GATE 2023 EE Q.26: solution and answer

Official answer: A, DSource: Official answer key, GATE 2023 EE (IIT Kanpur)

1. Required behaviour: in the OFF state the switch must block $V_{PQ}>0$ (P positive). In the ON state it must carry current both $P\to Q$ and $Q\to P$. Blocking a negative $V_{PQ}$ is not required.

2. (A) The antiparallel diode conducts $Q\to P$. With the gate off, $V_{PQ}>0$ reverse-biases that diode and the IGBT blocks, so the OFF requirement is met. With the gate on, current $P\to Q$ flows through the IGBT and current $Q\to P$ flows through the antiparallel diode. Both directions are carried, so (A) works.

3. (B) The series diode conducts only $Q\to P$, so current $P\to Q$ cannot pass through the IGBT branch. Current $Q\to P$ can flow through the series diode and then the antiparallel diode. Current therefore flows in one direction only, so (B) fails.

4. (C) The series diode conducts only $P\to Q$. Current $Q\to P$ would have to pass through this series diode, which blocks it, whether or not the antiparallel diode is available. Current flows in one direction only, so (C) fails.

5. (D) Current $P\to Q$ flows through the IGBT and the series diode, which conducts $P\to Q$. Current $Q\to P$ flows through the outer diode, which conducts $Q\to P$. With the gate off and $V_{PQ}>0$, the outer diode is reverse-biased and the IGBT and series diode branch is blocked by the IGBT. The OFF requirement is met, so (D) works.

6. Hence (A) and (D) realise the required switch.