Forced CCM vs DCM

Part 20 · Voltage Regulation · page 1 of 3

Key point: a sync FET can pull inductor current backward. A diode cannot. Same buck. The rectifier is what changes.

12 V to 5 V. D is 41.7 %. Gold dash is \(I_{L,\mathrm{pk}}\). Grey line is 0 A.

1
The triangle stays above zero.
IOUT 1.5 A, L 4.7 µH, 250 kHz. Diode box off. Δi_L is 2.48 A. I_L,pk is 2.74 A. I_L,min is 0.26 A. That peak is the switch current you size the FET for.
2
The valley crosses zero.
Same L and 250 kHz. IOUT 0.35 A. I_L,min is −0.89 A. I_L,pk is 1.59 A. The equation does not stop at zero.
3
A diode stops the current.
Tick Diode / DCM. Same 0.35 A. Current hits 0 and stays there. I_L,pk is 1.32 A. The rest of the period is idle. That idle is where SW rings, on the next page.
4
More L, and the valley is positive again.
Diode box off. L 22 µH, still 0.35 A. I_L,min is 0.08 A. I_L,pk is 0.62 A. Mode says FCM, still ≥ 0. You did not need reverse current.
5
A faster clock still reverses.
Back to 4.7 µH, fsw 500 kHz, FCM, 0.35 A. I_L,min is −0.27 A. I_L,pk is 0.97 A. The clock stayed fixed. You switch twice as often and the FET still pulls current backward. That is the light-load cost of forced CCM.

Valley current-mode, diode reverse recovery, and the loop are not in this sketch. It is the buck inductor current.

Buck cell · FCM vs DCM

12 V → 5 V · gold dash = \(I_{L,\mathrm{pk}}\) · grey = 0 A

IOUT A L µH fsw kHz

Diode / DCM

0 µs

Inductor current

Switch node

Mode—

I_L,pk—

I_L,min—