Lorandt explains what the saturation current of a power inductor is, and also how the saturation current is measured at Würth Elektronik eiSos.
This article explains what inductor saturation current is, how it is defined and measured, and how to use it correctly in practical converter design. For this Lorandt uses a magnetics analyzer to measure the inductance value at several DC currents, and creates the saturation current chart out of them.
Key Takeaways
- Inductor saturation current is a critical parameter affecting efficiency and ripple in DC-DC converters.
- Manufacturers define saturation current as the DC current at which inductance drops by a specified percentage from its zero-bias value, usually 10%, 20%, or 30%.
- Saturation current can be measured using an LCR meter under DC bias and helps in determining the performance limits of inductors.
- Designers should choose inductors with saturation current at least 20-50% above expected peak to ensure reliability during load transients.
- Understanding inductor saturation current is essential for proper power supply design and enhancing converter performance.
Introduction
Inductors are key energy-storage elements in modern switching power supplies, where their magnetic cores must operate well below saturation to maintain stable inductance and predictable converter behavior. Saturation current is one of the most critical parameters for power inductors, directly influencing efficiency, output ripple, and the safe operating area of DC-DC converters.
Fundamentals of Inductance and Saturation
An inductor stores energy in the magnetic field created by current flowing through its winding, with inductance L0 relating voltage and current via = L0 ⋅ di(t)dt). In real components, the inductance is not perfectly constant: as magnetic flux in the core approaches the material’s limits, the permeability drops and the inductance falls, indicating the onset of saturation.
Magnetic Core Behavior
Magnetic materials exhibit a nonlinear relationship between magnetic field strength H and magnetic flux density B, described by a hysteresis curve that flattens as the material saturates. In the linear region, small changes in current cause proportional changes in flux, while near saturation, large current changes produce only small additional flux, effectively reducing inductance.
Effective Inductance in Power Applications
In power converters, the effective inductance under DC bias determines current ripple and energy storage capability rather than the small-signal inductance at zero bias. As DC current increases toward the saturation region, the falling inductance increases ripple current and may push the converter outside its intended operating mode.
Definition of Saturation Current
Manufacturers commonly define saturation current as the DC current at which the inductance drops by a specified percentage from its initial zero-bias value, often 10%, 20%, or 30% depending on the catalog convention. This definition links a purely magnetic limit to a measurable change in inductance, making it practical for both characterization and design.
Percentage Drop Criteria
If the initial inductance at zero DC bias is L0, then a saturation current criterion of 10% drop corresponds to a current Isat,10% at which the measured inductance L(I) equals 0.9 x L0. Some vendors specify several saturation current levels, such as 10% and 30% inductance drop, to provide more information about the knee of the inductance-versus-current curve.
Measurement of Saturation Current
Saturation current is measured using an LCR meter or impedance analyzer with superimposed DC bias current, typically at a low AC frequency like 1 kHz and small AC amplitude (e.g., 250 mV) to approximate small-signal conditions. The setup involves sweeping the DC bias from 0 to a maximum expected current (e.g., 800 mA), recording inductance at discrete steps, and plotting the inductance versus DC current to identify the drop point.
Typical Measurement Setup
The measurement uses a Wayne Kerr LCR instrument with a DC bias load connected via prepared leads, ensuring the inductor is properly mounted for accurate readings. Conditions include room ambient temperature, as thermal effects are separate from saturation characterization.
Inductance vs. Current Graph Analysis
The resulting graph shows a linear region up to about 300 mA where inductance remains stable, followed by a gradual drop marking the saturation onset.[1] This knee indicates the limit for linear operation in DC-DC converters.[1]
| DC Bias Current (mA) | Inductance (μH) | Percentage Drop (%) |
|---|---|---|
| 0 | 10.0 | 0 |
| 100 | 10.0 | 0 |
| 300 | 9.95 | 0.5 |
| 500 | 9.0 | 10 |
| 800 | 7.0 | 30 |
Saturation Current vs. Rated Current
Saturation current characterizes the magnetic limit and is independent of temperature, while rated current is the maximum allowable RMS current based on thermal rise, typically lower than saturation current to prevent overheating. Designers must ensure the inductor’s saturation current exceeds the peak current in the converter to avoid nonlinear operation.
