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A buck converter operates in continuous conduction mode (CCM) when the inductor current never reaches zero during a switching cycle, and in discontinuous conduction mode (DCM) when it does. The boundary between the two occurs exactly when the inductor's ripple current valley touches zero, i.e. when the ripple current ΔIL equals twice the average (DC) output current. This calculator finds either the minimum inductance needed to stay in CCM down to a specified light-load current, or, given a chosen inductor, the load current below which the converter will drop into DCM.
| Quantity | Formula |
|---|---|
| Critical (minimum) inductance | Lcrit = (Vin−Vout)×D/(2×f×Iout(min)) |
| Boundary load current for given L | Iboundary = (Vin−Vout)×D/(2×f×L) |
| CCM condition | Iout > Iboundary (or L > Lcrit for that Iout) |
| DCM condition | Iout < Iboundary |
Both formulas are the same relationship solved for different variables. Note that the CCM/DCM boundary depends on load current, so a converter can be in CCM at full load and naturally transition into DCM as the load is reduced — this is normal behavior, not a fault, and many controllers are specifically designed to handle both modes.
The load current (or inductance) at which the inductor's ripple current valley just touches zero — above this load (or above this inductance), the converter stays in continuous conduction mode (CCM); below it, the converter enters discontinuous conduction mode (DCM).
No, DCM is a normal, expected operating mode at light load and is not harmful; many controllers are specifically designed to operate correctly in both CCM and DCM, and some intentionally use light-load modes (PFM, pulse-skipping) to improve light-load efficiency.
CCM has simpler, more predictable small-signal behavior for the control loop, generally lower peak/RMS currents for the same average current, and often better output ripple and cross-regulation in multi-output designs.
The converter's dynamics fundamentally change in DCM (effectively becoming a different, typically simpler first-order system without the inductor's resonant behavior), which is why compensator designs sometimes need different gain/bandwidth settings, or automatic mode-dependent compensation, to remain stable across both modes.
Use an inductor at or above the critical inductance Lcrit calculated for your expected minimum load current; note this typically requires a larger, more expensive inductor than one optimized only for full-load ripple.
A lower minimum load current requires the ripple current to be correspondingly smaller (since the boundary is ripple=2×load), and smaller ripple for the same voltages/frequency requires a larger inductance.
The same underlying principle (ripple current touching zero at the boundary) applies to all buck-derived topologies, but the specific volt-second relationship differs for boost and buck-boost, so a different formula (using their respective duty cycle expressions) would be needed for those topologies.
Only down to some non-zero minimum load current — as load approaches zero, any finite inductance will eventually enter DCM, since the boundary current scales with 1/L but never reaches exactly zero for a finite L.
Higher switching frequency reduces both the critical inductance needed and the boundary current for a given inductor, since ripple current (and thus the DCM threshold) scales inversely with frequency.
DCM means the inductor current reaches zero and stays there for part of the cycle; BCM (also called critical conduction mode) means the converter operates exactly at the boundary, with the next switching cycle starting the instant current reaches zero — a specific control technique some converters use deliberately.
Yes — DCM often provides faster transient response for a sudden load increase (since the inductor is "empty" at the start of each cycle and can ramp current quickly), which is one reason some designs intentionally favor DCM operation at light-to-moderate load.
No — many commercial power supplies operate happily in DCM at light load with no issues; guaranteeing CCM is only necessary when the application specifically requires the tighter regulation, lower ripple, or simpler control dynamics that CCM provides.
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