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?

The correct answer is MOSFET.

To determine the most suitable power semiconductor device, let us analyze the given operational parameters of the converter inside the charger:
1. Power Rating (P): 100 W (Low to moderate power level)
2. Voltage Rating (V): 20 V (Low voltage level)
3. Switching Frequency (f): 200 kHz (High switching frequency)

Now, let us calculate the operating current:
$I=\frac{P}{V}=\frac{100\text{W}}{20\text{V}}=5\text{A}$

Let us compare the characteristics of the given power devices under these operating conditions:

1. Thyristor:
Thyristors are designed for very high-power, high-voltage applications but are limited to very low switching frequencies (typically below 1 kHz). They cannot operate at 200 kHz.

2. BJT (Bipolar Junction Transistor):
BJTs are current-controlled devices. They suffer from minority carrier storage time, which slows down their turn-off speed. Consequently, their switching frequency is generally limited to around 10 kHz to 20 kHz.

3. IGBT (Insulated Gate Bipolar Transistor):
IGBTs combine the gate drive advantages of a MOSFET with the low conduction loss of a BJT. However, they are optimized for high-voltage (usually > 600 V) and medium-power applications, with typical switching frequencies limited to below 50 kHz due to the "current tail" during turn-off.

4. MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor):
Power MOSFETs are majority carrier devices, which means they do not suffer from minority carrier storage delays. This allows them to switch at very high speeds, making them ideal for high-frequency operations (typically from 100 kHz to several MHz). Furthermore, they have very low conduction losses (on-resistance) at low voltage levels (such as 20 V).

Therefore, for a low-voltage (20 V), low-power (100 W) converter operating at a high switching frequency of 200 kHz, the MOSFET is the best-suited power device.