Voltage regulator in a television apparatus

Abstract

An input signal at a horizontal frequency is coupled to a deflection circuit through an isolation transformer during a power-on operation but not during a power-off operation. A voltage at a horizontal deflection frequency is developed in a linearity inductor of the deflection circuit. The voltage in the linearity inductor is rectified to produce a rectified signal that is coupled to an input terminal of a regulator oscillator of a power supply. The rectified signal is coupled to the regulator oscillator in an electrically nonisolated manner with respect to electrical shock hazard isolation and is electrically isolated by the transformer from the input signal. The regulator oscillator generates trigger pulses for controlling the switching operation of an SCR switch of the power supply to produce an output supply voltage that energizes the deflection circuit. The regulator oscillator is energized during both power-up and power-off operation and is responsive to the rectified signal to cause the duty cycle of an output signal of the oscillator to increase during power-off operation. The increased duty cycle causes the anode-to-cathode voltage of the SCR switch to be small enough to prevent latching of the SCR switch during the power-off operation. By preventing SCR switch latching, audible parasitic vibrations in the power supply are avoided.