Bo H. Choi

General Unified Analyses of Two-Capacitor Inductive Power Transfer Systems: Equivalence of Current-Source SS and SP Compensations

A general and systematic comparison of eight compensation schemes in the inductive power transfer system (IPTS) of single magnetic coupling and two capacitors is proposed in this paper. The characteristics of series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP) compensation schemes for a voltage source or a current source are widely explored in terms of maximum efficiency, maximum power transfer, load-independent output voltage or current, magnetic coupling coefficient (k) independency, and allowance of no magnetic coupling (k = 0). Through comparative analyses using a general unified IPTS model, the current-source-type SS and SP are found to be superior to other compensation schemes in terms of the five criteria mentioned above, and they are found to have nearly the same efficiency, load power, and component stress characteristics for the same load quality factor. A design guideline for the current-source-type SS and SP is suggested and experimentally verified by a 200-W prototype of air coils at 100 kHz.

Dipole-Coil-Based Wide-Range Inductive Power Transfer Systems for Wireless Sensors

A dipole-coil-based extremely loosely coupled inductive power transfer system (IPTS) for wireless sensors over a wide range is proposed. The overall superiority of dipole coils for a long-distance power delivery over loop coils with an identical configuration of a square core is verified by comparing the magnetizing inductance between primary and secondary coils. Series-parallel resonant circuits were used to achieve a higher load voltage than that of the series-series scheme. Contrary to conventional IPTSs or coupled magnetic resonance systems, the quality factor Q was set as low as 100, guaranteeing a frequency tolerance of 1%, where a prototype of narrow dipole coils with a length of 2 m was used to deliver 10.3 W of power up to 7 m away at a low frequency of 20 kHz. The powering capabilities of the proposed IPTS were experimentally compared inside and outside of a metal container 8 m × 4 m × 2.5 m in size at different frequencies ranging from 20 to 150 kHz, where the proposed IPTS is highly likely to be surrounded by arbitrary distributed conductors in most wireless sensor usage environments. The coupling coefficient κ between the primary and secondary coils was measured over a long distance from 2 to 12 m for different secondary coil positions, where κ is mostly much less than 0.01. A comparative analysis of the maximum amounts of load power among different core materials, i.e., two ferrite cores and an amorphous core, was conducted in relation to the design of a secondary coil. The results were verified by experiments at a given switching frequency of 20 kHz.

Temperature-Robust LC 3 Passive LED Drivers With Low THD, High Efficiency and PF, and Long Life

New passive LED drivers that can reduce the total harmonic distortion (THD) significantly by LC parallel resonance are proposed. Using an inductor and three capacitors, called LC3, novel characteristics, such as high power efficiency and power factor (PF) with extremely long life time are achieved. The proposed LED drivers have a temperature-robust characteristic, because their power is hardly changed by temperature. By selecting the number of LEDs in series ns appropriately, the LED power variation caused by temperature change in LED can be zero. For the universal use of the proposed LED drivers in various countries with different frequencies, circuit configurations applicable to 50/60 Hz are proposed. To analyze the LED power and PF of the proposed LED driver, the phasor transformation was, for the first time, applied to nonlinear diode rectifier modeling. Although the proposed LED driver is a nonlinear switching circuit, the proposed analyses matched well with simulation and experimental results. A prototype LED driver showed a very high power efficiency of 95.2% at 70 W, meeting PF and THD regulations for source voltage variation of ±6% of 220 V, even when a reasonably small number of filters were used.

Lumped Impedance Transformers for Compact and Robust Coupled Magnetic Resonance Systems

An innovative coupled magnetic resonance system (CMRS), introducing two lumped impedance transformers, is proposed. There are three major magnetic couplings between coils in CMRS: source-transmitter (Tx), Tx-receiver (Rx), and Rx-load couplings. Except for Tx-Rx coupling, other couplings do not directly contribute to wireless power transfer. Hence, in this paper, this miscellaneous coupling is replaced with a lumped transformer with ferrite core. Because there is only a Tx-Rx coupling, the CMRS becomes compact in size and robust to ambient changes. Moreover, the design of CMRS is drastically simplified without complicated multiresonance tunings due to little magnetic flux linkage from the source coil or load coil. Coreless coils are used for Tx and Rx coils to examine the characteristics of CMRS with lumped transformers. A detailed static analysis on the explicit circuit model of the proposed CMRS and design procedures are fully established. Experiments for 1- and 10-W prototype CMRSs with a class-E inverter at the switching frequency of 500 kHz, where the quality factors are less than 100, verified the usefulness of the proposed model, achieving 80% of the maximum Tx coil-to-load efficiency. It is concluded in this paper that the conventional CMRS, in general, is just a special form of an inductive power transfer system where the quality factor is extremely high.

Gyrator-Based Analysis of Resonant Circuits in Inductive Power Transfer Systems

In this paper, first, it is found that not only the magnetically coupled inductors but also all inductive power transfer systems (IPTSs) inherently have the nature of a gyrator. Widely known characteristics of IPTSs such as impedance inversion and source-type conversion are proved to be the nature of the gyrator. A graphical approach that utilizes the gyrator is proposed for the modeling of IPTSs in general. The proposed graphical technique enables manipulations on the circuit diagram instead of on the circuit equations, which are difficult to handle when the system order is higher than 4. Hence, the equivalent model can be obtained almost by inspection conveniently, giving fruitful physical insights that are limitedly achieved with the equation manipulations. Steady-state analyses at any frequency are possible, and equivalent series resistances can also be included in the proposed model. Five selected electrical characteristics, i.e., source-to-load gain, load-independent output voltage/current characteristics, power factor at the source, sign of the source phase angle, and allowances of open/short loads are evaluated for three widely used IPTS topologies. Also, this technique is extended to the mistuned case for verifying the general use of the approach. An experimental prototype of the voltage-source-type inductor-capacitor-inductor secondary-parallel (V-LCL-P) topology was built to demonstrate the proposed approach for both perfectly tuned and mistuned situations at 85 W and 100 kHz.

