Imperial College London

DrChristopherKwan

Faculty of EngineeringDepartment of Electrical and Electronic Engineering

Visiting Researcher
 
 
 
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Contact

 

christopher.kwan Website

 
 
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Location

 

510Electrical EngineeringSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

19 results found

Arteaga JM, Kwan CH, Nikiforidis I, Pucci N, Lan L, Yates DC, Mitcheson PDet al., 2021, Design of a one-to-four isolated DC-DC converter using a 13.56 MHz resonant air-core transformer, 2021 IEEE Applied Power Electronics Conference and Exposition (APEC), Publisher: IEEE, Pages: 2580-2585

This paper showcases the design and development of a DC-DC converter with one input and four outputs using a high frequency resonant air-core transformer. The transmitter to receivers air-gap is 25 mm. Practical tuning equations were derived for multiple receivers which allow the converter to be optimised for overall efficiency and unity power factor at the transmit coil (i.e. zero reflected reactance). Experiments were conducted using two receive coil structures, one with four equally shaped adjacent coils in a single PCB, and the other with four differently-shaped coils featuring overlapping traces to maximise the coupling factor with the transmitter and minimise the coupling factor between the receivers. The two structures were tested and compared using the same transmitter, driven by a single-ended 13.56 MHz Class EF inverter. Single-ended Class D rectifiers were implemented at the receive side. Experiments were performed, first with equal AC test loads, and afterwards with the addition of the rectifiers and buck converters to regulate each of the four output voltages to 15 V independently. The results of the experiments implementing adjacent coils demonstrate that equal distribution of power can be achieved by modifying the tuning capacitances at the receivers with the AC loads; however, when the voltage-regulating buck converters were introduced at each output, it was only with the coil structure with overlapping traces that the required power of 10 W at each output was achieved.

Conference paper

Kwan CH, Arteaga JM, Pucci N, Yates DC, Mitcheson PDet al., 2021, A 110W E-scooter Wireless Charger Operating at 6.78MHz with Ferrite Shielding

This paper reports on the design, construction and integration of a wireless inductive charging solution for an electric scooter, operating at a frequency of 6.78MHz and providing an output power of 110 W. With the use of a push-pull Class EF inverter at the transmit end, as well as ferrite shielding and a voltage-doubler full-wave Class D rectifier at the receive end, this system achieved a DC-DC IPT efficiency of 69%-75% and exhibited good tolerance to misalignment at full charging power.

Conference paper

Kwan C, Arteaga Saenz J, Aldhaher S, Yates D, Mitcheson Pet al., 2020, A 600W 6.78MHz wireless charger for an electric scooter, IEEE PELS WoW 2020, Publisher: IEEE, Pages: 278-282

This paper presents a 600 W electric scooter wireless charging solution operating at a frequency of 6.78 MHz. At the transmitter end, a load-independent Class EF push-pull(differential) inverter with GaN transistors was used to drive a 33 cm square-shaped copper pipe coil. A full-wave voltage-triplerClass D rectifier with silicon Schottky diodes was connected to a24 cm-by-26 cm trapezoidal receiver coil (also made of copperpipe) mounted underneath the steel frame of the scooter. In order to reduce the eddy current and magnetic losses in the steel chassis, parts of the electric scooter frame were shielded with copper tape. With the battery recharging in situ at 600 W,the IPT system achieved a DC-to-DC efficiency of 84 %.

Conference paper

Lan L, Polonelli T, Qin Y, Pucci N, Kwan CH, Arteaga JM, Boyle D, Yates DC, Yeatman EM, Mitcheson PDet al., 2020, An induction-based localisation technique for wirelessly charged drones, Pages: 275-277

This manuscript proposes a technique to use an inductive power transfer system to perform last-stage localisation of drones for tracking and automated landing. This system is proposed to assist the final stage of landing by solely making use of the inductive charger and avoid using vision or other external sensors which would increase cost and complexity.The simplicity of the proposed method can help widen the practical implementation of automated drones. This proposed method is demonstrated with a high frequency (6.78 MHz) inductive charging system that can deliver up to 100 W of power to a DJI M100 drone when it lands at any position on the designed one-meter diameter charging pad.

Conference paper

Lan L, Kwan CH, Arteaga JM, Yates DC, Mitcheson PDet al., 2020, A 100W 6.78MHz inductive power transfer system for drones, 2020 14th European Conference on Antennas and Propagation (EuCAP), Publisher: IEEE, Pages: 1-4

This paper reports on the design and development of a wireless charging solution for a DJI Matrice 100 quadcopter drone. The system is based on a high frequency inductive power transfer system built with lightweight copper pipe air-core coils at both ends and lightweight electronics at the receive side. The developed system is capable of delivering power to the drone at the same rate as the original wired charger (100W) when landed at any position on the charging pad, regardless of the lateral misalignment or angular orientation. The charging pad is circular with a one-metre diameter, therefore allowing for a lateral misalignment of up to 25cm. The system has an average mains-to-battery efficiency of 70% and enables the drone missions to be completely autonomous as it eliminates the need for human interference for battery recharging or swapping.

