Thank you, Aaron and sorry for the inconvenience. I can't figure out how to attach images by using " [img][/img]".
Quote:for the dV/dt part neat zero, I think the author is simply saying that if there is a time shift (such that there is overlap), you don't want a large change in voltage as this would result in power being burnt in the transistor. i.e. change in voltage / change in time should be zero.
When the transistor is OFF, Vx will increase but it can't increase suddenly because of the presence of the capacitor C1.
We need dVx/dt = 0 around the time transistor switches from ON to OFF. According to second condition the desired value of Vx is the smallest.
Then, apply both second and third conditions, we need Vx at the time when transistor switches from ON to OFF is very very small. Is this right that the voltage Vx is very small at the time transistor switches from ON to OFF?