Buck Inductor Calculator

What is a Buck Converter?

A buck converter (also called a step-down converter) is one of the most common DC-DC power supply topologies. It efficiently converts a higher input voltage to a lower output voltage by rapidly switching a MOSFET on and off, then filtering the resulting square wave through an inductor and capacitor. Buck converters are everywhere: laptop chargers, USB power delivery, automotive ECUs, server VRMs. Their popularity comes from high efficiency (often 90%+), simple control, and wide availability of integrated controller ICs.

Why Inductance Matters

The inductor is the primary energy storage element in a buck converter. During the on-time, energy is stored in the inductor's magnetic field as current ramps up. During the off-time, this energy is released to the output. The inductance value directly controls the ripple current — the triangular AC component superimposed on the DC output current. More inductance means less ripple but a bigger part. Less inductance saves board space but increases peak currents and demands more from the output capacitor.

The inductance is set by your input/output voltages, switching frequency, and how much ripple you're willing to tolerate:

A typical design targets 20-40% ripple ratio (ΔI_L / I_out), balancing inductor size against efficiency and output capacitor requirements. The ripple ratio also affects the inductor's peak current, which determines whether the core saturates, and the RMS current, which determines copper losses and heating.

CCM vs DCM Operation

Buck converters operate in one of two fundamental modes. In Continuous Conduction Mode (CCM), the inductor current never reaches zero — it ramps up during the on-time and ramps down during the off-time, but always stays positive. CCM is the normal operating mode at moderate to high loads and provides predictable, well-controlled behavior.

In Discontinuous Conduction Mode (DCM), the inductor current reaches zero before the switching period ends, creating a "dead time" where no current flows. DCM naturally occurs at light loads when the ripple current exceeds twice the average current. While DCM simplifies control loop design, it produces higher peak currents and larger output voltage ripple for the same average current.

The critical inductance (L_crit) marks the boundary between these modes:

This calculator defaults to CCM-optimized inductance and shows you the operating mode automatically. For intentional DCM designs (common in light-load or low-power applications), enable the DCM override in the Advanced section.

Key Design Parameters

• Target inductance: The minimum inductance to achieve your desired ripple ratio. Select the nearest standard value equal to or greater than this target.
• Peak current (I_pk): The maximum instantaneous current through the inductor. This must not exceed the inductor's saturation current rating, which is typically specified at a percentage of inductance drop (e.g., 30% drop).
• RMS current (I_rms): Determines copper losses (I²R heating) in the inductor winding. The inductor's rated RMS current must exceed this value with adequate thermal margin.
• Energy stored: E = ½LI_pk² — the peak energy stored in the inductor's magnetic field each cycle. Useful for comparing inductor sizes and understanding core utilization.
• Volt-microsecond product (V·µs): The voltage-time integral applied to the inductor during each on-time, proportional to core flux excursion. This parameter is critical for selecting core materials and ensuring the core does not saturate.
• Duty cycle: The fraction of the switching period the high-side switch is on. For a buck converter, D = V_out / V_in (ideal).

Practical Design Considerations

• Saturation current: Choose an inductor whose saturation current exceeds I_pk with at least 20% margin. Saturation causes inductance to drop sharply, leading to runaway current.
• Core material: Ferrite cores offer low losses at high frequencies but saturate sharply. Powder iron cores have softer saturation curves but higher core losses. For frequencies above 200 kHz, ferrite is typically preferred.
• DC resistance (DCR): Lower DCR reduces conduction losses but generally requires a larger component. Balance DCR against board space constraints.
• Standard values: Inductors are manufactured in standard values (E12 or E24 series). Round up to the next standard value above your calculated target — a slightly larger inductance reduces ripple, which is generally beneficial.
• Temperature derating: Ferrite inductance typically drops 10-30% at elevated temperatures. Verify the inductor meets requirements at your expected operating temperature, not just at 25°C.