Partial circuit design of color TV switching power supply

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As shown in the figure, some circuits in the color TV switching power supply are composed of VT806, VT802, N801 and VT803. The sampling voltage of the voltage regulator circuit is taken from the output terminal of the switching power supply 115V, and the VT806 emitter is connected to the 6.2V voltage. Pressure tube VD812. When the output voltage of the switching power supply rises, the base electrode potential of the VT806 rises, the collector current increases, the current flowing through the light-emitting diode of the photocoupler N801 increases, and the luminous intensity increases, and the phototransistor of the N801 leads. The current increases, the voltage drop on R809 increases, the base potential of VT802 decreases, the collector current increases, the base potential of VT803 rises rapidly, the conduction current of VT803 increases, and the shunt current of the switching tube VT804 increases. , let VT804 exit the saturation state in advance. The on-time of the switch tube is shortened, and the secondary output voltage of the switching power supply is lowered to return to the normal value. When the output voltage of the switching power supply drops, the working process of the voltage stabilizing circuit is opposite to the above process, thereby ensuring the stability of the output voltage. Therefore, the photocoupler N801 plays a controlling role while isolating the mains from the regulated output.