Question 8-18B2

In a switching power supply, a Field-Effect Transistor (FET) functions as a high-speed electronic switch. When the FET is turned fully "on," its channel becomes conductive, offering a specific, very low resistance (known as Rds(on)) between the drain and source. This allows current to flow with minimal voltage drop, similar to how a pure, low-value resistor conducts current. When the FET is turned completely "off," its channel effectively blocks current flow, presenting an extremely high impedance (ideally infinite). This characteristic of preventing current passage is analogous to an open mechanical relay, which physically breaks a circuit. Other options are incorrect because inductors and capacitors store energy or oppose changes in current/voltage rather than providing a simple resistive path or complete block. While 1-Megohm or 10-Megohm resistors represent high resistance, an "off" FET's impedance is significantly higher, approaching an open circuit, which a mechanical relay best represents.