A charger supplies 100 W at 20 V for charging the battery of a laptop. The power devices, used in the converter inside the charger, operate at a switching frequency of 200 kHz. Which power device is best suited for this purpose?
IGBT
Thyristor
MOSFET
BJT
ANSWER
The MOSFET is best suited, so the answer is option (C).
Official answer: CSource: Official answer key, GATE 2022 EE (IIT Kharagpur)
Operating point: at , so the output current is . This is a low-voltage, low-power converter.
The switching frequency is . The device must switch fast, because switching loss grows in proportion to .
Thyristor (SCR): it latches on, has no gate turn-off, and needs turn-off times of tens of microseconds. It cannot switch at 200 kHz, so it is ruled out.
BJT: storage time and the current-controlled base drive limit it to a few tens of kHz, so it is not suitable.
IGBT: tail current at turn-off limits it to roughly 20-50 kHz. It is also meant for high voltage and high current, and its conduction drop of about 1.5-2 V is large compared with 20 V.
Power MOSFET: a majority-carrier device with no tail current, switching in tens of ns and working well at hundreds of kHz. A low-voltage MOSFET has a very small , so the conduction loss is small at 5 A.
Hence the MOSFET suits a low-voltage, low-power, high-frequency converter. Answer: (C).
KEY IDEA
Choose the power device from the voltage level and the switching frequency. For a low voltage of about 20 V and a frequency of 200 kHz, the MOSFET is the right choice. It is a fast, majority-carrier device with low and no tail current.
WATCH OUT FOR
Picking the IGBT because it is the usual modern power switch. Its turn-off tail current limits it to about 20-50 kHz, and its 1.5-2 V conduction drop is a large share of a 20 V supply. Another slip is thinking the BJT or thyristor can reach hundreds of kHz.
From the GATE 2022 EE paper. Every question of this paper · Download the full paper · All 9 Switches questions, with notes
