Separately excited switching regulated power supply circuit three

The figure shows a separately excited switching power supply circuit using a self-excited multivibrator as a pulse generator. In the figure, VTl is the switching tube; VT2 and VT3 are push tubes; VT4 is the emitter follower. The separately excited oscillation circuit is a self-excited multivibrator composed of VT5, VT6, C2, C3, R3, R4, etc. VT7 and VT8 form an emitter-coupled differential amplifier for error amplification. The sampling circuit is composed of resistors R7, R8 and potentiometer R9 connected in series. The reference voltage source circuit consists of zener diodes VD4 and R5. The diodes VDl and VD2 are used to prevent the emitter junctions of the push tubes VT2 and VT3 from being broken down by reverse voltage. VD3 is the freewheeling diode; L is the energy storage inductor. The conduction and cutoff of the switching regulator tube are flipped by the multivibrator and controlled by the emitter follower VT4. The switching time of the multivibrator is determined by the error amplifier. In other words, the flip time of VT5 from cut-off to on is determined by the charging current of VT8 to C3. The greater the current of VT8, the longer the cut-off time of VT5. In the same way, the flip time of VT6 from cut-off to on is determined by the charging current of VT7 to C2. The greater the current of VT7, the shorter the VT5 cut-off time. When the output voltage drops for some reason, the collector current of the VT8 tube of the differential amplifier decreases, and the collector current of the VT7 tube increases, which shortens the conduction time of VT5 and lengthens the cutoff time, causing the conduction time of VT4 to become longer. The cut-off time becomes shorter, and ultimately the conduction time of the control switch tube is lengthened and the cut-off time is shortened, allowing the output voltage to rise and maintain the stability of the output voltage.