CCM vs DCM
What are CCM and DCM? How do they change the conversion ratio, efficiency and control loop of a buck converter?
| Level | ●● onsite technical |
| Track | Power |
| Really testing | Light-load behaviour and boundary condition. |
Going deeper
Extra explanation that is not in the book: worked numbers, where people go wrong, and how to say it in the room.
Where the boundary is — worked. 12 V → 3.3 V, 500 kHz, 4.7 µH gives ΔI ≈ 1.0 A, so the converter goes discontinuous below about 0.5 A of load. Double L and the boundary halves.
What changes in the loop. In DCM the inductor starts every cycle empty, so no energy is carried from cycle to cycle. The LC double pole disappears and the power stage behaves like a single pole — the compensation designed for CCM can be too slow at light load. In a boost, the right-half-plane zero moves to very high frequency in DCM.
What changes in the conversion ratio. In DCM, Vout at a fixed duty cycle rises as the load falls. The loop corrects it by cutting D — you'll see narrow pulses and, eventually, skipped pulses.
Trap in the interview: "A forced-PWM synchronous buck never enters DCM." True — because the low-side FET lets the current go negative. That keeps the ripple low and the loop the same, at the cost of light-load efficiency.
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