A double pulse measurement for an inductively loaded circuit controlled by the IGBT switch is carried out to evaluate the reverse recovery characteristic of the diode, D, represented approximately as a piecewise linear plot of current vs time at diode turn-off. L _par is a parasitic inductance due to the wiring of the circuit and is in series with the diode. The point on the plot (indicate your choice by entering 1, 2, 3 and 4) at which IGBT experiences the highest current stress is _________.

The correct point on the plot at which the IGBT experiences the highest current stress is 3.

Understanding Circuit Dynamics during Reverse Recovery:
In a double-pulse test setup for an inductively loaded circuit controlled by an IGBT switch, when the IGBT turns back on (second pulse), the load current I_load is initially freewheeling through the diode, D, and parasitic inductance, L_par. As the IGBT turns on, the diode undergoes reverse recovery.

Current Breakdown:
By Kirchhoff's Current Law (KCL) at the node connecting the IGBT emitter, the load branch, and the diode branch:

$i_{\text{IGBT}}\left(t\right)=I_{\text{load}}-i_{\text{D}}\left(t\right)$

During diode reverse recovery:
1. At point 1, the diode current begins to decline from its positive forward current.
2. At point 2, the diode current crosses zero, meaning $i_{\text{D}}=0$, so the IGBT current equals the load current ($i_{\text{IGBT}}=I_{\text{load}}$).
3. At point 3, the diode current reaches its peak negative value, known as the reverse recovery current ($i_{\text{D}}=-I_{\text{rm}}$). Substituting this negative peak into the equation yields:

$i_{\text{IGBT, max}}=I_{\text{load}}-(-I_{\text{rm}})=I_{\text{load}}+I_{\text{rm}}$

Therefore, the IGBT carries the sum of the load current and the maximum peak reverse recovery current of the diode at point 3, subjecting it to the maximum current stress.