High voltage switching regulated power supply circuit

The picture shows a high-voltage switching regulated power supply circuit. It is a switching power supply for 80P color TV sets, using self-excited, pulse-width modulation circuits. Its output is isolated from the grid. The power supply has two DC voltage outputs of 110V and 18V, and is composed of a power frequency rectifier filter circuit, a starting oscillation circuit, a control circuit and a DC output circuit. Capacitors C309, C30l and inductor L30l form an input filter, which is used to weaken the power grid interference from the power supply and also prevent power interference from polluting the power grid. The power frequency rectifier filter circuit composed of VD30l ~ VD304 and C308 converts the 220V AC voltage into a DC voltage of about 300V. C304~C307 are used to protect the diodes connected in parallel with them. The 300V DC voltage is reduced by the starting resistor R302 and then added to the base of the switching regulator VT304, so that it is forward biased and turned on. At the same time, current flows through the ⑨~② terminals of the input coil of the pulse transformer T301, causing the feedback coil 11~⑩ terminals to obtain positive feedback, providing forward bias to the base of VT304 to further conduct it. The collector current increases linearly, producing a lower positive and upper negative voltage drop through resistor R313. On the other hand, the voltage on terminals 13 to 12 of the sampling winding is rectified and filtered by VD306 and C312, and divided by R306, R308 and R309 to add a bias voltage to the base of VT302. When the algebraic sum of these two voltages is a certain negative number, VT302 begins to conduct, and VT303 also conducts accordingly, causing the positive potential of C314 to drop almost to zero, the potential of the negative pole to become a negative value, and VT304 cuts off. The output coil stores energy when VT304 is turned on, and transmits energy to the two output coils at terminals ⑤~⑥ and ④~⑥ when VT304 is turned off. There is a distributed capacitance Cp between the input coils. This capacitance is connected in parallel with the ⑨~② terminals of the coil to form a resonant circuit. When VT304 is turned off, the direction of its oscillating current is shown as the dotted line in the figure. When the current passes through the terminals ②~⑨ of the coil, the induced electromotive force generated at the terminals ⑩~⑩ of the feedback coil causes VT304 to continue to be cut off. As time changes, when the direction of the oscillating current is opposite to the direction of the dotted line in the figure, the direction of the current generated in the feedback coil is also opposite. It is injected into the base of VT304 to change it from cut-off to conduction. When VT304 is turned on, the collector current increases linearly and generates a constant voltage in terminals ⑩~11 of the feedback coil, allowing VT304 to remain turned on. The switching regulator tube cycles on and off again and again, forming an oscillation. During the cut-off period of VT304, the energy stored on terminals ⑨~② of the input coil is transmitted to the output coil and coupled to terminals 13~12 of the sampling coil. After rectification and filtering, it is sent to the error amplifier tube VT301, and passes through VT30l, VT302, VT303 A control circuit is formed to adjust the pulse width of the switching tube VT304 to stabilize the output voltage.