Power Electronics Circuits
In an ideal buck converter (freewheeling through a diode) with an input voltage of 12 V, an output voltage of 3 V, an inductance of 15 µH and a switching frequency of 300 kHz, the load current is gradually reduced. Which of the following is the minimum output current that keeps continuous conduction mode (the boundary with discontinuous conduction mode)?
- A0.25 A
- B0.5 A
- C1 A
Show answer and explanation
Answer: A. 0.25 A
The inductor current is a triangular wave around its average (the output current) with a peak-to-peak ripple of ΔI = 9 × 0.25/(15 µH × 300 kHz) = 0.5 A. Its valley is "output current − ΔI/2", so when the output current falls below ΔI/2 = 0.25 A, the valley reaches zero and the converter enters discontinuous conduction mode. For the 3 V output this happens when the load resistance exceeds 12 Ω (a lighter load). In a synchronous buck converter the inductor current can also flow in the negative direction, so it can keep operating in continuous conduction mode even at light load.
- A. Correct. The boundary is half the ripple, ΔI/2 = 0.25 A.
- B. This is the ripple itself, twice the boundary value.
- C. This is half of the 2 A ripple obtained when the duty cycle is forgotten.