Richard J. Charles

Design issues for the transformer in a low-voltage power supply with high efficiency and high power density

Circuit model, design feasibility, and design tradeoffs are investigated for the transformer in 1.5-5 V power supplies with high efficiency and high power density. The transformer is constructed from a single or a matrix of pot cores and from interleaved planar windings. It has been determined theoretically and verified experimentally that such a transformer is realizable as long as the loss constraint is not severe (e.g. less than 0.5 W transformer loss per 100 W output). The primary source of loss is the winding, not the core, in 1.5 V/turn design. Measures to reduce the transformer height tend to increase transformer loss or volume. >

High efficiency, high density MHz magnetic components for a low profile converter

A highly efficient (99.5%) transformer and resonant inductor with very high power density (1500 W/in/sup 3/) and low profile (height<0.4 in) are described. The design of these components is based on a tradeoff study which establishes the optimum operating frequency to be around 1 MHz. The transformer utilizes highly efficient thin flex circuit windings. The windings are novel folded copper patterns, eliminating the need for vias and resulting in very low DC resistance. The winding arrangements have a high degree of interleaving between the primary and secondary windings, resulting in very low AC conductor losses due to skin and proximity effect. The core is a low profile design with very low core losses. The resonant inductor, instead of using low permeability material, uses highly efficient high permeability material. The desired low permeability is then achieved by introducing a large number (>40) of small gaps to reduce the effect of fringing fields. The high-frequency loss measurements for such highly efficient inductors require a special ring down measurement setup.<<ETX>>

Evaluation of trade-offs in transformer design for very-low-voltage power supply with very high efficiency and power density

Design methodology and trade-offs have been presented for a transformer in low-voltage (1.5-5 V) power supplies intended to meet 95% efficiency and 100 W/in/sup 3/ power density. The design is based on the pot core geometry, which has low core loss, and the planar toroidal winding geometry, whose mechanical and electrical parameters are highly controllable and reproducible by modern fabrication processes. To achieve high power density in the presence of skin and proximity effects, the primary and secondary winding layers are interleaved to increase the effective thickness. Leakage inductance is reduced, and winding capacitance is increased as a result. The design equations have been coded into a computer program that selects the transformer with the lowest volume over specified ranges of optimizing parameters, such as power loss and flux density.<<ETX>>

Strength and Microstructure of Dense, Hot-Pressed Silicon Carbide

The microstructural and strength characteristics of dense SiC, hot-pressed from submicron SiC powders with small additions of boron, are shown to be strongly dependent on the control exercised over oxygen content during pressing. Uniform, equiaxed grain structures of SiC are obtained if sufficient oxygen is present to allow detectable particles of SiO2, whereas uniform, elongated grain structures of β SiC are obtained if excess carbon is utilized to reduce oxygen content to undetectable limits. At intermediate oxygen levels, free silicon was formed and large, tabular α-SiC grains in a fine-grained β-SiC matrix were obtained.

Megahertz transformers for high density power conversion

Transformer designs that achieve efficiencies greater than 99% operating at 2 MHz at power densities in excess of 400 W/in/sup 3/ for the power range of 50-100 W with output voltages of 1.5 V are discussed. To achieve these results, copper density must be increased beyond what is possible with more conventional Litz wire constructions. Instead, multilayer windings are made of copper foil using flex circuit technology. The elimination of external connections between the winding layers by incorporating the interconnections as an integral part of the winding design is described. This results in Z-folded winding structures that can be ultimately interleaved to achieve high copper density, low copper loss, and very low leakage inductance. >

MHz transformers for high density power conversion

The authors discuss transformer designs that achieve efficiencies greater than 99% operating at 2 MHz at power densities in excess of 400 W/in/sup 3/ for the power range of 50 to 100 W with output voltages of 1.5 V. To achieve these results copper density must be increased beyond that possible with more conventional Litz wire constructions. Instead, multilayer windings are made of copper foil using flex-circuit technology. A particular innovation described is the elimination of external connections between the winding layers by incorporating the interconnections as an integral part of the winding design. This results in Z-folded winding structures that can be intimately interleaved to achieve high copper density, low copper loss, and very low leakage inductance.<<ETX>>