Conference paper

Arteaga Saenz JM, Lan L, Kwan CH, Yates DC, Mitcheson PDet al., 2020, Characterisation of high frequency inductive power transfer receivers using pattern recognition on the transmit side waveforms, IEEE Applied Power Electronics Conference and Exposition (APEC), Publisher: IEEE, Pages: 825-831, ISSN: 1048-2334

This paper demonstrates the characterisation of inductively coupled receivers for high frequency inductive power transfer (HF-IPT) systems using pattern recognition on the inverter waveforms at the transmit side. The impedance reflected by the candidate receivers to the transmit coil was estimated using a model programmed to associate the experimental drain-voltage waveforms of the inverter when it drives a receiver under test to those when driving known loads. The necessity of employing this technique is due to the difficulty of accurately measuring current and voltage across the coil given the parasitic effects of probing and the precise skewing required to measure an impedance, especially at high Q-factor. The proposed technique is convenient for characterising and comparing the impedance reflected by candidate receivers for a particular application where there is a choice to be made with respect to the rectifier topologies and semiconductor technologies. Experimental results, using a 13.56 MHz 100 W inductive power transfer system, were obtained for a full-wave Class D rectifier using silicon (Si) and silicon carbide (SiC) Schottky diodes, and two Class E rectifiers using SiC diodes.

Conference paper

Pucci N, Kwan CH, Yates DC, Arnold AD, Keene D, Whinnett ZI, Mitcheson PDet al., 2020, Effect of fields generated through wireless power transfer on implantable biomedical devices, 2019 IEEE PELS Workshop on Emerging Technologies: Wireless Power Transfer (WoW), Publisher: IEEE, Pages: 1-5

This paper assesses the safety of pacemakers when exposed to the electromagnetic (EM) field generated by high frequency inductive power transfer (HF-IPT) systems. It includes both simulation and experimental results, showing temperature variations to ensure conformity with the EN standards, changes in detected lead impedance and determining whether EM field strength can affect the operating mode of the device. This is the first time the interaction between 6.78MHz, 100W HF-IPT systems and pacemaker devices was tested up to distances of 5 cm to 10 cm, Temporary decrease of detected lead's impedance and interruption of communications are the most relevant effects recorded through in-vitro tests. No permanent alteration of the device's operation was recorded, indicating good early stage evidence of safety for pacemaker users in proximity of this new emerging technology.

Conference paper

Kwan C, 2020, ., .

Conference paper

Pucci N, Kwan CH, Yates DC, Mitcheson PDet al., 2020, Multi-Megahertz IPT Systems for Biomedical Devices Applications, 19th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (Power MEMS), Publisher: IEEE

Conference paper

Nikiforidis I, Arteaga JM, Kwan CH, Yates DC, Mitcheson PDet al., 2020, Design and Modelling of Class EF Inverters for Wireless Power Transfer Applications, 19th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (Power MEMS), Publisher: IEEE

Conference paper

Arteaga Saenz J, Aldhaher S, Kkelis G, Kwan C, Yates D, Mitcheson Pet al., 2019, Dynamic capabilities of multi-MHz inductive power transfer systems demonstrated with batteryless drones, IEEE Transactions on Power Electronics, Vol: 34, Pages: 5093-5104, ISSN: 0885-8993

This paper presents the design of a multi-MHz inductive power transfer (IPT) system showcasing lightweight and energy-efficient solutions for non-radiative wireless power transfer. A proof of concept is developed by powering a drone without a battery that can hover freely in proximity to an IPT transmitter. The most challenging aspect, addressed here for the first time, is the complete system level design to provide uninterrupted power-flow efficiently while allowing for variable power demand and highly variable coupling factor. The proposed solution includes the design of lightweight air-core coils that can achieve sufficient coupling without degrading the aerodynamics of the drone, and designing newly-developed resonant power converters at both ends of the system. At the transmittingend, a load-independent Class EF inverter, which can drive a transmitting-coil with constant current amplitude and achieves zero-voltage switching (ZVS) for the entire range of operation, was developed; and at the receiving-end, a hybrid Class E rectifier, which allows tuning for large changes in coupling and power demand, was used. For the demo, the range of motion of the drone was limited by a 7.5 cm nylon string tether, connected between the centre of the transmitting-coil and the bottom of the drone. The design of the IPT system, including all the power conversion stages and the IPT link, is explained in detail. The results on performance and specific practical considerations required for the physical implementation are provided. An average end-to-end efficiency of 60% was achieved for a coupling range of 23% to 5.8%. Relevant simulations concerning human exposure to electromagnetic fields are also included to assure that the demo is safe according to the relevant guidelines. This paper is accompanied by a video featuring the proposed IPT system.

