switch opens and closes at a periodic rate wherein said periodic rate is a function of the rate at which said capacitor charges and discharges;
• and
• means for maintaining said first switch in said current limit state until the voltage across said capacitor drops below a second predetermined value.

In an electrical power converter having a rectifier means for converting an input AC voltage to an intermediate DC voltage, said intermediate DC voltage appearing on first and second output terminals of said rectifier means, and a DC-to-DC converter for converting said intermediate DC voltage into one or more regulated output DC voltages, including first and second input terminals, a transient overvoltage protection circuit comprising:

• a current limit means connected in series between said first output terminal of said rectifier means and said first input terminal of said converter creating a conductive path between said respective output and input terminals, said current limit means being in a fully conductive state upon normal operation of said electrical power converter;
• means for electrically coupling said second output terminal to said second input terminal;
• a capacitor connected between said first and second input terminals of said converter;
• and
• overvoltage detection means for detecting when the voltage across said capacitor exceeds a maximum voltage level, said current limit means being responsive thereto to switch to a partially conductive state to limit the amount of current in said conductive path to below a maximum current value, said current limit means remaining in said partially conductive state until the voltage across said capacitor returns to a normal voltage level.