Example 9: Low-Inductance, High-Current SMD Inductor

This inductor in a 5050 package consists of a powder core material with a distributed air gap. The manufacturer’s rated inductance of 220nH is significantly exceeded at the start (280 nH). The thermally permissible RMS current is specified at 66A, and the saturation current at a 20% drop in inductance is 68A.

With an LCR meter, the datasheet specification for L0 (220nH @ 100kHz, 0.25V) cannot be verified without a special test adapter. Even with a carefully calibrated test adapter, it is difficult. The actual initial inductance L0 is systematically higher than specified, which is confirmed by measurements with the Power Choke Tester DPG10.

The saturation curve of the powder core material is much softer than that of ferrite cores with an air gap. This makes this core material less problematic for use in switched-mode applications, if the current rises far above the rated current in the event of a fault. Even at 200A, this particular model still has an inductance of over 140nH.

The main reason for the discrepancies between the manufacturer’s specifications and the measurements taken with the Power Choke Tester DPG10 lies in the different measurement methods. The Power Choke Tester DPG10 series uses application oriented rectangular large-signal measurement pulses with the same amplitude, frequency, and waveform as in real-world applications. The manufacturer uses a LCR meter with DC bias, which is based on small-signal measurements with sinusoidal voltages in the µV or mV range and currents in the µA or mA range. Such measurement signals normally do not occur in real-world power electronics applications. However, the core materials behave more or less differently in the unrealistic small-signal measurement, where the core material only undergoes tiny hysteresis loops around the DC operating point.

With such extremely low inductance values, even the smallest differences in the geometric test setup play a significant role. Even small deviations in the test point on the component or the type of current feed (e.g. planar or punctiform) can lead to different measurement results. In order to obtain reproducible test results, the geometry of the test setup must always be exactly the same. This cannot be guaranteed without special measurement adapters, which is why we developed our MABX and MABXSMD measurement adapters.