Measurement Standards

Inductors and Coupled Inductors

Introduction

Data is only as good as the testing process and environment in which it is gathered. The measurements described hereafter are performed in an ISO/IEC17025 accredited lab using high-end, calibrated equipment. The tests are performed using standard and consistent test setups for a true 1-to-1 comparison. The data is collected and recorded with software to increase efficiency and minimize human error. A sample size of ten pieces is used to perform the measurements for each part number, and the median value is used to characterize the part in the final results.

Measurement Tests

Basic Testing
Fixed inductance and DC Resistance measurements are performed for a simple comparison of how components relate to their datasheets and to other components.
DC Resistance is measured at room temperature of 20 °C using a programmable digital micro Ohmmeter with a 0.04% basic accuracy.
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Inductance is measured using a Precision LCR Meter with a 0.05% basic accuracy. The test equipment is set up with the test frequency and bias voltage as they are defined on the component manufacturer’s datasheet, for example 100 kHz / 100 mV.
The test condition from a manufacturer datasheet:formula
Equipment test conditions are set to match:formula
The leakage inductance is also tested for coupled inductors. The leakage inductance is tested in the same manner as inductance, except the second winding is shorted. The test frequency and bias voltage are set as they are defined on the component manufacturer’s datasheet.
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Frequency Tests
Inductance (L), impedance (Z), resistance (R), phase angle (θz), and quality factor (Q) are measured with respect to frequency. Components with a larger inductance value are measured over a lower frequency range and components with a smaller inductance are measured over a higher frequency range. For coupled inductors, these tests are performed separately for each winding. Inductance > 15 µH is tested from 1 kHz – 120 MHz Inductance < 15 µH is tested from 1 MHz – 3 GHz
A 2-port Vector Network Analyzer (VNA) is used for testing. This is used in conjunction with an adjustable spring clip fixture to ensure the integrity of the connection and consistent measurements.
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Saturation Current
Saturation current (ISAT), or the current which causes a certain inductance drop (often 20%), is a key parameter which is used to define how much current a component can carry before it stops functioning as intended.
This inductance vs. current measurement is taken using a precision LCR meter in conjunction with a series of DC bias current units. A testing fixture is used to ensure good connections and consistent measurements. The inductance is measured at an increasing current level until the inductance drop reaches approximately 80%. The test conditions for measuring the inductance are set to the frequency and bias voltage as defined in the manufacturer’s datasheet, such as the 100 kHz / 100mV example in the Basic Testing section. For coupled inductors, this test is measured over the primary winding.
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Rated Current
The use of the term Saturation Current is often interchanged with the term Rated Current (IRATED). However, the common usage of Rated Current is the current that a component can carry which causes a certain temperature rise (often 40 °C) that the component is able to withstand.
Manufacturers have widely varying test setups to measure rated current, making it difficult to truly compare one product with another by simply comparing the specifications on the datasheets. Since the rated current measurement is highly dependent upon the test setup, we chose a test setup that will closely resemble the performance in the customer application.
For our test setup, the component is placed on an PCB that has a higher current capability than the component being tested. This PCB is chosen so that the traces are as wide as the component, so the PCB does not adversely affect the temperature rise of the component. However, care is also taken to ensure that the PCB is not too large, so that a very large copper area does not act as a heat synch for a small component. The test PCBs have trace widths of 1 mm, 2 mm, 3 mm, 5 mm, 7 mm, 11 mm, 16 mm or 22 mm.
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The dimensions of the actual test PCBs used are as follows:
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The component and PCB are placed in an enclosed chamber to minimize any external influences of temperature or air movement. A thermocouple is affixed to the component to measure the temperature rise as current is passed through the component using a DC Power Supply. The current is gradually incremented and the temperature rise is measured at multiple current steps. The current level is held at each step until the temperature reading is stable and at least five minutes have passed. To avoid skewed results from components that are on the fringes of the specification, components chosen for testing include the lowest, highest and median inductance values from our sample. For coupled inductors, this test is measured over the primary winding.
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Conclusion

The table below provides a summary of the measurement tests and the test setup used. The tests are performed in a controlled, consistent lab environment while minimizing external influences. Maintaining a controlled test process and environment in an accredited lab allows for a true and undistorted comparison between similar products.
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