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Most high frequency (> 15 MHz) medical ultrasound systems are based on single element transducers mechanically scanned. These systems can provide images with excellent resolution. However, single element transducers are often limited by the fixed focal point and small depth of field. Annular arrays consisting of concentric rings of elements are focused electronically. These arrays are desirable to avoid the fixed focal point of the single element transducers and improve the depth of field. This paper reports the design, fabrication, and characterization of a 5-element equal-area annular array transducer. After electrical impedance matching, the average center frequency was 20 MHz and -6 dB bandwidths ranged from 34 to 42%. The ILs for the matched annuli ranged from 6.1 to 26.5 dB. [ABSTRACT FROM AUTHOR]

This paper presents the design of new high-frequency transformer isolated bidirectional dc-dc converter modules connected in input-series-output-parallel (ISOP) for 20-kVA-solid-state transformer. The ISOP modular structure enables the use of low-voltage MOSFETs, featuring low on-state resistance and resulted conduction losses, to address medium-voltage input. A phase-shift dual-half-bridge (DHB) converter is employed to achieve high-frequency galvanic isolation, bidirectional power flow, and zero voltage switching (ZVS) of all switching devices, which leads to low switching losses even with high-frequency operation. Furthermore, an adaptive inductor is proposed as the main energy transfer element of a phase-shift DHB converter so that the circulating energy can be optimized to maintain ZVS at light load and minimize the conduction losses at heavy load as well. As a result, high efficiency over wide load range and high power density can be achieved. In addition, current stress of switching devices can be reduced. A planar transformer adopting printed-circuit-board windings arranged in an interleaved structure is designed to obtain low core and winding loss, solid isolation, and identical parameters in multiple modules. Moreover, the modular structure along with a distributed control provides plug-and-play capability and possible high-level fault tolerance. The experimental results on 1 kW DHB converter modules switching at 50 kHz are presented to validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper presents a novel measuring system for partial discharge (PD) measurements in Gas Insulated Systems (GIS) using high frequency current transformers (HFCT). The system is based on the measurement of the induced PD currents in the GIS enclosure. In opposition to the existing antenna technologies that measure the radiated energy in the very high frequency/ultra-high frequency (VHF/UHF) range, the proposed system measures the PD conducted currents in the high frequency (HF) range and below. The foundation of the measurements together with a detailed explanation of the sensor installed conveniently at the bolts of the GIS spacer are presented. An experimental study on the current distribution in the GIS enclosure is described to evaluate the impact of the sensor on the measurements. Laboratory experiments have been performed that show the suitability of this method to properly measure particle discharges caused by corona, surface and free moving particle discharges in SF6. Discharges in the range of 1 to 4 pC have been properly measured. An analysis to evaluate the performance of the method is shown, in comparison to VHF/UHF antenna measurements. The potential benefits of this novel technique rely on the small attenuation of PD signals in the GIS components in the HF range and sample rate reductions. Finally, a discussion on the potential applicability of present cluster and charge calculation techniques to the proposed PD GIS measurement using HFCT is presented. [ABSTRACT FROM AUTHOR]

An integrated balancing transformer is proposed for a high frequency parallel AC-LED drive circuit using a LLC resonant converter. The design and implementation of the integrated balancing transformer are presented. The experimental results for the converter with the integrated transformer are provided to show the validity of the proposed technique. [ABSTRACT FROM AUTHOR]