In a switching power supply, what is the role of a flyback diode?

• A. Blocks current during on-state
• B. Increases switching frequency
• C. Provides a path for inductive kick when the switch opens; prevents voltage spike; stores energy in inductor
• D. Dissipates energy as heat in the switch

Answer: (C)

The key idea is that the flyback diode gives the inductor a safe, continuous path for current when the switching transistor turns off. As the switch opens, the current in the inductor wants to continue, so it generates a voltage to keep that current flowing. The diode conducts, directing that energy into the output capacitor and load, which both transfers the stored magnetic energy to the output and clamps the voltage to a safe level for the switch. This protects the switch from a damaging voltage spike and enables efficient energy transfer from the inductor to the output. The diode isn’t about increasing frequency or dissipating energy in the switch; it’s about providing the return path for the inductive current and delivering energy to the output during the off-time.