Abstract: A circuit for synchronizing an operating clock of a switching power supply (SPS) system is provided. The SPS has an oscillator, a primary control circuit, a secondary voltage generation circuit, a feedback control circuit. The oscillator outputs an oscillation signal to the primary control circuit. The feedback control circuit, in response to a direct current (DC) voltage from the secondary voltage generation circuit, generates a DC feedback voltage signal. The synchronization circuit comprises an isolation device, a signal separation circuit and a triangle-wave generation circuit. The isolation device has an input terminal and an output terminal. The input terminal receives a horizontal synchronization signal and the DC feedback voltage signal. A mixed signal is generated at the output terminal as a result of amplitude-modulating the DC feedback voltage signal by the horizontal synchronization signal. The input terminal is voltage-isolated from the output terminal.

Abstract: A control circuit for a switching power supply. First and second transformers for outputting different voltages are controlled by first and second switches. The switches are controlled by first and second control means which are turned on when the applied voltage exceeds a specified start-up voltage, and off when the voltage drops below a lower shut-down voltage. A reference voltage charges a capacitor, which is then added to the operating voltage of the circuit so that the start-up voltage can be applied even though the operating voltage of the circuit is lower than the start-up voltage. A third control means controls the change from one switch to the other at the desired voltage levels. The use of lower operating voltages allows the circuit to more rapidly drop below the shut-down voltage when abnormal operation occurs.