It is well known that single-phase pulse width modulation rectifiers have second-order harmonic currents and corresponding ripple voltages on the dc bus. The low-frequency harmonic current is normally filtered using a bulk capacitor in the bus, which results in low power density. However, pursuing high power density in converter design is a very important goal in the aerospace applications. This paper studies methods for reducing the energy storage capacitor for single-phase rectifiers. The minimum ripple energy storage requirement is derived independently of a specific topology. Based on the minimum ripple energy requirement, the feasibility of the active capacitor's reduction schemes is verified. Then, we propose a bidirectional buck-boost converter as the ripple energy storage circuit, which can effectively reduce the energy storage capacitance. The analysis and design are validated by simulation and experimental results.

A High Power Density Single-Phase PWM Rectifier With Active Ripple Energy Storage

This paper describes the design and performance of a 6-kW, full-bridge, bidirectional isolated dc-dc converter using a 20-kHz transformer for a 53.2-V, 2-kWh lithium-ion (Li-ion) battery energy storage system. The dc voltage at the high-voltage side is controlled from 305 to 355 V, as the battery voltage at the low-voltage side (LVS) varies from 50 to 59 V. The maximal efficiency of the dc-dc converter is measured to be 96.0% during battery charging, and 96.9% during battery discharging. Moreover, this paper analyzes the effect of unavoidable dc-bias currents on the magnetic-flux saturation of the transformer. Finally, it provides the dc-dc converter loss breakdown with more focus on the LVS converter.

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Design and Performance of a Bidirectional Isolated DC–DC Converter for a Battery Energy Storage System

It is expected that in the near future the use of high-voltage dc (HVDC) transmission and medium-voltage dc (MVDC) distribution technology will expand. This development is driven by the growing share of electrical power generation by renewable energy sources that are located far from load centers and the increased use of distributed power generators in the distribution grid. Power converters that transfer the electric energy between voltage levels and control the power flow in dc grids will be key components in these systems. The recently presented modular multilevel dc converter (M2DC) and the three-phase dual-active bridge converter (DAB) are benchmarked for this task. Three scenarios are examined: a 15 MW converter for power conversion from an HVDC grid to an MVDC grid of a university campus, a gigawatt converter for feeding the energy from an MVDC collector grid of a wind farm into the HVDC grid, and a converter that acts as a power controller between two HVDC grids with the same nominal voltage level. The operation and degrees of freedom of the M2DC are investigated in detail aiming for an optimal design of this converter. The M2DC and the DAB converter are thoroughly compared for the given scenarios in terms of efficiency, amount of semiconductor devices, and expense on capacitive storage and magnetic components.

Comparison of the Modular Multilevel DC Converter and the Dual-Active Bridge Converter for Power Conversion in HVDC and MVDC Grids

THIS paper explores opportunities and challenges in power conversion in the VHF frequency range of 30-300 MHz. The scaling of magnetic component size with frequency is investigated, and it is shown that substantial miniaturization is possible with increased frequencies even considering material and heat transfer limitations. Likewise, dramatic frequency increases are possible with existing and emerging semiconductor devices, but necessitate circuit designs that either compensate for or utilize device parasitics. We outline the characteristics of topologies and control methods that can meet the requirements of VHF power conversion, and present supporting examples from power converters operating at frequencies of up to 110 MHz.

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Opportunities and Challenges in Very High Frequency Power Conversion

A comparison of on-chip inductors with magnetic materials from previous studies is presented and examined. Results from on-chip inductors with magnetic material integrated into a 90 nm CMOS processes are presented. The inductors use copper metallization and amorphous Co-Zr-Ta magnetic material. Inductance densities of up to 1700 nH/mm2 were obtained thanks to inductance increases of up to 31 times, significantly greater than previously published on-chip inductors. With such improvements, the effects of eddy currents, skin effect, and proximity effect become clearly visible at higher frequencies. Co-Zr-Ta was chosen for its good combination of high permeability, good stability at high temperature (> 250degC), high saturation magnetization, low magnetostriction, high resistivity, minimal hysteretic loss, and compatibility with silicon technology. The Co-Zr-Ta alloy can operate at frequencies up to 9.8 GHz, but trade-offs exist between frequency, inductance, and quality factor. Our inductors with thick copper and thicker magnetic films have dc resistances as low as 0.04 Omega, and quality factors of up to 8 at frequencies as low as 40 MHz.

Review of On-Chip Inductor Structures With Magnetic Films

An improved calculation of ferrite core loss for nonsinusoidal waveforms separates a flux trajectory into major and minor loops via a new recursive algorithm. It is highly accurate and outperforms two previous methods for our measured data. The only characteristics of the material required are the standard Steinmetz-equation parameters.

/pdf/accurate-prediction-of-ferrite-core-loss-with-nonsinusoidal-3bljaupm8k.pdf

Accurate prediction of ferrite core loss with nonsinusoidal waveforms using only Steinmetz parameters

Thin-film V-groove inductors for microprocessor power delivery are discussed. A 10-MHz, 3.3-V to 1.1-V, 7-A dc-dc converter with V-groove inductors has been predicted to have an efficiency of 88% and power density of 60 W of output power per cm/sup 2/ of substrate area. The first prototype inductors with composite granular Co-ZrO/sub 2/ thin-films were fabricated with dimensions smaller than those required for a 7-A converter in order to fabricate them on standard 25-mm silicon wafers. For these prototype devices, we predicted that a 10-MHz, 3.3-V to 1.1-V, 2.1-A dc-dc converter would have 60% efficiency. Measurements on the prototypes indicate that a 2.1-A converter would have 55% efficiency with a power density of 28 W/cm/sup 2/. Measured losses were 23% higher than predicted. We have demonstrated that V-groove inductors can be fabricated with performance close to predicted results.

/pdf/measured-electrical-performance-of-v-groove-inductors-for-1pr3jrryid.pdf

K. Venkatachalam

Papers

Accurate prediction of ferrite core loss with nonsinusoidal waveforms using only Steinmetz parameters

Measured electrical performance of V-groove inductors for microprocessor power delivery