Upon further investigation, you are able to reverse engineer the converter, and arrive at the schematic drawing of Fig. 1. Moreover, you notice that the transistor labeled Q1 is operating with a fixed frequency of 100 kHz, and fixed duty cycle of 0.5 (for all input voltages). The transformer is also labelled Lu1 = 200 H. You conclude that somehow the switches S1-S4 are managing to keep the rms output voltage fixed by adjusting their phase angle