Coreless power supply rails compatible with both stationary and dynamic charging of electric vehicles

A universal wireless power transfer (U-WPT) system which can be used for not only roadway-powered electric vehicles (RPEVs) but also stationary charging of electric vehicles (EVs) is newly proposed in this paper. Contrary to previous WPT systems for RPEVs, the proposed U-WPT can also be used for pick-ups fabricated by the SAE J2954 standard for stationary charging. The proposed U-WPT adopts a new coreless power supply rail to increase its operating frequency from the conventional level of 20 kHz to 85 kHz, as specified in the SAE J2954 standard. With the adaptation of U-WPT, EVs in the future will simply be called wireless electric vehicles (WEVs) regardless of whether they use stationary or dynamic charging systems. A detailed comparison between coreless and with-core power supply rails is thoroughly made, with the results verified in simulations and experiments. This comparison shows that, with the same power rating, the proposed coreless power supply rail is superior to the conventional with-core power supply rail due to its low cost, less insulation voltage stress, lack of core loss, and only slight sensitivity to lateral misalignment, mitigating the condition of less induced voltage.

Temperature-robust LC 3 LED driver with low THD, high efficiency, and long life

A new type of passive LED driver that can reduce total harmonic distortion (THD) significantly by LC parallel resonance is proposed in this paper. Using an inductor and three capacitors, called LC3, novel characteristics such as high efficiency and power factor (PF) with extremely long life time are achieved. The proposed LED driver has a temperature-robust characteristic because its power is hardly changed by temperature, selecting the number of LED in series ns appropriately so that the LED power variation due to temperature change in LED can be zero. For analyzing LED power of the proposed LED driver, the phasor transformation technique is applied, which is firstly applied to a non-linear diode rectifier modeling. Nevertheless this non-linear switching, the proposed analyses agreed well with simulation and experiment results. A prototype LED driver showed very high power efficiency of 96.7 % at 60 W, meeting high PF of 0.95 and low THD of 10.1 %, though a reasonably small filter was used.

Versatile LED Drivers for Various Electronic Ballasts by Variable Switched Capacitor

An LED driver compatible with various electronic ballasts that are currently commercially used is newly proposed, which adopts a variable switched capacitor by controlling the switching duty cycle for LED power regulation. The resonant frequency of an LC resonant tank of electronic ballasts can be changed, which makes the proposed LED driver versatile for electronic ballasts for various switching frequencies. In this way, the fluorescent lamp is replaced with an LED lamp, where the electronic ballast in the lighting infrastructure remains unchanged. The zero voltage switching is applied for the variable switched capacitor in the electronic ballast, operating at a frequency of 30-60 kHz. Neither an inductor nor a transformer is introduced in the proposed LED driver, which leads to compact size and high efficiency. Furthermore, no electrolytic capacitor is used, which is beneficial for the long lifetime of LED drivers. A prototype LED driver of 16 W was implemented and verified for the three types of electronic ballasts that are most popular in markets. The LED power was well regulated for a wide range of the source voltage variations between 180 and 270 V, and the power efficiencies of the proposed LED driver were 95.8%, 96.1%, and 94.1% for the instant start, rapid start, and programmed start types of the electronic ballasts, respectively.

Synthesized Magnetic Field Focusing Using a Current-Controlled Coil Array

A novel synthesized magnetic field focusing (SMF) technology is proposed and nonradiative but focused magnetic field is experimentally demonstrated as a one-dimensional example. Contrary to common belief, a magnetic field can be focused arbitrarily by increasing the number of current-controlled transmitting coils in a manner similar to that of conventional beam-forming. Moreover, the proposed SMF is independent of frequency from dc to rf. Chronic problems associated with conventional wireless power transfer and magnetic induction tomography technologies, i.e., poor resolution and electromagnetic interference due to the extent of the magnetic field, can be resolved with the proposed SMF technology. The spatial resolution of the magnetic field of an experimental kit composed of 10 coils spaced 5 cm apart with a 90 cm long ferrite core was 12.5 cm at a distance of 10 cm, which is 4 times that of a nonfocusing coil and which may be improved if more coils are used.

Motor Health Monitoring at Standstill Through Impedance Analysis

Induction motors are the most common driving force for active mechanical components such as pumps and valves. When these motors are used in safety systems, which are normally on standby, monitoring the motors through nondestructive evaluation techniques is needed to guarantee their operability and system integrity. In this context, this study suggests a method for motor health monitoring at standstill through impedance analysis. Impedance change tendencies for various motor failure cases are investigated. To identify the tendencies, first an equivalent circuit (EC) of an induction motor is developed, and probable impedance changes for each failure case are estimated based on the developed EC. Then, the estimates are experimentally verified by reproducing the failures and measuring the impedance with an impedance analyzer. As the equivalence between the estimates and the experimental results is confirmed, unique tendencies in impedance changes for each failure case are characterized. The suggested method is expected to be useful in areas with access restriction, such as nuclear power plants (NPPs), as motor impedance can be measured from a distance without any disruption on-site.