DC-supply arrangement for a telecommunication line

Claim 1

An improved switched-mode DC power supply circuit for a telecommunications line on which common mode currents may occur, such circuit being connectable to a DC source having a pair of terminals one of which is earthed and the other of which is a DC supply voltage terminal, such circuit having a pair of output terminals between which an output capacitor is connected, a terminal of said capacitor being directly connected to said earthed terminal of said DC source, said capacitor being charged from said supply voltage terminal by a first switched DC converter comprising a first switching element which is periodically switched by a first periodic control signal having a variable duty cycle so as to maintain the voltage across the output terminals of said circuit at a preselected level, said capacitor also being charged by said common mode currents; such improvements being characterized in that said power supply circuit further comprises a second switched DC converter having a second switching element and inductor connected in series across said capacitor, said second converter further having a diode connecting the junction of said second switching element and said inductor to said supply voltage terminal, said second switching element being periodically switched by a second periodic control signal which becomes operative when the duty cycle of said first periodic control signal has been reduced substantially to zero; said second converter transferring charge from said capacitor to said DC source so as to maintain the voltage across the output terminals of said circuit at said preselected level despite charging of said capacitor by said common mode currents.