Sai Chun Tang

Evaluation of the shielding effects on printed-circuit-board transformers using ferrite plates and copper sheets

This paper presents an effective shielding technique and a simple structure for printed-circuit-board (PCB) transformers. The performance of PCB transformers using the proposed shielding technique is evaluated using thin ferrite plates and copper sheets. Without affecting the transformer energy efficiency, the shielding method under investigation can achieve 28 dB shielding effectiveness (SE), which is much higher than the SE (about 4 dB) obtained by shielding the transformer windings with only two ferrite plates. The proposed PCB transformer structure is very simple and has high energy efficiency (>90%) for Megahertz operation.

Coreless printed circuit board (PCB) transformers for power MOSFET/IGBT gate drive circuits

Gate drive circuits for modern power electronic switches, such as MOSFET and insulated gate bipolar transistor (IGBT), often require electrical isolation. This paper describes the modeling and experimental results of some coreless printed circuit board (PCB)-based transformers that can be used for MOSFET and IGBT devices at high-frequency (500 kHz to 2 MHz) operation. PCB-based transformers do not require the manual winding procedure and thus simplify the manufacturing process of transformer-isolated gate drive circuits. With no core loss, coreless transformers are found to have favorable characteristics at high-frequency operations. This project demonstrates an important point that the size of the magnetic core can approach zero and become zero when the frequency is sufficiently high.

A low-profile power converter using printed-circuit board (PCB) power transformer with ferrite polymer composite

A new design of low-cost and low-profile power transformer is presented in this paper. The manufacturing cost of a power transformer can be reduced using the proposed printed-circuit board (PCB) transformer. The transformer windings are etched on the opposite sides of a double-sided PCB. Self-adhesive ferrite polymer composite (FPC) sheets are stuck on the two PCB surfaces to shield the magnetic flux induced from the transformer windings. The PCB transformer does not require manual winding and bobbin. A power converter prototype employing the PCB transformer has been implemented. The technique of choosing the optimum switching frequency of the power converter using the PCB transformer is addressed in this paper. The maximum power delivered from the prototype is 94 W. The maximum efficiency of the power converter is 83.5%.

Development of a multistage current-controlled switched-capacitor step-down DC/DC converter with continuous input current

This paper presents the design and analysis of a low-profile multistage current-controlled switched-capacitor (SC) step down DC/DC converter. The converter not only exhibits all advantages of classical SC converters, but also features good regulation capability and continuous input current waveform, resulting in low conducted electromagnetic interference with the supply network. The concept of energy transfer is achieved by paralleling the input and output of two step-down converter cells and operating them in antiphase. The voltage conversion ratio is determined by controlling the charging trajectories of the capacitors in each cell. As it is unnecessary for the circuit to use any inductive elements, possibilities of integrated circuit fabrication and high power density are promising. By applying the state-space averaging technique, a third-order state-space model for an n-stage converter is derived. Static and small-signal dynamic behaviors of the converter are investigated. A 70 W 48 V/12 V converter is implemented. Analytical predictions are verified with experimental measurements.

Thermal modeling of Lundell alternators

Thermal analysis of Lundell alternators used in automobiles is presented. An analytical thermal model for Lundell alternators is proposed, and procedures for acquiring the model parameters are elucidated. Based on the thermal model, the temperature profile of an operating Lundell alternator can be predicted analytically. The predicted alternator temperatures are found to be consistent with the experimental measurement. The presented models and measurement methods are useful for embedding switched-mode power electronics into the alternator with low manufacturing cost.

On the use of current control scheme for switched-capacitor DC/DC converters

This paper presents an investigation into the use of a current control scheme (CCS) and a comparison with a classical switching scheme for switched-capacitor (SC) step-down DC/DC converters. With the CCS, capacitors are charged with near-constant current, controlled by the gate-source voltage of MOSFETs. By paralleling two SC cells, the converter input current becomes continuous, resulting in much reduced conducted electromagnetic interference with other circuits fed by the same power supply. All MOSFETs are operated for half of the switching period, in order to improve the regulation capability. Static and dynamic behaviors of the converter with the CCS are predicted and confirmed in an experimental 36 W 12 V/9 V prototype.