Key Differences
Saturation current focuses on inductance stability, whereas rated current ensures thermal management in continuous operation.[1] For optimal design, select inductors where saturation current is at least 1.5 times the peak ripple current.
| Parameter | Saturation Current | Rated Current |
|---|---|---|
| Definition | DC current for 10% inductance drop | Max RMS current for 40°C rise |
| Temperature Dependence | None | High (derates with heat) |
| Design Use | Peak current margin | Average power handling |
Design Implications for DC-DC Converters
In buck or boost converters, operating near saturation increases output voltage ripple and reduces efficiency due to higher peak currents and core losses. Proper selection requires calculating peak inductor current as Ipeak = Iout + ΔI2, ensuring Isat > Ipeak.
Practical Selection Guidelines
Choose inductors with saturation current 20-50% above expected peak for margin against load transients.[1] Simulate or measure under actual bias conditions for validation.
Conclusion
Understanding and properly applying saturation current specifications is essential for reliable power supply designs, preventing failures from unexpected inductance variations. By selecting inductors with adequate saturation margins and distinguishing them from thermal-rated currents, engineers can achieve efficient and stable DC-DC converter performance. Future advancements in core materials may extend linear ranges, but core principles remain critical for current applications.
FAQ about Saturation Current of an Inductor
Saturation current is the DC current at which the inductance of a power inductor starts to drop significantly because the magnetic core approaches its flux density limit, causing the material permeability to decrease and the inductance to fall from its initial value.
In practice, saturation current is usually defined as the DC current at which the inductance has dropped by a fixed percentage, often around 10%, from the initial inductance measured at zero DC bias.
The saturation current is measured using an LCR meter or magnetics analyzer with a superimposed DC bias, sweeping the DC current from zero up to the target value while recording inductance at each step and then reading the current at the specified inductance drop point.
Typical conditions include a small-signal AC test at about 1 kHz and around 250 mV AC level, with DC bias swept from 0 up to several hundred milliamps, performed at room ambient temperature.
The inductance versus DC bias current graph usually shows a flat, almost constant inductance region at low current, followed by a bend where the inductance starts to decrease as current approaches the saturation range of the core material.
In a DC-DC converter, the inductor sees a peak current that includes the ripple component, so the saturation current must be higher than the maximum ripple peak current to keep operation linear and to ensure the inductor can store the required energy without collapsing its inductance.
If an inductor is driven above its saturation current, the inductance drops, current rises faster than expected, output ripple increases, and the converter can become unstable or overstressed, potentially damaging components.
Saturation current is defined by the magnetic behavior of the core and an inductance drop criterion, while rated current is determined by thermal limits and specifies the maximum current that the inductor can carry continuously without exceeding a given temperature rise.
How to Measure Saturation Current of an Inductor
- Step 1: Prepare the test setup
Connect the inductor to the LCR meter or magnetics analyzer using a suitable fixture or short leads, ensuring good contact and minimal parasitic effects, then connect the DC bias module in series according to the instrument manual.
- Step 2: Configure measurement conditions
Set the AC test frequency to around 1 kHz and select a small AC level of approximately 250 mV so that the instrument measures the small-signal inductance at each DC bias point.
- Step 3: Define the DC bias sweep
Program the DC bias current sweep to start from 0 A and increase up to the maximum expected operating current, for example up to about 800 mA, using reasonable current steps that allow a smooth inductance versus current curve.
- Step 4: Run the measurement sweep
Start the sweep so that the instrument applies each DC current level, measures inductance at that point, and records the data; the sweep typically advances automatically in fixed time steps until it reaches the programmed maximum current.
- Step 5: Generate the inductance vs. current graph
After the sweep finishes, use the instrument’s software to generate an inductance versus DC bias current graph, which shows how inductance stays nearly constant at low current and then begins to drop as saturation effects appear.
- Step 6: Measure the saturation current
From the graph or the measurement table, locate the current where inductance has decreased by the specified percentage, such as 10% lower than the initial zero-bias value, and record this current as the saturation current of the inductor.
- Step 7: Compare with converter ripple current
Compare the measured saturation current with the maximum ripple peak current expected in the target DC-DC converter and ensure that the saturation current is clearly higher to maintain linear inductance and reliable operation.
