Patent ID: 7385356

Claim:
Startup circuit for a power supply, said startup circuit comprising: an input for connecting a source of high voltage thereto, an output rail for feeding rectified voltage to the power supply, a first capacitor coupled to the output rail for storing energy when voltage is first applied to the input, a second capacitor coupled to an output of the power supply for storing energy when a voltage appearing at the output of the power supply reaches substantially steady state, and a switching circuit coupled to the first energy storage device and to the second energy storage device and being responsive to the first energy storage device having sufficient energy for transferring said energy to the second energy storage device and disconnecting the first energy storage device from the output rail; wherein: the power supply is a universal or variable power supply that is adapted to operate over a range of power supply voltages fed to said input, a starting resistor is coupled between the output rail and the first energy storage device for sourcing current to the first energy storage device, a current source is connected to the input for charging the first capacitor, and a first input of a first comparator is coupled to an output of the first capacitor and a second input of the first comparator is coupled to a first reference voltage for generating a first switching signal when the output of the first capacitor exceeds the first reference voltage; a first input of a second comparator is coupled to an output of the second capacitor and a second input of the second comparator is coupled to a second reference voltage for generating a second switching signal when the output of the second capacitor exceeds the second reference voltage; the switching circuit is responsive to the first capacitor having sufficient energy for disconnecting the starting resistor from the output rail; and the switching circuit is responsive to the first switching signal for changing from an initially open circuit wherein the first capacitor is isolated from the second capacitor to a closed circuit whereby the first capacitor is connected in parallel with the second capacitor; and is responsive to the second switching signal for disabling charge flow to the first capacitor.