Patent ID: 8629631

Claim:
A switch mode power supply (SMPS) having improved start-up time at reduced input voltage, comprising: a rectifier circuit for converting an alternating current (AC) power source to a direct current (DC) power source; a SMPS transformer having a primary winding thereof coupled to the DC power source and a controller supply winding for powering a SMPS controller; a power transistor switch coupled to the primary winding of the SMPS transformer; the SMPS controller having a drive output and a Vcc power supply input, wherein the drive output is coupled to the power transistor switch such that the SMPS controller controls when the power transistor switch turns on or off; a Vcc power supply capacitor coupled to the Vcc power supply input of the SMPS controller; a first rectifier diode coupled between the controller power winding of the SMPS transformer and the Vcc power supply capacitor; and a start-up constant current source coupled between the DC power source and the Vcc power supply capacitor, wherein the start-up constant current source provides a constant current charge to the Vcc power supply capacitor until a threshold voltage is reached thereon, then the start-up constant current source shuts off and charging voltage to the Vcc power supply capacitor is supplied by the controller power supply winding of the SMPS transformer through the first rectifier diode.