Switching regulator off-line starting circuit

Claim 1

In a switching regulator power supply having a d-c input, a level shifting transformer, a pulse width modulated switch for modulating and controlling the energy supplied from said d-c input to said transformer, an output rectifier-filter combination for supplying a d-c output, and pulse generating circuits for supplying pulse width modulated pulses to said switch whereby said d-c output can be fed to said pulse generating circuits to said d-c output, a starting circuit, for initiating operation of said power supply when it is energized, comprising: an isolated capacitor of sufficient size to store sufficient energy to power said pulse generating circuits during a start interval; a winding on said transformer and a series connected rectifier coupled for developing a nominal charge voltage on said isolated capacitor; means for coupling said pulse generating circuits to be energized by said nominal charge voltage; a first resistor coupled between said d-c input and said capacitor and having a value high enough to be incapable of operating said pulse generating circuits, said first resistor being low enough in value to charge said capacitor when said pulse generating circuits are inoperative; a plurality of zener diodes coupled in series across said isolated capacitor with the combined zener voltage of said plurality substantially exceeding said nominal charge voltage while being a fraction of said d-c input whereby a zener current will flow when said charge on said isolated capacitor exceeds said combined zener voltage; and means responsive to the onset of said zener current for starting said pulse generating circuits.