Chroma ATE : How to use RF impedance measurements to ensure the quality of miniaturized power inductors

Miniaturization Trends in Power Inductors

The trend toward miniaturization in electronic products continues unabated, driven by the pursuit of lightweight and compact designs. Meanwhile, with the demand for energy efficiency and low power consumption, the operating conditions of integrated circuits (ICs) are gradually shifting towards low voltage and high current modes. This has led to an increased need of small Point-of-Load (POL) power supplies situated near the ICs, with some even incorporating POL loops internally. The power inductors used in these applications are subject to ongoing miniaturization and low leakage flux requirements. This trend has in turn caused molded inductors using nanometer-grade metal magnetic powder to become the mainstream of this type of power inductors.

Why is high-frequency testing necessary?

Most high-frequency and miniaturized power inductors are metal molding-type inductors. The magnetic metal powder surfaces are usually treated with an oxide layer to reduce eddy current losses at high frequencies, enhancing the usable frequency range and reducing losses and temperature rising. However, users typically cannot determine the particle size of the magnetic material and the integrity of the oxide layer after molding and sintering through visual inspection alone. Traditional testing based on the nominal frequency of the inductor measures the inductance value and Q value. Since the impedance state of high-frequency miniaturized inductors is low, the Q value is often influenced by the contact resistance of the probe. Therefore, by increasing the test frequency, and consequently the inductive reactance, we can reduce the impact of probe contact resistance. This improvement enhances the detection rate of abnormalities in the magnetic material and insulation of the wire, a method commonly adopted by first-tier manufacturers. Besides issues with the original magnetic material (large metal particles/faulty iron oxide layer), both damage to the local iron oxide layer or coil insulation after impulse winding testing and direct contact with the magnetic material can cause a decrease in the Q value. In subsequent use, this easily causes quality issues such as reduced conversion efficiency, overheating or burnout due to large losses.

The equivalent circuit and relationship between inductance (Ls) and Q at high frequencies is shown in Figure 1. Generally, if a manufacturer uses an Ls close to the inductor's self-resonant frequency (SRF) to detect the quality defects discussed previously, this is essentially a test of whether its Q value is low. For a normal product, the inductance value (Ls) near the SRF is higher due to parasitic capacitance. Defective products suffer from excessive losses and have a low Q value, and the apparent Ls is also low. However, because of the low impedance state, if only tested at the nominal frequency, this is often difficult to distinguish due to the probe contact resistance affecting the Q value. Therefore, testing at frequencies significantly higher than the operating frequency substantially improves the detection rate.

▲ (Figure 1) High-frequency equivalent circuit for inductor and the Ls-Q formula		▲ (Figure 2) Sample measurements and explanatory curves

Solution: Chroma 11090-030 RF LCR Meter + A110901 SMD Test Fixture

The Chroma 11090-030 RF LCR Meter provides high-frequency measurement and evaluation of passive components such as SMD chip inductors and RF filters. With a testing frequency range starting from 100kHz, it covers the nominal frequency range of typical power inductor tests (beyond the capability of regular RF LCR meters) and can achieve frequencies as high as 300MHz. This wide frequency range not only meets the increasing demand for nominal frequency testing of components like POL and small DC-DC converters, but also addresses quality defects that can only be detected at ultra-high frequencies. Additionally, it can fulfill common 100MHz impedance testing needs for components like EMI filters and ferrite beads. The 11090-030 utilizes the RF-IV measurement method to measure the voltage and current of the device under test (DUT). Compared to conventional network analyzers, the 11090-030 provides more accurate measurements over a wider impedance range, and is capable of measuring inductance values as small as a few nH (see Figure 2). For automated production testing, if multiple test frequencies are required, the Chroma 11090-030 offers a testing speed of up to 0.5mS/point, completing the testing process within a few milliseconds to enhance equipment throughput.

▲(Figure 2) Measurements of characteristic curves

Furthermore, the Chroma 11090-030 RF LCR Meter features a comprehensive and user-friendly touchscreen interface, test signal monitoring functions, and an open/short/load correction kit for manual measurements. The 11090-030 can be paired with the A110901 SMD Test Fixture (patent M681245), specially designed for a wide range of small-sized SMDs. The fixture features an improved push-down actuation method, allows 90-degree rotation, and enables replacing the DUT in only three steps (actual testing takes about 40 seconds), reducing the load and unload time. Whether for the development and characterization of products such as high-frequency inductors, LCR components (EMI Filters, Ferrite Beads, etc.), and other passive components, rapid testing on automated production lines, or various IQC and OQC scenarios, Chroma 11090-030 offers a comprehensive test solution.

▲(Figure 3) 11090-030 LCR Meter + A110901 SMD Test Fixture

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Chroma 11090-030 RF LCR Meter