Actual circuit of step-down chopper switching regulated power supply

The picture shows the actual circuit of a step-down chopper switching regulated power supply. VTl is the adjustment switch tube, and VT2 and VT3 are two-stage push tubes. The diodes VDl and VD2 connected to the emitters of the two push tubes are to prevent reverse breakdown of the emitter junctions of the transistors. The turn-on and cut-off time of VTl is determined by the self-excited multivibrator composed of VT5 and VT6, which is isolated and amplified by the VT4 emitter follower. The switching time of the multivibrator depends on the emitter-coupled differential amplifier composed of VT7 and VT8. The cut-off time of VT5 from cut-off to conduction is determined by the charging current of VT8 to C3. If the current of VT8 is large, the cut-off time of VT5 will be short; similarly, the flip-over time of VT6 from cut-off to conduction is determined by the charging current of VT7 to C2. The current of VT7 is large, and the cut-off time of VT6 is short. The magnitude of the currents of VT7 and VT8 depends on the potential difference between the bases of the two tubes. The base of VT7 is connected to the reference voltage source circuit composed of zener diode VD4 and resistor R5, and the base of VT8 is connected to the sampling circuit. When the output voltage Uo drops, the current of VT8 decreases, the current of VT7 increases, the cut-off time of VT5 is extended, and the conduction time is shortened. The opposite is true for VT6, so the conduction time of VT4 is lengthened and the cut-off time is shortened. At the same time, the tube VT2, The conduction time of VT3 switch tube VT1 is lengthened and the cut-off time is shortened, causing the output voltage to rise. To achieve the purpose of voltage stabilization. When the output voltage of this circuit changes by ±20%, the output changes within ±0.5%, and the efficiency reaches 93%.