How to change the output voltage of ringing switching power supply

In a simple ringing choke switching regulator circuit, the output voltage Uo is proportional to the negative bias voltage Uc. If you want to change the output voltage, you must find a way to change Uc, as shown in the figure below. The collector of transistor VT2 is connected to the negative terminal of capacitor C2. Once Uc2 rises, the base current through the zener diode VD2 and transistor VT2 increases, causing VT2 to conduct. Since the VT2 collector current shortens the conduction time of the switching transistor VTl than before, it can be cut off early, thereby reducing the output voltage Uo. Conversely, once Uc decreases, the circuit changes to the cut-off direction of VT2, causing the base current of VT1 to increase and the conduction time to extend, so the output voltage increases. During voltage stabilization control, the expression of Uc is: It can be seen that changing the ratio of ruler 1 and R2 can change the output voltage. If R, and R2 are replaced with potentiometers, the output voltage Uo can be adjusted arbitrarily.