Power Electronics Circuits
In a synchronous buck converter, which of the following correctly describes the path of the inductor current during the dead time when both the upper and lower MOSFETs are off?
- AThe inductor current stops completely, but only during the dead time
- BIt flows through the body diode of the lower MOSFET (or a Schottky barrier diode in parallel with it)
- CIt passes through the gate insulator of the upper MOSFET and flows back from the gate-driver supply to the input supply
Show answer and explanation
Answer: B. It flows through the body diode of the lower MOSFET (or a Schottky barrier diode in parallel with it)
The inductor current cannot stop abruptly, so while both MOSFETs are off, the body diode of the lower MOSFET conducts and keeps the current flowing. The body diode has a large forward voltage and a long reverse recovery time, so too long a dead time increases losses. Too short a dead time, on the other hand, lets the upper and lower MOSFETs be on together, and a shoot-through current short-circuits the input. The dead time is therefore kept to the minimum the devices allow, and a Schottky barrier diode with a low forward voltage is sometimes placed in parallel.
- A. Trying to stop the inductor current abruptly produces a high voltage. The current keeps flowing through another path.
- B. Correct. During the dead time the body diode takes over the current.
- C. The gate is insulated and is not a path for the main current.