Zhigang Dang

Modeling and investigation of magnetic resonance coupled wireless power transfer system with lateral misalignment

Lateral misalignment between the transmitter (Tx) and receiver (Rx) in a wireless power transfer (WPT) system significantly reduces its power transmission efficiency. This paper first investigates the lateral misalignment in the magnetic resonance coupled (MRC) WPT system and identifies the High Efficiency Range (HER). The HER is a high efficiency area on the transmission efficiency versus Rx lateral misalignment amount curve. In the HER, the transmission efficiency is nearly constant at a maximum value before it sharply drops down to zero (or a very small value). The identification of the HER is verified by simulation results obtained from a developed ANSYS® HFSS® 3-D physical model. Simulation results of the ANSYS® HFSS® 3-D physical model with 5-turn, 60cm outer diameter spiral shape MRC-WPT system show that when the vertical distance (DIS) between the Tx and Rx ranges from 0.1 m to 1 m, an HER exists at each DIS value. When 0.3m ≤ DIS ≤ 0.6m, nearly constant high efficiency of ~90% could be maintained when the lateral misalignment is not larger than 50cm (83.3% of the Rx diameter).

Reconfigurable Magnetic Resonance-Coupled Wireless Power Transfer System

This paper presents a method for a reconfigurable magnetic resonance-coupled wireless power transfer (R-MRC-WPT) system in order to achieve higher transmission efficiency under various transmission distance and/or misalignment conditions. Higher efficiency, longer transmission distance, and larger misalignment tolerance can be achieved with the presented R-MRC-WPT system when compared to the conventional four-coil MRC-WPT (C-MRC-WPT) system. The reconfigurability in the R-MRC-WPT system is achieved by adaptively switching between different sizes of drive loops and load loops. All drive loops are in the same plane and all load loops are also in the same plane; this method does not require mechanical movements of the drive loop and load loop and does not result in the system volume increase. Theoretical basis of the method for the R-MRC-WPT system is derived based on a circuit model and an analytical model. Results from a proof-of-concept experimental prototype, with transmitter and receiver coil diameter of 60 cm each, show that the transmission efficiency of the R-MRC-WPT system is higher than the transmission efficiency of the C-MRC-WPT system and the capacitor tuning system for all distances up to 200 cm (~3.3 times the coil diameter) and for all lateral misalignment values within 60 cm (one coil diameter).

Permanent Magnet toroid power inductor with increased saturation current

This paper presents a power inductor with toroid core that utilizes a Permanent Magnet (PM) in its gap (PMTPI) in order to increase saturation current while keeping the same size and inductance. After discussing the PMTPI structure, the results of ANSYS®/Maxwell® physical model simulation results for a design example are presented. The design example considered in this paper is for a toroid-core power inductor with dimensions of 12.5mm × 12.5mm × 6.5mm and inductance of ~592nH. The saturation current is increased from 14A in the conventional design to 28A in the proposed design.

Evaluation of High-Current Toroid Power Inductor With NdFeB Magnet for DC–DC Power Converters

This paper presents an experimental evaluation of a high-current toroid power inductor (TPI) with a NdFeB permanent magnet (PMTPI). By adding a small piece of a fabricated NdFeB-N35EH magnet (the magnet volume is ~0.36% of the PMTPI ferrite core volume) in the air gap of the TPI, the saturation current of the PMTPI is doubled with the same size and inductance value. The desired dimensions of the NdFeB-N35EH permanent magnet (PM) are calculated before fabrication, and then, the fabricated PM is characterized. The ~6.8-μH PMTPI is tested in a 5-2-V buck power converter. Results show that, compared with a TPI with the same size and inductance (~6.8 μH) without the PM, the saturation current of the PMTPI is doubled (from 7 to 14 A). Compared with another TPI with a larger size needed to double the saturation current, the ferrite core weight of the PMTPI is reduced to 53.6%, and the core volume is reduced to 59.2%. Experimental results also show that the addition of the NdFeB-N35EH PM does not introduce additional power losses and increase of temperature for the PMTPI and does not affect the power converter efficiency.

Elimination method for the Transmission Efficiency Valley of Death in laterally misaligned wireless power transfer systems

This paper first identifies the Transmission Efficiency Valley of Death (TEVD) in laterally misaligned magnetic resonance coupled (MRC) wireless power transfer (WPT) system. The paper then presents a method to eliminate the TEVD by angularly rotating the transmitter coil (Tx) or receiver coil (Rx). Simulation results of the ANSYS® HFSS® 3-D physical model with 5-turn spiral shape four-coil MRC-WPT system with 60cm outer diameter show that when the Rx is 30cm vertically away from the Tx, TEVD exists when the lateral misalignment value (MIS) ranges from 50cm to 70cm. The presented method eliminates the TEVD and extends the high efficiency range from MIS = 50 cm (83.3% of the Rx diameter) to MIS = 70 cm (117% of the Rx diameter).

