Power Inductors
Power inductors store magnetic energy while carrying substantial current. They are essential parts of buck, boost, flyback-related, multiphase, and many other switching power circuits. Their job is not only to provide a stated inductance, but to do so with acceptable loss, temperature rise, and saturation margin.
Energy storage
The inductor repeatedly stores and releases energy as the switching devices turn on and off. In a buck converter, for example, the inductor smooths the pulsed switch-node voltage into a more continuous output current.
Current ripple
For an approximately constant voltage across an inductor during one switching interval:
More inductance produces less current ripple for the same voltage and time. Smaller inductance permits faster current change but may increase ripple, losses, peak current, and output-capacitor stress.
Saturation current
A magnetic core loses incremental permeability as it approaches saturation. The inductance therefore drops as current increases. Manufacturers often quote an Isat value at which inductance has fallen by a stated percentage.
RMS current and temperature rise
The winding's DCR causes copper loss:
An RMS-current rating is usually tied to an allowed temperature rise under specified test conditions. Ambient temperature, board copper, airflow, nearby heat sources, and switching loss all affect the actual temperature.
Core loss
An alternating magnetic field also heats the magnetic core. Core loss depends on flux swing, frequency, material, temperature, and waveform. In high-frequency converters, core loss can be as important as copper loss.
Ferrite versus powdered/composite cores
Ferrite power inductors often maintain inductance fairly well until a distinct saturation knee is reached. Powdered-iron and composite materials often show a softer, more gradual reduction in inductance with current. Neither is automatically superior; the converter's frequency, ripple, peak current, temperature, size, and efficiency goals determine the best choice.
Shielded and unshielded packages
A shielded inductor confines more of its magnetic field, reducing interaction with nearby circuits. An unshielded part may be smaller, cheaper, or better cooled, but can radiate more magnetic flux into sensitive traces and components.
DCR versus size
Lower DCR generally requires thicker copper or a larger winding area. This creates a familiar design tradeoff: smaller inductors save board space, while larger parts often provide lower resistance, lower temperature rise, or greater current capability.
Choosing a power inductor
- Required inductance at the actual DC bias current.
- Peak current and saturation margin.
- RMS current and temperature rise.
- DCR and copper loss.
- Core loss at switching frequency and ripple level.
- Operating and maximum temperature.
- Shielding and magnetic-field sensitivity of nearby circuitry.
- Package size, mounting, vibration, and reliability requirements.