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GATE 2018 EE Q.4: solution and answer

GATE 2018 · EEQ.4 · 1 mark · MCQ

Four power semiconductor devices are shown in the figure along with their relevant terminals. The device(s) that can carry dc current continuously in the direction shown when gated appropriately is (are)

GATE 2018 EE question 4: Four device symbols, each with an upward arrow labelled I drawn at the bottom terminal, showing the current entering the…Tap to open full size
Figure from the GATE 2018 EE paper. It is part of the question.
Figure description

Four device symbols, each with an upward arrow labelled I drawn at the bottom terminal, showing the current entering the lower terminal and flowing upward through the device. (1) Thyristor: anode A at top, cathode K at bottom, gate G on the left side of the cathode end; the arrow I enters at K. (2) Triac: MT1 at top, MT2 at bottom, gate G at lower left; the arrow I enters at MT2. (3) GTO: A at top, K at bottom, gate G at lower left drawn with a cross-bar (GTO symbol); the arrow I enters at K. (4) MOSFET: gate G on the left, drain D at top, source S at bottom; the channel arrow points toward the channel from the source side, so it is an n-channel enhancement MOSFET; the arrow I enters at S, flowing from S to D.

  1. (A)

    Triac only

  2. (B)

    Triac and MOSFET

  3. (C)

    Triac and GTO

  4. (D)

    Thyristor and Triac

Answer and solution

ANSWER

Option (B): Triac and MOSFET.

Official answer: BSource: Official answer key, GATE 2018 EE (IIT Guwahati)

  1. The marked current enters each device at its lower terminal and flows upward. That is from K to A in the thyristor and the GTO, from to in the triac, and from S to D in the MOSFET.

  2. Thyristor: it is a unidirectional device. It conducts only from A to K, and in the K to A direction it blocks. It cannot carry the marked current, whatever the gate signal.

  3. GTO: it is also asymmetric. It conducts only from A to K, and gate turn-off action does not change that. It cannot carry current from K to A.

  4. Triac: it is a bidirectional switch, equivalent to two thyristors in anti-parallel. With a suitable gate pulse it conducts in either direction, so it can carry the current from to .

  5. MOSFET: the gated channel conducts from D to S. The device also has an intrinsic body diode with anode at S and cathode at D. This diode conducts from S to D whatever the gate signal, so the MOSFET can carry the marked current continuously. In the reverse direction the channel, once turned on, also conducts and shares the current.

  6. The devices that can carry the current in the direction shown are therefore the triac and the MOSFET, which is option (B).

KEY IDEA

Compare the direction of the marked current with each device's conduction direction. A thyristor and a GTO conduct only from anode to cathode. A triac conducts both ways. A MOSFET has an intrinsic body diode that conducts from source to drain.

WATCH OUT FOR

Treating the MOSFET as a one-way device from D to S and forgetting its body diode. Also assuming the GTO can carry current both ways because it can be turned off by the gate.

From the GATE 2018 EE paper. Every question of this paper · Download the full paper · All 9 Switches questions, with notes

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