Permanent-Magnet Coupled Power Inductor for Multiphase DC–DC Power Converters

Using a coupled power inductor (CPI) in a multiphase dc–dc power converter instead of using multiple single-phase power inductors (PIs) reduces the inductor size and achieves better steady state and transient performances. To further reduce the inductor size and weight used in multiphase power converters, this paper presents a two-phase CPI that utilizes a permanent magnet (PM) to achieve almost doubled saturation current with the same size or equivalently achieve significant size and weight reduction. Operation principle of the presented permanent magnet coupled power inductor (PMCI) and required PM dimensions are derived and used as a design guide. The three-dimensional physical model of the PMCI is developed by using ANSYS/Maxwell software package to “visualize” the saturation current doubling. The fabricated PMCI design, with specifications of 24 A/phase, ∼4.25 μH/phase equivalent steady-state inductance, and ∼2.3 μH equivalent transient inductance, is tested in a 4–8 V two-phase dc–dc boost power converter with up to 50 A input current. Results show that compared to a conventional CPI design with the same size, weight, and inductance, the fabricated PMCI almost doubles the saturation current (from 13 to 24 A/phase). Compared to another CPI with a larger size but with even a smaller saturation current (18 A/phase), the core volume of the PMCI is reduced to 51.9% and the core weight is reduced to 51.2%. In addition, the PMCI achieves ∼76.3% core size and ∼73.4% core weight reductions compared to two separate single-phase PIs with the same steady-state inductance and similar saturation current (∼22 A) due to the dual flux cancelation effect from the coupling and PM realized by the presented PMCI structure.

Range and misalignment tolerance comparisons between two-coil and four-coil wireless power transfer systems

Two-coil and four-coil magnetic resonance coupled wireless power transfer (MRC-WPT) system configurations are analyzed and compared in this paper based on the simplified circuit model and 3-D physical model simulations. Physical model simulation results of symmetrical MRC-WPT systems show that when the sizes of transmitter (Tx) coil and receiver (Rx) coil are equal and without any external capacitors, the four-coil system achieves longer transmission distance and larger misalignment tolerance with relatively lower efficiency at close distance compared to the two-coil system. The designed 10-turn, 400 mm outer diameter spiral shape four-coil WPT system achieves operation distance of 500 mm with nearly constant maximum transmission efficiency of 85%, while the two-coil system achieves ~96% of efficiency up to distance of 35 0mm. When the vertical Tx to Rx distance is fixed at 150 mm, the four-coil system has a larger lateral misalignment tolerance of 275 mm compared to the two-coil system which has misalignment tolerance of 200 mm at the same distance.

Extended-range two-coil adaptively reconfigurable wireless power transfer system

This paper presents a two-coil reconfigurable wireless power transfer (WPT) system topology in order to optimize transmission efficiency under different transmission distance (DIS) and lateral misalignment (MIS) conditions. The reconfigurable WPT system includes one transmitter (Tx) side and one receiver (Rx) side but it could switch between different circuit configurations (which are made up of different values of series and shunt capacitors) at Tx side and/or Rx side. Design guidelines of the two-coil reconfigurable WPT system are devised based on an equivalent circuit model. A proof of concept prototype with three different adaptive configurations at Tx side and one configuration at Rx side is built and experimentally compared with conventional two-coil WPT system. Results show that when the system is perfectly aligned, the reconfigurable system improves the transmission efficiency by up to 20%. When system is laterally misaligned, the transmission efficiency is improved by up to 19%.

On-chip coupled power inductor for switching power converters

This paper proposes a design of an on-chip coupled power inductor (OCPI) structure which consists of a layer of ferrite core material and another layer of coupled spiral windings on a silicon substrate. The flux cancellation effect of the two inversely coupled windings significantly reduces the net fluxes in the ferrite layer, which helps increasing the saturation current of the power inductor. An ANSYS®/Maxwell® 3-D physical model of a 5.6mm×5.6mm two-phase OCPI is developed and simulation results are obtained. Simulation results show that the saturation current (7A/phase) of the OCPI is twice the saturation current (3.5A) of the non-coupled power inductor with the same inductance density. Based on a 3.3V/1.5V 4MHz two-phase buck converter, the equivalent steady state inductance and transient inductance of the OCPI are calculated to be equal to 78.1nH and 22.2nH, respectively.

Dynamic efficiency tracking controller for reconfigurable four-coil wireless power transfer system

This paper presents a dynamic efficiency tracking control strategy for reconfigurable four-coil wireless power transfer (R-WPT) system. The proposed R-WPT system includes an array of drive loops, a transmitter (Tx coil), a receiver (Rx coil), an array of load loops, two relay switch arrays, a rectifier and a control unit. By sensing voltage and/or current from the power source and the load, Tx and Rx side switches are adaptively controlled such that one drive loop out of several drive loops with different diameters is connected to the power source and one load loop out of several load loops with different diameters is connected to the load. Since drive loops and load loops have different sizes, the optimum coupling factor with corresponding drive loop and load loop is adaptively varied and selected, such that the maximum efficiency is achieved at a given distance/misalignment condition. Proof of concept experimental results demonstrate and verify the operation of the system achieving high efficiency.