Patent ID: 7633776

Claim:
A DC-DC converter comprising a primary winding of a transformer and at least one switching element connected in series to a DC power source; a main rectifying smoother connected to a secondary winding or said primary winding of said transformer; a primary control circuit for producing drive signals to turn said switching element on and off to produce DC output of a controlled given level through said main rectifying smoother; and a drive winding provided in said transformer for providing drive power for said primary control circuit through an auxiliary rectifying smoother connected between said drive winding and primary control circuit; wherein said primary control circuit comprises an impedance controller and a protective circuit for ceasing operation of said primary control circuit when a power source voltage on said primary control circuit exceeds a predetermined voltage level; and said impedance controller comprises: a potential detector for picking out the power source voltage to said primary control circuit to produce a detection signal; an impedance adjuster for adjusting power input impedance in said primary control circuit in response to the detection signal from said potential detector; a first resistor having one end connected to a power source input terminal of said primary control circuit; a constant-voltage element having one end connected to the other end of said first resistor; a second resistor having one end connected between the other end of first resistor and one end of said constant-voltage element; and a switching element having a first main terminal connected to the other end of said second resistor, a second main terminal connected to a ground, and a control terminal connected to the other end of said constant-voltage element.