Separately excited switching regulated power supply part 2

The picture shows a separately excited switching regulated power supply circuit using a single-junction transistor as a pulse generator. VDl~VD4 and C1 are rectifier and filter circuits, which convert the 220V AC grid voltage into 300V DC voltage. The pulse generator consists of a single-junction transistor VT1, resistors R3, R4, R5, a potentiometer Rp and a capacitor C3. Changing the resistance of the potentiometer Rp can change the pulse frequency. C4 is the coupling capacitor of the output pulse. R1 and R2 divide the 300V DC voltage to provide a working voltage of 20~30V to the single junction tube VTl. VT2 is an inverting amplifier tube, which inverts and amplifies the positive polarity pulse output by the pulse generator, and delivers a negative polarity pulse to the switching tube VT4, so that it cuts off when VT1 outputs a positive polarity pulse. VT3, R7 and C5 form the protection circuit of switching tube VT4. When the power is turned on and the pulse generator has not started working, VT4 is in the conductive state, and the voltage drop on R7 increases, which increases the forward bias of the base of VT3, causing its collector current Ic3 to increase and the tube voltage drop Uce3 to decrease. The base potential of the VT4 tube is reduced, which limits the collector current Ic4 of VT4 and keeps it below the safe value. When the circuit is working normally, the pulse generator outputs a pulse, which is invertedly amplified by VT2 and sent to the base of VT4 to cut it off; after the pulse disappears, VT4 is turned on again. It can be seen that when the grid voltage increases, the duty cycle of the pulse generated by the pulse generator increases. After inversion amplification by VT2, the duty cycle of the pulse output by the control switch tube VT4 decreases, causing the output voltage to decrease; conversely, When the grid voltage decreases, the duty cycle of the pulse generated by the single-junction tube VTl decreases, while the duty cycle of the output pulse of VT4 increases, causing the output voltage to rise again, thereby maintaining the stability of the output voltage. This power supply is suitable for fixed loads. , because it only regulates changes in grid voltage.