Standing-Wave Suppression for Transcranial Ultrasound by Random Modulation

Low-frequency transcranial ultrasound (¿1 MHz) is being investigated for a number of brain therapies, including stroke, tumor ablation, and localized opening of the blood-brain barrier. However, lower frequencies have been associated with the production of undesired standing waves and cavitation in the brain. Presently, we examine an approach to suppress standing waves during continuous-wave (CW) transcranial application. The investigation uses a small randomization in the frequency content of the signal for suppressing standing waves. The approach is studied in an ex-vivo human skull and a plastic-walled chamber, representing idealized conditions. The approach is compared to single-frequency CW operation as well as to a swept-frequency input. Acoustic field scans demonstrate that the swept-frequency method can suppress standing waves in the plastic chamber and skull by 3.4 and 1.6 times, respectively, compared to single-frequency CW excitation. With random modulation, standing waves were reduced by 5.6 and 2 times, respectively, in the plastic chamber and skull. It is expected that the process may play a critical role in providing a safer application of the ultrasound field in the brain and may have application in other areas where standing waves may be created.

A low-profile low-power converter with coreless PCB isolation transformer

Very small manually wound transformers for sub-watt DC-DC converters are notorious for their relatively high cost and low reliability. In this paper, an isolated low-profile low-power 8 MHz soft-switching power converter using a coreless printed circuit board (PCB) transformer is described. Coreless PCB transformers eliminate several problems of their core-based counterparts in low-power applications. The diameter of the coreless PCB transformer is merely 0.46 cm. The converters power output is about 0.5 W with a typical transformer efficiency of 63%. The high-frequency capability, high reliability and the low-profile structure make coreless PCB transformers a viable and attractive option for reliable mega-hertz switching converters and micro-circuits.

A harmonic cancellation technique for an ultrasound transducer excited by a switched-mode power converter

The aim of this study is to evaluate the feasibility of using harmonic cancellation for a therapeutic ultrasound transducer excited by a switched-mode power converter without an additional output filter. A switching waveform without the 3rd harmonic was created by cascading two switched-mode power inverter modules at which their output waveforms were pi/3 phase-shifted from each other. The waveform and harmonic contents of the acoustic pressure generated by a 1-MHz self-focused piezoelectric transducer with and without harmonic cancellation have been evaluated. Measured results indicated that the acoustic 3rd harmonic-to-fundamental ratio at the focus was small (-48 dB) with harmonic cancellation, compared to that without harmonic cancellation (-20 dB). The measured acoustic levels of the 5th harmonic for both cases with and without harmonic cancellation were also small (-46 dB) compared to the fundamental. This study shows that it is viable to drive a piezoelectric ultrasound transducer using a switched-mode power converter without the requirement of an additional output filter in many high intensity focused ultrasound (HIFU) applications.

An Intraoperative Brain-shift Monitor Using Shear-mode Transcranial Ultrasound: Preliminary Results

A device that uses the shear mode of transcranial ultrasound transmission for intraoperative monitoring has been designed, constructed, and tested with 10 human subjects. Magnetic resonance (MR) images were obtained with the device spatially registered to the MR reference coordinates. Peaks in both the ultrasound and MR signals were identified and analyzed both for spatial localization and signal-to-noise ratio (SNR). The first results aimed towards validating the prototype device against MRI have demonstrated excellent correlation (n = 38, R2 = 0.9962) between the structural localization abilities of the two modalities. In addition, the overall SNR of the ultrasound backscatter signals (n = 38, SNR = 25.4plusmn5.2 dB) was statistically equivalent to that of the MR data (n = 38, SNR = 22.5plusmn4.8 dB).