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actually the Vd has same shape of the transformers primary side's waveform. look at my drawing. the amplitude is proportional to the ratio of transformer n1:n2. the switching element's maksimum voltage(Vsw max) is about 2 x Vdc voltage in theory. but it will be a little higher in practise.
please search forward converter waveforms if i was wrong
Curiously, you edited the original post and presented a completely new circuit. No problem in this case, because nobody referred to
the original circuit yet, but a bad practice anyway.
Second comment: An "oscilloscope measuring point" is undefined without telling what's the ground reference. If your waveform is
meant as the transformer secondary voltage, why do you name it vd rather than vsec?
Third comment: If the transformer polarity is drawn correct, you have a forward converter without a path for the flyback voltage.
The waveform at the transformer must be expected to show a high flyback peak in this case, very different from your claimed waveform.
So the new circuit makes the waveform look even more unlikely.