Two transistor, diode clamped single-ended forward converter circuit

The figure below is a two-transistor, diode-clamped single-ended forward converter circuit. VTl and VT2 are turned on and off at the same time by PWM pulse excitation. When they are turned off, the polarity of the voltage on N1 is reversed, diodes VD3 and VD4 conduct, and the potential is clamped. Therefore, the voltage amplitude applied between the collector and emitter during the cut-off period of VTl and VT2 is equal to the input voltage Ui.