principle
R1 is a fuse, and the power supply is integrated into DC through D1 and C1.
Dc through R2 to make Q1 slightly on, in the transformer to produce positive and negative inductive voltage (not electromotive force), similarly in the feedback stage to produce positive and negative inductive voltage, feedback voltage through C4 to Q1 positive feedback (up), so that Q1 fully on.
After Q1 is fully switched on, the current of the main winding does not change (the maximum), the magnetic flux does not change, the feedback induced voltage is 0, the base of Q1 returns to the starting state (only the current of R2, and will charge a little more to C4), Q1 starts to turn off slightly, and then the induced voltage will reverse, and a positive induced voltage will be generated at the feedback stage. Then through C4 positive feedback (downward), accelerate Q1 off.
After Q1 is completely turned off, the current of the main winding does not change (0), the magnetic flux does not change, the feedback induced voltage is 0, the base of Q1 only has the current of R2, Q1 starts to slightly conduct.