Current limiting scheme for the AC input circuit to a switch mode power supply

Abstract

A DC switch mode power supply operating from an AC input power source typically has an input stage which includes a rectifier for rectifying the AC input power waveform to provide a DC voltage to the switch mode power supply. This rectifier includes a capacitive storage circuit for storing the DC power to be supplied to the switch mode power supply. To protect the circuit against current surges during start-up, normally an open relay is provided in parallel with a current limiting resistor. This resistor can burn out under some circumstances. This invention replaces the resistor with a capacitor divider circuit containing at least one capacitor in series between the storage capacitor and one of the terminals from the input power supply. A second capacitor can also be provided in parallel with the storage capacitor and one of the diodes of the rectifier. Another diode of the rectifier is in series with the storage capacitor. The resulting circuit limits the initial current surge to the storage capacitor and also functions as a charge pump to make possible the storage of a charge on the storage capacitor greater than the peak input voltage.

Dependencies are inferred from the claim text.