Flyback switching power supply with two sets of transistors and diode clamps

The picture below is a flyback switching power supply with two sets of transistors and diode clamps. Since VTl and VT2 are excited at the same time by the control pulses, when they are turned on, the input voltage Ui is applied to the primary winding N1 of the transformer T; when VTl and VT2 are cut off, the polarity of the voltage on the winding N1 is reversed, and when it reaches Ui , diodes VD2 and VD3 are turned on, and the voltage on N1 is clamped at Ui. Therefore, the voltage amplitude applied across each transistor will not exceed the input voltage Ui. If the circuit works directly under grid rectification, the maximum DC input voltage is about 342V. Transistors with a withstand voltage of 450V can be used. The disadvantage of this circuit is that it uses a large number of transistors, has a large driving power, and needs to be turned on and off at the same time.