Pulse modulated switching power supply application circuit

As shown in the figure is an example of a pulse width modulated switching power supply, which is the Kaige 4D17U model produced by Shanghai Radio No. 4 Factory. Stabilized power supply for 35cm black and white TV. Its main technical indicators are as follows: Output voltage: 12V; Output current: 1A; Power consumption <20W; Efficiency>64%; Internal resistance <0.075Ω; Ripple <15mV; Mains voltage: 160~240V. In the figure, 6VDl and 6VD2 form a full-wave rectifier circuit; 6C3 is a filter capacitor; 6C1 and 6C2 are used to weaken the surge current; 6VT7, 6VT2, 6VT3 and 6R4 and 6C9 form a self-excited multivibrator, and 6VT7 is a switch. Tube. When the 12V output voltage decreases for some reason, the voltage obtained by the base of the error amplifier tube 6VT5 from the sampling circuits 6R6, 6R9, ​​and 6R8 also decreases, the collector voltage of 6VT5 increases, and the base potential of 6VT3 also increases, and the switch The output pulse of the regulating tube 6VT7 becomes wider, causing the reduced voltage to rise again. On the contrary, when the output voltage decreases, the opposite adjustment is performed according to the above process to keep the output voltage stable.