Abstract: In a switched mode power supply during start-up, an increasing charge on a soft-start capacitor controls a pulse width modulator for increasing the length of drive pulses provided to a pair of switching transistors until a reference voltage level is reached, after which stable power supply operation ensues. Detection circuitry senses a low power supply output voltage such as arising from input line interruptions and rapidly discharges the soft-start capacitor and prevents the recurrence of the soft-start mode of operation before the soft-start capacitor is fully discharged to prevent overloading of power supply output semiconductor devices upon the resumption of normal input voltage to the power supply.

Abstract: The power supply of the present invention regulates the output of a DC voltage source, such as a storage battery, and is adapted to operate in combination with a rectified AC voltage to continue to provide a level DC output voltage in the event AC line power is lost. The switching regulator power supply automatically turns on in response to an AC line outage and provides a DC output voltage regulated by a feedback control arrangement which precisely controls the DC output voltage level. When the rectified AC line voltage is restored, the switching regulator power supply automatically turns off thus reducing DC voltage source usage and prolonging storage battery operating lifetime. The switching regulator power supply of the present invention is particularly adapted for low current applications including providing a well-regulated DC voltage to a volatile memory device of the general complementary-metal-oxide semiconductor type (CMOS) on a standby basis in the event the primary DC source is lost.