Switching regulated power supply for Jinxing TV sets

There are generally three forms of switching power supplies with input and output isolation, namely transformer isolation, photoelectric coupling isolation, and sampling and output isolation. The picture shows the switching regulated power supply of a Venus brand TV. It uses two transformers for isolation. The sampling circuits R1, Rp, and R2 are directly connected to the output end of the power supply, and a certain proportion of the 120V output voltage is sampled and added to the base of the error amplifier tube VTl. After the voltage is compared and amplified by VTl with the reference voltage provided by VD1 and R5, the error voltage is output by the collector and is directly coupled to the pulse width modulation circuit composed of Schmitt triggers VT2 and VT3 through R7 to become a pulse. After amplification, the excitation tube VT4 is coupled to the switching tube VT5 through the isolation transformer T1, thus realizing the isolation between the error amplifier and the switching output stage. Transformer T2 separates the DC output end from switching tube VT5, achieving all isolation except the switching output stage. The isolation effect is very good, but one more transformer T1 is needed.