Switch mode power supply start-up circuit

Claim 1

A switch mode power supply comprising: means for converting an AC input voltage to a DC voltage, including an AC input line and a first positive temperature coefficient thermistor series connected in said line; a transformer having a primary winding, primary winding output, and secondary winding; means for generating a series of pulses; means responsive to said pulses for switching said DC voltage across the primary winding of said transformer; a charge storage device connected to a power input of said pulse generating means, said charge storage device comprising a storage capacitor connected between said start-up circuit and a primary DC return line; an auxiliary voltage generator connecting the primary winding output of said transformer to said power input of said pulse generating means, said auxiliary voltage generator comprising a transistor and filter, said filter including an inductor connected between said primary winding output and transistor, said transistor and relay coil being series connected between said filter and primary DC voltage return line and said transistor having a control electrode connected through a first resistor to the junction of said filter and transistor; a start-up circuit connecting said AC input voltage to said charge storage device; and switch means responsive to the current in said primary winding output for disconnecting said start-up circuit from said charge storage device, said switch means including a relay having a coil and armature, said armature having a first and second position, wherein said first position series connects said first thermistor with said start-up circuit, wherein said second position short circuits said first thermistor, and wherein said armature assumes said second position in response to a current in said coil.