Ringing switching power supply overcurrent protection circuit

In the ringing choke switching power supply, the output current Io and the primary current Ic are not in a one-to-one correspondence. That is to say, for the same collector current Ic, as the input voltage increases, Io also increases. Therefore, in the ringing switch circuit, in addition to adopting startup protection measures, a protection circuit must also be installed, as shown in the figure. Add RA and RB to the base branch of VT2. Since the voltage on C3 is a positive voltage proportional to the input voltage, once the input voltage Ui increases, the current flowing through the resistor RA increases, accelerating the conduction of VT2 and playing a current limiting role. Therefore, the output voltage Uo and the voltage Uc2 of C2 will decrease, so the negative current flowing through RB will decrease, the base voltage of VT2 will increase, and VT2 will be deeply saturated and turned on, thus reducing the output current.