Inductor specification

  • Inductance L (\mu H) (tested at a certain test conditions for example 100 KHz, 0.1 V_{rms}, 0 A DC)
  • Inductive tolerance: It is the allowed amount of variation from the nominal value specified by the manufacturer (e.g. ±20%).
  • Rated operating voltage (across inductor)
  • DC Resistance (DCR): The resistance of the inductor winding measured using DC current. The resistance in a component due to the length and diameter of the winding wire used.

R_{DC}=\rho \frac{l}{s}

\rho \ - \ resistivity \ (material \ dependent \ factor), [\Omega m]

l \ - \ length,\ [m]

s \ - \ cross \ section, [m^2]

The DC resistance has a constant value. The bigger the cross section, the lower the DC Resistance (DCR), the lower the copper losses. The smaller the cross section, the higher the DC resistance (DCR), the higher the copper losses.

  • AC Resistance (ACR)
  • Maximum DC current I_{DC}: Maximum DC current is the DC current at which the inductance falls to 90% of its nominal value or until its temperature rise reaches 30 °C.

Figure: Inductance vs DC Bias Load (or DC Bias Characteristic)

DC Bias current relates to a constant current element that is added to the AC signal.

  • Incremental Current Rating: The DC bias current that causes an inductance drop of 5% from the initial zero DC bias inductance value.
  • I_{rms} or RMS current:
    • I_{rms} for a 20°C rise above 25°C ambient temperature
    • I_{rms} for a 40°C rise above 25°C ambient temperature
  • Saturation current I_{SAT}: The DC bias current that causes the inductor to drop by a specified percentage (e.g. 10% or 20%) from its value without current. See Figure Inductance vs DC Bias Load (or DC Bias Characteristic)
  • Q factor or Quality factor: The measure of the relative losses in the inductor.

Q=2 \pi \frac{maximum \ energy \ stored}{energy \ dissipated \ per \ cycle}=2 \pi \frac{\frac{1}{2}LI_{max}^2}{(\frac{I_{max}}{\sqrt{2}})^2R_{e}T}=2 \pi \frac{L}{R_{e} \frac{1}{f}}=\frac{2 \pi f L}{R_{e}}=\frac{X_{L}}{R_{e}}

Quality factor is defined as the ratio of the inductive reactance X_{L} to the effective resistance R_{e}. Both X_{L} and R_{e} are functions of frequency. The test frequency must be given when specifying Q .

  • Self-Resonant Frequency (SRF) or f_{o} in Hz
  • Curie temperature T_{C} (in degrees Celsius): It is the temperature at which the core material start to lose its magnetic properties.
  • Inductance temperature coefficient: The change in inductance per unit temperature change. Measured under zero bias conditions and expressed in parts per million (ppm).
  • Resistance temperature coefficient: The change in DC wire resistance per unit temperature change. Measured at low DC Bias (<1 VDC) and expressed in parts per million (ppm).
  • Magnetic saturation flux density B_{SAT}: At this value of flux density, all magnetic domains within the core are magnetized and aligned.
  • Shielding
    • with shield
    • without shield
  • Electromagnetic interference (EMI): It refers to the magnetic field radiated away from the inductor into space. The magnetic field may cause interference with other magnetically sensitive components.
  • Core material
    • Ferrite cores
    • Iron powder cores
  • Storage temperature range
  • Operating temperature range
    • Ambient temperature range not including self-temperature rise
    • Product temperature range including self-temperature rise. The operating temperature T_{Op} is equal to the ambient temperature T_{Amb} plus component’s self-heating \Delta T. The maximum allowable temperature for an inductor is the maximum ambient temperature plus the maximum temperature rise.
  • Moisture Sensitivity Level (MSL)