Journal article

Kwan CH, Arteaga JM, Yates DC, Mitcheson PDet al., 2019, Design and Construction of a 100 W Wireless Charger for an E-Scooter at 6.78 MHz, Pages: 186-190

This paper demonstrates the design and construction of a multi-MHz inductive power transfer (IPT) system with air-core coils, integrated in an electric scooter, whose on-board battery is required to be charged at 100 W. With respect to the magnetics, integration challenges arise due to the proximity of the receive coil to the steel frame of the scooter, which significantly change the coil and link parameters from those calculated and observed in air. The design of the power electronic circuits at both ends of the system are detailed and explained. The battery of the electric scooter was able to be charged with a de-to-battery efficiency of 65.6 % with an air-gap of 17 cm, where most of the losses are found in the battery charger. A losses distribution diagram of the entire system is included. Simulations of human exposure to the electromagnetic fields demonstrate that the designed system is well within the general exposure limits given by ICNIRP.

Conference paper

Kwan CH, Yates DC, Mitcheson PD, 2019, Reducing human body heating and temperature rises due to inductively-powered implantable medical devices, 18th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588

Conference paper

Lan L, Ting NM, Aldhaher S, Kkelis G, Kwan CH, Arteaga JM, Yates DC, Mitcheson PDet al., 2018, Foreign Object Detection for Wireless Power Transfer, 2nd URSI Atlantic Radio Science Meeting (AT-RASC), Publisher: IEEE

This paper presents foreign object detection (FOD) methods for MHz wireless power transfer (WPT) systems. Unlike current FOD implementations, the presented methods can operate without requiring a feedback loop from the wireless power receiver to the transmitter. This allows complete decoupling of the transmitter and receiver and therefore reduces the design complexity and cost of the system. The developed FOD methods were implemented on a 13.56 MHz WPT and experimental results are presented showing successful detection of a wide range of objects regardless of the loading condition of the system.

Conference paper

Kwan CH, Pinuela M, Mitcheson P, Yates D, Lawson Jet al., 2016, Inductive power transfer system, WO2016050633 A3

There is provided a near-field inductive power transfer system (10), comprising a power transmission device (100) arranged to transmit power wirelessly at a first frequency, f0, and a power reception device (200) arranged to receive power transmitted by the power transmission device (100). The power reception device (200) is moveable relative to the power transmission device (100) and comprises a receiver circuit (210) configured to receive power for powering a variable load (230) when the power reception device (200) is in a near-field region of the power transmission device (100), the receiver circuit being a resonant circuit with a resonant frequency, fR, such that 0.2 < f0/fR < 3. The power reception device (200) also includes an impedance emulator (220) for providing the received power to the variable load (230), the impedance emulator being arranged to suppress a variation in an impedance presented to the receiver circuit (210) by the load when the load varies during use of the near-field inductive power transfer system (10).

Patent

Plazzotta G, Kwan C, Boyd M, Colijn Cet al., 2016, Effects of memory on the shapes of simple outbreak trees, Scientific Reports, Vol: 6, ISSN: 2045-2322

Genomic tools, including phylogenetic trees derived from sequence data, are increasingly used to understand outbreaks of infectious diseases. One challenge is to link phylogenetic trees to patterns of transmission. Particularly in bacteria that cause chronic infections, this inference is a ected by variable infectious periods and infectivity over time. It is known that non-exponential infectious periods can have substantial e ects on pathogens' transmission dynamics. Here we ask how this non-Markovian nature of an outbreak process a ects the branching trees describing that process, with particular focus on tree shapes. We simulate Crump-Mode-Jagers branching processes and compare di erent patterns of infectivity over time. We nd that memory (non-Markovian-ness) in the process can have a pronounced e ect on the shapes of the outbreak's branching pattern. However, memory also has a pronounced e ect on the sizes of the trees, even when the duration of the simulation is xed. When the sizes of the trees are constrained to a constant value, memory in our processes has little direct e ect on tree shapes, but can bias inference of the birth rate from trees. We compare simulated branching trees to phylogenetic trees from an outbreak of tuberculosis in Canada, and discuss the relevance of memory to this dataset.

Journal article

Kwan CH, Yates DC, Mitcheson PD, 2016, Design Objectives and Power Limitations of Human Implantable Wireless Power Transfer Systems, IEEE Wireless Power Transfer Conference (WPTC), Publisher: IEEE, ISSN: 2474-0225

Conference paper

Kwan CH, Kkelis G, Aldhaher S, Lawson J, Yates DC, Luk PC-K, Mitcheson PDet al., 2015, Link efficiency-led design of mid-range inductive power transfer systems, Pages: 1-7

Conference paper

Kwan CH, Lawson J, Yates DC, Mitcheson PDet al., 2014, Position-insensitive long range inductive power transfer, 14th International Conference on Micro- and Nano-Technology for Power Generation and Energy Conversion Applications (PowerMEMS), Publisher: IOP PUBLISHING LTD, ISSN: 1742-6588

Conference paper

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