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When selecting an inductor for any application it is crucial to understand what test frequency was used to arrive at the data sheet specification.

Figure 1. Inductance and Impedance for a 100 nH Wirewound Inductor

Manufacturer specified tolerances can vary widely, depending on the type of inductor and its intended application. Chip inductor specifications, for example, can be as tight as ±1% at room temperature. That’s very appropriate for high frequency, narrow-band tuned circuits, but it must be considered that the 1% inductance tolerance is specified only at room temperature and the specific test frequency. Typical wire-wound air core and ceramic core chip inductors (non-magnetic) have a temperature coefficient of inductance (TCL) in the range of +25 to +125 ppm/°C. Inductors with ferrite cores will have more variation, with temperature, with TCLs up to 700 ppm/°C or higher.

Irms current ratings are generally derived from measuring inductor temperature rise due to current flow without consideration of frequency effects. The Irms rating is not intended to capture all possible forms of power loss due to application conditions. The power dissipation is simply = I

Since Irms is a measure of self-heating, the limitation is generally the temperature rating of the insulating materials in the inductor. Therefore the Irms rating is a measure of how much current may be allowed in the inductor to ensure safe, reliable operation without heat damage to the inductor.

Calculating losses with frequency effects is not always straightforward, depending on many factors in both the winding and inductor core. To quantify the impact of operating frequency, inductor manufacturers provide a variety of graphs, charts and calculator tools to determine the impact of skin effect, proximity effect, and magnetic core losses.

For example, the RF Inductor/Choke Finder and Analyzer generates ESR vs frequency curves for chip and air core inductors. The Power Inductor Finder and Analyzer provides a measure of core and winding losses for power inductors.

Isat current rating is a different consideration. Whereas the Irms rating deals with heat rise and reliability due to average inductor current, the Isat rating is a performance indicator related to the instantaneous inductor current. Ferrite core inductors, for example, tend to have very flat inductance vs current curves until the “knee” where inductance drops off rapidly, as shown in Figure 2. Powdered iron or composite core inductors have more slowly sloping “soft” saturation curves. At current above the saturation current rating (Isat) inductance decreases with additional current. Typical data sheet specifications show the current at which the inductance drops 10%, 20%, and 30% of its initial value at low current.

Some inductor types with “open” magnetic paths such as unshielded rod core, drum core, or unshielded chip inductors may not have published Isat ratings because there are no meaningful saturation effects at currents as high as 2× or 3× the Irms current so the usable current is effectively limited by the Irms rating and it is assumed there is no saturation at currents at that higher level.

Figure 2: Saturation curve comparison between ferrite and powdered iron inductors

An approximate thermal resistance can be calculated from the data sheet Irms rating.

Power dissipation: Pdc = Irms

so, Rth (in °C/W) = Temp Rise / Pdc.

Expected temperature rise can thus be calculated for any power dissipation, and the resulting part temperature can be compared to the maximum inductor temperature rating.

The Power Inductor Comparison + Analysis Tool allows users to compare up to six inductors for core and winding losses under any combination of operating frequency and current. The calculator estimates the resulting temperature rise (per the above calculations) and compares the resulting part temperature to the maximum part temperature rating. In the graphical results display, sliders can be manipulated to view how frequency, current, or ambient temperature affects the result. When searching for a specific power inductor for your application, The Coilcraft Power Inductor Finder tool incorporates all of these calculations and lists every inductor that meets the user specified conditions.

- How Current and Power Relates to Losses and Temperature Rise, Document 1055, Coilcraft, Inc., 09/04/2012
- RF Inductor Comparison Tool, Online Tool: RF Inductor/Choke Finder and Analyzer Coilcraft, Inc.
- Power Inductor Comparison + Analysis Tool, Online Tool: Power Inductor Finder and Analyzer

- Assistance with Safety Agency Approvals
- Basics of Inductor Selection (from Electronic Design magazine)
- Calibration, Compensation, and Correlation
- Current and Temperature Ratings
- Hipot Testing of Magnetic Components
- How Current and Power Relates to Losses and Temperature Rise
- Measuring Self Resonant Frequency
- Operating Voltage for Inductors
- Selecting Current Sensors and Transformers
- Simulation Model Considerations: Part I
- Simulation Model Considerations: Part II
- S-parameters for High-frequency Circuit Simulations
- Testing Inductors at Application Frequencies
- Working Voltage Ratings Applied to Inductors

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- Determining Inductor Power Losses
- Ferrite Vs Pressed Powder-core Inductors
- Forward or Flyback? Which is Better?
- Notes on Thermal Aging in Inductor Cores
- Selecting Coupled Inductors for SEPIC Applications
- Selecting Inductors to Drive LEDs
- Selecting the Best Inductor for Your DC-DC Converter
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