Power Inductor
A power inductor stores energy in a magnetic field and releases it as a current source when the switch in a DC–DC converter opens. It is the bulk energy reservoir that lets switching regulators move charge from input to output efficiently.
Many turns of insulated copper wire are wound on a ferrite or powdered-iron core. When voltage is applied across the winding the magnetic flux ramps up at di/dt = V/L, storing energy ½·L·I². When the source is removed the field collapses and the winding becomes a current source whose voltage flips polarity to maintain the same current — that's what charges the output cap of a buck converter every cycle.
In plain terms
A flywheel for current — once electrons get going through it they don't want to stop, which is exactly what a buck/boost regulator needs to ferry energy each cycle.
Why designers use it
- Form the energy-storage element of every buck and boost regulator.
- Smooth pulsating current in audio amplifier rails.
- Filter EMI on motor leads.
Best for
- Buck / boost
- EMI filter
- DC-DC core
Key specifications
- Inductance: 0.1 µH – 1 mH
- Saturation current Isat: 1 A – 100 A
- DCR: 1 mΩ – 500 mΩ
- Self-resonant: 1 MHz – 200 MHz
- Core type: Ferrite, alloy powder, metal composite
When not to use it
- On signal lines where a ferrite bead is enough — power inductors are big, expensive, and saturate hard.
- Above a few MHz of switching frequency unless the core is a high-frequency NiZn or air-core spiral.
- Where the saturation current sits below the worst-case peak — a buck inductor in saturation looks like a wire and lets current run away.
Common mistakes
- Specifying by inductance only and ignoring Isat — a 4.7 µH part rated 1 A in a 3 A converter saturates before the load even hits steady-state.
- Choosing the smallest case to save area, then watching the converter burn 20 % efficiency as DCR copper losses dominate.
- Routing the high-side switch node trace under the inductor — radiated coupling injects noise into the feedback divider and the loop oscillates.
Where you will find it
- A metal detector's search coil is essentially a large power inductor: the controller drives it with a pulsed current to create a magnetic field, then listens to the decaying field during the off-time — a buried metal object distorts that decay in a measurable way, revealing its presence.
- A laptop's CPU power stage uses a pair of shielded power inductors in a two-phase buck: each inductor stores and releases energy on alternate half-cycles, halving the output ripple and allowing the regulator to respond to the processor's 100 A load steps in microseconds.
- An electric vehicle's on-board charger uses a large ferrite-core inductor in its PFC boost stage to draw sinusoidal current from the AC mains — without it, the rectifier would gulp current in narrow spikes that overload the building's wiring and fail harmonic-content regulations.
A short history
A power inductor is part of the energy-transfer and filtering circuit in many switching DC-DC converters. Working with a capacitor, it helps smooth the switched waveform into a useful DC output. The chosen inductance affects ripple current and the converter's response when its load changes.
Good to know
- A 'shielded' power inductor wraps the winding in ferrite to keep stray flux from coupling into nearby traces — the test for a good layout is to bring an unshielded inductor next to the SMPS and watch jitter on a scope.
- Saturation behaviour matters more than rated current: an inductor pushed past Isat suddenly looks like a wire and lets current explode in microseconds.
- Metal-composite cores (powdered iron in epoxy) replaced ferrite in many converters in the 2010s for better DC-bias linearity and quieter operation.