Power Electronics Circuits
In a step-down chopper (buck converter), which of the following correctly describes the path of the inductor current while the switch is off?
- AIt keeps flowing from the input supply through the switch into the inductor
- BIt freewheels through the diode and keeps flowing from the inductor to the output
- CThe inductor current drops to zero the instant the switch turns off and flows nowhere
Show answer and explanation
Answer: B. It freewheels through the diode and keeps flowing from the inductor to the output
In a buck converter, while the switch is on, current flows from the input through the inductor to the output and energy is stored in the inductor. When the switch turns off, the inductor tries to keep its current flowing, so the diode (freewheeling diode) conducts and current flows from ground through the diode and the inductor to the output. The current on the output side is therefore uninterrupted, which keeps the output ripple small. Replacing this diode with a MOSFET is synchronous rectification.
- A. While the switch is off, no current flows from the input supply. The input current of a buck converter is pulsating.
- B. Correct. The inductor current keeps flowing along the path through the diode.
- C. The inductor current cannot change abruptly. Trying to stop it suddenly produces a high voltage.