An inductor datasheet may list nominal inductance, inductance at rated current, saturation current, and temperature-rise current. These values are not interchangeable because magnetic materials are nonlinear. The distinction between incremental inductance and large-signal inductance is particularly important when a converter carries substantial DC current with switching ripple.
Small-signal inductance
An LCR meter typically applies a small AC test signal around zero DC bias. The result is useful for production turns checks and initial component comparison. It does not necessarily predict ripple at full load because permeability can fall under DC magnetizing force and vary with AC amplitude, frequency, and temperature.
Incremental inductance under bias
Incremental inductance describes the local slope of flux linkage versus current around a DC operating point. It determines how a small ripple current changes in response to a small applied voltage at that bias. In a switching converter, this value strongly influences current-ripple slope near the operating current.
A bias fixture or impedance analyzer may superimpose a small AC signal on DC current. The reported value depends on AC test amplitude and frequency. If the ripple in the real converter is not small, the measurement may not capture the full excursion.
Large-signal or amplitude inductance
Large-signal inductance relates the total flux linkage or stored energy to current over a wider excursion. Different conventions include secant inductance, amplitude inductance, and energy-equivalent inductance. The exact definition should be stated because the same nonlinear curve can produce different numerical values.
For transient energy, current limiting, and peak flux, a large-signal description may be more relevant than a local incremental value. For control-loop ripple around a bias point, incremental inductance may be the better quantity. A robust specification can include both.
Saturation is not always a sharp knee
Gapped ferrite may retain relatively stable inductance until core flux approaches a knee, after which the slope changes rapidly. Powder cores have a distributed gap and often show gradual permeability roll-off with DC bias. Calling one point the saturation current requires a criterion, such as a 10, 20, or 30 percent inductance decrease.
That criterion should be linked to the circuit. A 20 percent drop may be acceptable in one converter and destabilizing in another. Peak current, ripple, semiconductor limit, acoustic behavior, and transient duration all matter.
Temperature changes the curve
Ferrite saturation flux generally decreases with temperature. Powder-core permeability and loss also vary with temperature. Copper resistance rises and changes current and heating. Inductance-versus-current data at room temperature alone may therefore overstate margin at the winding or core hot spot.
Pulse measurement in the application range
A practical method applies a known voltage pulse and measures current slope. Instantaneous differential inductance is voltage divided by current slope after accounting for winding resistance and switch or fixture voltage. Probe bandwidth, delay, parasitic ringing, source impedance, and pulse duration must be controlled.
The pulse should reach relevant current without overheating the winding or core. Repetition rate must allow thermal control. The test winding voltage should be measured at the component terminals, not inferred from the power supply setting.
What to put on a custom inductor specification
- Nominal inductance and tolerance with test frequency and amplitude.
- Minimum incremental inductance at defined DC current, AC amplitude, and temperature.
- Peak-current or large-signal criterion and pulse duration.
- Current waveform, ripple, frequency range, transient count, and duty cycle.
- Maximum DC resistance and component temperature in stated cooling.
- Core-loss, acoustic, insulation, size, and lifetime requirements.
BaoHui Tech uses the actual converter waveform to choose which inductance definition controls the design. This avoids approving a power inductor from a low-level LCR value that does not represent its operating point.
Frequently asked questions
Is saturation current the same as thermal current?
No. Saturation current is based on magnetic inductance reduction or flux behavior. Thermal current is based on temperature rise. Either can be the limiting rating.
Why does pulse-derived inductance differ from an LCR meter?
The pulse uses different bias, amplitude, frequency content, temperature, and measurement definition. Both values can be correct for their test conditions.