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Magnetic Characterization Technique and Material Comparison for Very High Frequency IVR

Fig. 1. Improved permeability measurement setup with external DC source and AC voltage cancellation.
To efficiently power multi-core processors in today%u2019s computing devices, integrated voltage regulators (IVR) show significant energy saving ability by dynamic voltage and frequency scaling. One key aspect in developing IVR is to design power inductors with a small size and small loss at very high frequency. However, very high-frequency magnetic characterization is a major obstacle to accurately designing and testing the IVR inductors. In this work, the magnetic characterization technique in tens of MHz is investigated, and the issue and solution in permeability and loss measurement are demonstrated. Low-temperature co-fired ceramic (LTCC) and NEC flake materials are characterized and compared at very high frequencies for IVR inductor design.

Fig. 1 shows an improved permeability measurement setup based on an Agilent 4294A impedance analyzer. The impedance analyzer has only 0.1A internal DC bias ability, which is not adequate to provide the required DC bias level for the core under test. Therefore, an external DC source is added. Meanwhile, an AC voltage cancellation mechanism is also implemented to diminish the error induced by the impedance of the DC source.
Fig. 2 (a) shows an improved loss measurement setup to enable very high frequency magnetic loss characterization in tens of MHz. It is based on a CPES-developed partial cancellation measurement method, and implements minimized driving and sensing loops and high bandwidth (120MHz) current-sensing probe to facilitate the measurement of very high frequencies. Using this setup, the LTCC and NEC flake magnetic materials are characterized up to 40MHz, and the results are summarized in Fig. 2 (b). The figure shows that the core loss density of LTCC increases dramatically at higher flux density Bm, especially at very high frequencies, while the core loss density change of the NEC flake keeps the same trend (as indicated by the straight lines in Fig. 2 b) as Bm increases.

By comparing the testing results from LTCC and NEC flake materials, we can see that the NEC flake shows advantages over LTCC materials in terms of both permeability and core loss density for very high-frequency IVR applications.

Fig. 2(a). Improved loss measurement setup with minimized loops and high bandwidth current.
Fig. 2(b). The core loss measurement results of LTCC and NEC flake magnetic materials.
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