In this question we will cover the switching transient of a power diode with some modified

modeling assumptions. Following are the assumptions to be considered for analysis in this

question. Neglect other non-idealities such as parasitic inductance, etc.:

i. The device has a fixed depletion capacitance, Cj=1nF, under reverse bias and no

effect under forward bias.

ii. The device is considered to be in reverse bias when Cj is charged.

iii. The device has a fixed diffusion capacitance, Cd=50nF, for buildup of injected charge

under forward bias, 100nC≥Qd≥0 and then does not accumulate any more charge.

iv. The ON-resistance of the diode is 0.5Ω for Qd<60nC and reduces to 10mΩwhen

Qd≥60nC due to conductivity modulation.

v. Diode has zero reverse saturation current.

The diode is initially reverse biased with VD=-1000V at the terminals and is turned on

with a step current of Io=100A at t=0.

a) What are the values of Qd, Qj, Cd and Cj at t<0. [2]b) What is the voltage VD at the diode terminals at t=0+.