Inductors#

A coil of wire around a core (or air). Inductors store energy in magnetic fields and resist changes in current. They’re essential in power conversion, filtering, and RF — and the component most likely to produce unexpected behavior.

The Basic Relationship#

V = L × (dI/dt)

The voltage across an inductor is proportional to the rate of change of current through it. Constant current = zero voltage. Changing current = voltage. Sudden interruption of current = very large voltage (because dI/dt is huge).

This is why inductors create voltage spikes when current is switched off abruptly, and why flyback diodes are essential on inductive loads.

Stored Energy#

E = ½LI²

The energy is in the magnetic field. It’s proportional to current squared, so doubling the current quadruples the stored energy. When the current path is interrupted, this energy has to go somewhere — typically into a voltage spike that can destroy semiconductors.

Where Inductors Live#

Power Conversion#

Switching regulators (buck, boost, buck-boost) use inductors as the energy storage element. The inductor charges during one phase of the switching cycle and delivers energy during the other. Inductor selection (inductance, saturation current, DCR) is critical to converter performance.

Filtering#

LC filters provide sharper roll-off than RC filters. Used in power supply output filtering, EMI filtering, and RF signal conditioning. The LC resonant frequency is f = 1 / (2π√(LC)).

RF and Signal#

RF circuits use inductors for impedance matching, tank circuits, and chokes. At RF frequencies, every wire is an inductor, and PCB trace inductance matters.

Saturation: The Main Caveat#

Every inductor with a magnetic core has a saturation current. Above this current, the core can’t store more magnetic energy, and the inductance drops dramatically — often to a fraction of its rated value.

What happens when an inductor saturates:

  • Inductance drops suddenly
  • Current rises rapidly (because V = L × dI/dt, and L just got much smaller)
  • In a switching converter, this means a current spike that can blow the switching transistor or trip overcurrent protection
  • The inductor might not be damaged, but the circuit around it can be

Always check the saturation current rating and ensure worst-case peak current stays below it. Datasheets specify saturation as the current where inductance drops by some percentage (typically 20-30%).

Soft vs. Hard Saturation#

  • Ferrite cores — Hard saturation. Inductance drops sharply at the saturation point. Little warning
  • Powdered iron / composite cores — Soft saturation. Inductance rolls off gradually as current increases. More forgiving, often used in power inductors for this reason

DCR (DC Resistance)#

The winding wire has resistance. This is DCR — the inductor’s series resistance at DC. It causes:

  • I²R losses (heat)
  • Voltage drop under load
  • Reduced efficiency in power converters

Lower DCR means higher efficiency but usually means larger physical size or fewer turns (and thus lower inductance). It’s always a tradeoff.

Core Losses#

At AC, the core material dissipates power through:

  • Hysteresis loss — Energy lost each cycle as the magnetic domains reverse. Proportional to frequency and flux swing
  • Eddy current loss — Currents induced in the core material. Proportional to frequency squared

Core losses add to DCR losses. At high switching frequencies, core losses can dominate. This is why inductor datasheets specify a maximum operating frequency or provide loss curves vs. frequency.

Tips#

  • Check both saturation current and thermal current ratings — use the lower of the two
  • For switching converters, calculate peak current (DC + half ripple) and ensure it’s below saturation
  • Use shielded inductors in noise-sensitive layouts to contain the magnetic field

Caveats#

  • Inductors fight current changes — When a switch in series with an inductor opens, the inductor tries to maintain current flow. The voltage will rise until it finds a path (flyback diode, arc across switch contacts, or semiconductor breakdown). Always provide a current path
  • Parallel inductors need care — Unlike resistors, paralleling inductors is complicated by mutual inductance. Two inductors close together on a board may couple magnetically, changing the effective inductance
  • Self-resonant frequency — Like capacitors, inductors have parasitic capacitance (between turns). Above the SRF, the “inductor” becomes capacitive. For RF work, always check SRF vs. operating frequency
  • Current rating vs. saturation rating — Some datasheets list two current ratings: thermal (current before overheating due to DCR losses) and saturation (current before inductance drops by 20-30%). Use the lower of the two as the design limit

In Practice#

  • An inductor that gets hot under DC current has DCR losses — check if the current is within the thermal rating
  • A switching converter with excessive output ripple may have an inductor operating near or in saturation
  • Voltage spikes on switch nodes that exceed the transistor’s rating often indicate inadequate flyback protection on inductive loads
  • Audible whine from an inductor suggests it’s being driven at an audible frequency (or subharmonic) — common in switching converters with burst mode or pulse skipping
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