Waveform at the blocking MOSFET transistor gate
The somewhat weird waveform at the gate of the driving MOSFET in the blocking output stage is shown below. Instead of what one would await (a rectangular pulse), the shape seems to indicate some capacitors that trickle in at one point.
Reading application note AN11158 from NXP, the answer is revealed in Section 2.6.2.1, where the three transistor charges are presented. The answer lies in the Miller capacitance. The flat region on both rising and falling edges of the gate voltage is called the Miller plateau and is due to the MOSFET's capacitance changing on the threshold voltage. This change in capacitance causes the voltage to stay constant. After some time, the change in capacitance stops and the gate voltage increases further.
The length in time of the Miller plateau is highly dependent on the circuit, as both the drain current, the drain-to-source voltage and the transistor load (in our case, inductive), affect its shape.
The same section of the NXP application note offers pointers on how to decrease this Miller plateau. By decreasing the drain-to-source voltage the length of the plateau is shortened, and by decreasing the drain current the threshold voltage is lowered.
Theodor-Adrian Stana, Jul. 2014