SWITCHED MODE POWER SUPPLY HAVING A DISCHARGE CIRCUIT FOR AN AUXILIARY POWER SOURCE

摘要

An SMPS including a primary rectifying circuit connected to an AC power supply, a transformer connected to the primary rectifying circuit, a secondary rectifying circuit connected to the secondary side of the transformer, a voltage fluctuation sensing section connected to the output of the secondary rectifying circuit for sensing the fluctuation of the output voltage, a switching signal generator for generating switching signals according to the output signals of the voltage fluctuation sensing section, a switching transistor inserted between the transformer and ground for executing switching operations repeatedly according to the switching signals, a protective circuit for detecting the state of the output voltage and stopping the operation of the switching signal generator in abnormal state, and an auxiliary power circuit for supplying the operating power to the switching signal generator and the protective circuit. The SMPS further comprises an AC power cut-off detecting circuit for detecting whether the AC power is interrupted or not and a discharge circuit for discharging the residual electric charge of the auxiliary power circuit according to the output of the detecting circuit.