Buck converter inductor selection is often reduced to reading a value from a reference design and ordering that number of microhenries. The value sets the ripple current, which is only the first of several decisions. Saturation current, RMS heating current, DC resistance and core material determine whether the part you chose works reliably or quietly limits the whole converter.
What the inductance value actually controls
In continuous conduction the inductor current is a triangle riding on the DC output current. The peak-to-peak height of that triangle is set by the voltage across the inductor, the time the switch is on, and the inductance. A larger inductance produces a smaller triangle and a smaller inductance a larger one. Designers usually target a ripple that is a moderate fraction of the maximum output current, because that choice balances competing effects. Too much ripple raises the peak current the switch and the inductor must handle, increases core loss, and increases output ripple voltage. Too little ripple means a physically large inductor with more turns, more winding resistance and worse transient response. The worst-case ripple usually occurs at maximum input voltage, so calculate there rather than at the nominal input.
Three current ratings, not one
The peak current, which is the DC output current plus half the ripple, must stay below the inductor’s saturation rating with margin, and that rating must be read at the highest core temperature the design allows. The RMS current, close to the DC output current for modest ripple, must stay below the inductor’s heating rating, which is defined by a stated temperature rise. These are independent limits with independent failure modes: exceeding saturation collapses the inductance and lets current run away within a switching cycle, while exceeding the thermal rating slowly cooks the winding and its insulation. A third consideration is the current during abnormal conditions, including startup inrush, a load step and short circuit recovery, where the peak can far exceed anything in the steady-state calculation.
Resistance and loss are not free either
Winding resistance appears directly in the efficiency figure, dissipating the square of the RMS current times the DC resistance, and it also adds to the output voltage error in converters that sense current through the inductor. But quoting DC resistance alone understates the loss, because at switching frequencies the current crowds toward the surface of the conductor and toward the edges of a flat winding, raising the effective resistance. Core loss adds to this, depending on the ripple amplitude and the switching frequency, and it does not appear in any DC measurement. Two inductors with identical inductance and identical DC resistance can therefore differ noticeably in a working converter. Manufacturers of power inductors usually publish loss or temperature-rise data as a function of ripple and frequency for exactly this reason.
Shielding, layout and the noise consequences
An unshielded inductor radiates a magnetic field that couples into nearby traces and components, which becomes a problem when the converter sits close to a sensitive analog circuit, a radio, or a high-resolution converter. Shielded and semi-shielded constructions reduce this at some cost in size, price and sometimes efficiency. Placement matters as much as the part: keep the switching loop small, keep the feedback network away from the inductor’s field, and avoid running sensitive traces underneath the component. Mechanical noise deserves a mention too, since some inductor constructions can emit audible sound when the converter operates at a light-load frequency that falls in the hearing range. That is not a fault, but it is a product problem if the equipment is meant to be silent.
Verifying the choice on the bench
Measure the inductor current directly with a current probe or a small series sense element and look at the waveform under worst-case conditions. A straight-sided triangle means the core is in its linear region; any upward bend near the peak means saturation is beginning. Check at maximum load, minimum input, maximum input, during startup and during a load step, with the enclosure closed and at the highest ambient the product must tolerate. Measure the inductor’s own temperature rise at the same time, separately from the converter’s other hot parts. Then compare the measured ripple against the calculation, because a significant discrepancy usually means the inductance at working current is lower than the marked value, which the manufacturer’s inductance-versus-current curve will confirm.
What to check before you act
- Calculate ripple at maximum input voltage, where it is largest.
- Compare peak current with saturation rating and RMS current with the thermal rating.
- Look at loss data at your switching frequency, not just DC resistance.
- Scope the inductor current during startup, load steps and fault recovery.
Common questions
Can I substitute an inductor with the same value from another manufacturer?
Only after comparing saturation current, RMS rating, DC resistance, loss at your frequency and shielding. Identical inductance says very little about how a part will behave in a converter.
What ripple current should I design for?
A moderate fraction of maximum output current is the usual starting point, chosen to balance peak current against inductor size and transient response. The controller datasheet normally recommends a range for its own architecture.
The practical takeaway
Use the inductance value to set ripple, then select the actual part on saturation current, thermal rating and loss at your switching frequency. Confirm the choice by looking at the real current waveform under worst-case load, input and temperature, and treat any bend near the peak as a reason to change the part.
References and further reading
- Department of Energy: how electronics losses turn into wasted energy and heat
- FCC: electromagnetic compatibility and radio frequency safety questions answered
- ENERGY STAR: what a product must demonstrate to earn certification
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
