Sang-Hoon Cheon

Circuit-Model-Based Analysis of a Wireless Energy-Transfer System via Coupled Magnetic Resonances

A simple equivalent-circuit model is developed for a wireless energy-transfer system via coupled magnetic resonances, and a practical design method is also provided. Node equations for the resonance system are built with the method, expanding on the equations for a transformer, and the optimum distances of the coils in the system are derived analytically for optimum coupling coefficients for high transfer efficiency. In order to calculate the frequency characteristics for a lossy system, the equivalent model is established at an electric-design automation tool. The model parameters of the actual system are extracted, and the modeling results are compared with measurements. Through the developed model, it is seen that the system can transfer power over a midrange of a few meters and that impedance matching is important to achieve high efficiency.

Optimization of wireless power transmission through resonant coupling

We investigated an optimization condition for a maximum power transmission between coils wirelessly. Due to the resonant coupling nature of the system, for the highest power transmission, there is an optimum distance between the power (receiving) coil and sending (load) coil for a fixed distance between the sending coil and receiving coil. This effect cannot be explained by a conventional magnetic induction theory. We made a model of a four-coil system employing a complex coupling constant to explain the effect.

24 GHz circularly polarized Doppler radar with a single antenna

24 GHz circularly polarized Doppler front-end with a single antenna is developed. The radar system is composed of 24 GHz circularly polarized Doppler radar module, signal conditioning block, DAQ unit, and signal processing program. 24 GHz Doppler radar receiver front-end IC which is comprised of 3-stage LNA, single-ended mixer, and Lange coupler is fabricated with commercial InGaP/GaAs HBT technology. To reduce the chip size and suppress self-mixing, single-ended mixer which uses Tx leakage as a LO signal of the mixer is used. The operation of the developed radar front-end is demonstrated by measuring human vital signal. Compact size and high sensitivity can be achieved at the same time with the circularly polarized Doppler radar with a single antenna.

Demonstration of on-chip appended power amplifier for improved efficiency at low power region

A new power amplifier topology which can achieve improved efficiency at power backoff region is demonstrated in this paper. In this topology, the output stage of the amplifier is appended with a secondary transistor in a parallel way through a 3-port interstage matching circuit and an impedance transforming network. By careful selection of the ratio of device active area, this appended transistor achieves earlier saturation at lower output power level than the output transistor, which is a basic requirement in achieving high efficiency. The power amplifier has been realized with InGaP/GaAs HBT technology and showed efficiency improvement of 81% at output power of 16.7 dBm. The proposed topology enables one-chip integration, hence is very attractive for portable communication terminals.

Modeling and Characterization of Sub-nanosecond Impulse Response of High-Voltage Heterojunction Bipolar Transistors

Sub-nanosecond impulse response of high-voltage InGaAs/GaAs HBTs was characterized by using a time-domain sampling oscilloscope with the cable delay and loss corrected in frequency domain. A peak voltage of 10 V with a width of 200 ps was obtained, which was attributed to the delayed onset of the Kirk effect. The Agilent HBT model was modified to account for the stronger-than-expected field and temperature dependence of the Kirk effect. The modified model correctly predicts the impulse response of the HBT and, hence, can be used in the design and simulation of impulse-based ultra-wideband circuits.

Wireless charging technology using magnetically coupled resonators

We are successful to charge the iPod touch using magnetically coupled resonators. The wireless charging was possible within the distance variation within 10 cm. The signal was generated and amplified with the balanced power amplifier structure which is useful for isolating the reflected power near the operation frequency of 9.4 MHz. To reduce the size of the receiving part, we implanted the thin coil on the polycarbonate substrate using ultrasonic. The receiving circuits are made of rectifying circuit using schottky diode and over voltage protection circuit.

Smart approach to liquid electrolyte-based multi-colored electrochemiluminescence for lighting applications

As potential lighting-emitting devices, electrochemiluminescence (ECL) devices are promising in terms of device structure and fabrication and involve low processing cost compared to organic light-emitting devices (OLEDs). Herein, liquid electrolyte-based multi-colored ECL devices (ECLDs) were prepared with organic materials dissolved in electrolyte solution, which generated red, yellow, green, and blue emissions under an applied square-wave AC bias voltage. Among the four different emissive colors, yellow showed a strong and stable emission, which enabled quantitative measurements, including a luminance of 12 cd m−2 at 4.5 V.

A Novel Harmonic Noise Frequency Filtering VCO for Optimizing Phase Noise

InGaP/GaAs HBT LC-VCOs designed with and without the proposed harmonic noise frequency filtering (HNFF) technique are presented here. To optimize phase noise, the HNFF technique is compared using noise frequency filtering capacitors. The HNFF-VCO produced ultra low phase noise performance. The tuning range of this VCO is 261 MHz with an output power of -10.25 dBm at 1.721 GHz carrier. Furthermore, the phase noise of this VCO -133.96 dBc/Hz at 1 MHz offset. The HNFF-VCO is designed with a total chip area of 0.9 - 0.9 mm2. Its phase noise performance is enhanced by 8.9 dBc at 1 MHz offset by comparison with a N-VCO without the HNFF technique

Study of quasi-two dimensional hole gas in Si/SixGe1−x/Si quantum wells

Abstract Quasi-two-dimensional hole gas in a strained Si/Si x Ge 1− x /Si quantum well structure is investigated both theoretically and experimentally. The hole effective mass and the charge distribution in the structure are achieved from the self-consistent solution of the Schrodinger-Poisson equations. The band mixing effect with split-off band is included by using the 3 × 3 multiband effective mass model. High quality Si/Si 0.8 Ge 0.2 /Si p -type modulation doped quantum well has been grown by molecular beam epitaxy and the electrical properties have been measured at different temperatures. Hole mobility as high as ∼ 10,400 cm 2 /V with a sheet carrier concentration of ∼1.1 × 10 12 and the effective mass of ∼ 0.3 m 0 are obtained at T = 4 K. These values are in fairly good agreement with theoretical predictions.

Stimuli-responsive magneto-/electro-chromatic color-tunable hydrophobic surface modified Fe3O4@SiO2 core–shell nanoparticles for reflective display approaches

In order to provide stimuli-responsive properties under magnetic- and electric-forces enabling reflective color-changes in a liquid medium, the surface of silica coated-iron oxide core–shell nanoparticles (Fe3O4@SiO2) is successfully modified with two-different types of silane coupling agents to improve the dispersion stability as well as the color change strength as a result of the effectively increased repulsion. A stimuli-responsive magneto- and electro-chromatic ink is prepared by mixing hydrophobic surface modified Fe3O4@SiO2-Fx (x = 0 and 13) core–shell nanoparticles and a dispersing agent in a low dielectric medium (LDM). The magneto-chromatic and electro-chromatic stimuli-responsive properties are visually confirmed. Moreover, the optical reflective color spectra and color gamut calculated from the CIE chromaticity coordinates of the hydrophobic surface modified Fe3O4@SiO2-F13 core–shell nanoparticles (with “F moiety” in the molecular structure) under an applied voltage range of 0 V to 10 V show that the hydrophobic surface modified magnetite nanoparticles are promising candidates for reflective display applications.