PhD Thesis Proposal: Kishalay Datta

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"Analysis and design of multi-winding current ballasted magnetics for integrated DC-DC conversion"

Abstract

Power conversion circuits have become a bottleneck in terms of reducing the size and cost of modern electronic systems in a vast number of applications ranging from personal devices, automotive systems, to data centers. This has motivated designs at higher frequencies where smaller passive components can be used. Unfortunately, magnetic components scale poorly to smaller dimensions. In addition, the AC resistance of magnetics deteriorates due to current crowding effects at high frequencies. Consequently, highly and fully-integrated power converters have moderate to poor efficiency and low power density.

In this work, we investigate high-frequency DC-DC converter circuit techniques that can help solve these challenges. These techniques are especially applicable to air-core magnetics, which can be integrated in standard CMOS processes. One of the main concepts we explore is the use of multi-winding current ballasting. This involves splitting inductor or transformer windings into multiple separate windings with width comparable to the skin depth at the operating frequency. A unique current is forced through each winding to reduce the detrimental high-frequency effects. We resonate each winding with a unique capacitor to ballast the desired current profile across the windings. This is implemented by connecting a unique switched capacitor (SC) converter to individual windings, giving rise to a network of distributed hybrid SC stages to form the overall power converter.

Using this technique, we have fabricated two fully integrated DC-DC converters. First, a non-isolated converter with a multi-winding inductor, achieving 84.7% peak efficiency and 0.55 Ω effective resistance in 130 nm SOI technology. Next, a bidirectional isolated converter with a multi-winding transformer that achieves 42.5% peak efficiency at a state-of-the-art 69.6 mW/mm2 power density, fabricated in 180 nm SOI process.

These converters use results from theoretical analyses performed on optimal current distribution for both multi-winding inductors and transformers. A current distribution ensuring equal voltage across all windings minimizes loss in inductors. For transformers, the power transfer efficiency is maximized by ensuring that the ratio of power transfer to power loss for every winding is the same. Implications of these results for planar spiral magnetics are discussed in detail.

Thesis Committee

• Jason T. Stauth (Chair)
• Charles R. Sullivan
• Minh Q. Phan

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu.