Load-independent Output Voltage Research Articles

Simple Enhancement of Series–Series-Compensated Inductive Wireless Power Transfer Links Operating With Load-Independent Voltage Output at Fixed Frequency to Attain Zero Inverter Phase Angle

It is well-known that series-series compensated resonant inductive wireless power transfer links (SS-IWPTL) may be designed to attain load independent voltage output (LIVO) for given operating frequency and coupling coefficient. Unfortunately, corresponding inverter output impedance is either highly-inductive or highly-capacitive (inductive region is typically preferred due to natural zero-voltage switching (ZVS) of inverter transistors), significantly increasing inverter volt-ampere (VA) rating. The letter demonstrates that designing the SS-IWPTL for operation in the capacitive region and then adding a shunt inductor across AC-side terminals of either inverter or rectifier while allows attaining inverter zero phase angle (ZPA) thus minimizing its VA rating.

An Unsymmetrical Driving Scheme for Inductive Power Transfer Systems Using Decoupled Transmitter Coils

This paper explores a novel and unsymmetrical driving scheme for an inductive power transfer system using decoupled transmitter (TX) coils. Instead of driving each TX in the same way, the proposed driving scheme would employ various compensations for different coils. When one TX coil is series compensated, the other TX coil is compensated by the LCC network, and these two compensated coils are driven by a cascaded inverter instead of two bridge ones. Through circuit analysis, the power flow of each TX is closely coupled and help avoid over load conditions. Meanwhile, it would maintain the benefits of a multiple-TX system like high misalignment tolerance, and enable the system design and control like a single-TX one, e.g., having load-independent output voltage and optimal loads. The experiment demonstrates the waveform of normal mode and protection mode, load-independent output voltage, power distribution conditions, and the optimum efficiency.

Explicit Design of Impedance Matching Networks for Robust MHz WPT Systems With Different Features

This article develops explicit design of an impedance matching network (IMN) to enhance the robustness of megahertz (MHz) wireless power transfer (WPT) systems against variations in their final load and coil coupling. Unlike the existing trial-and-error-based solutions, analytical derivations are conducted to calculate the parameters of a two-port T- or <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\bf \Pi$</tex-math></inline-formula> -IMN. The load-pull technique is first applied to determine a target impedance ray that represents a desired loading condition of the power amplifier. Based on the analytical derivations, the three degrees of the design freedom of the IMN are then fully utilized to accurately match the starting point and direction of the target impedance ray. Different features of a final MHz WPT system, such as load-independent output voltage and high tolerance to coil misalignment, are further achieved by having different target impedance rays. Performance enhancement of an experimental 6.78-MHz WPT system is investigated under largely varied final load (10–50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf{\Omega }$</tex-math></inline-formula> ) and coil coupling (0–53%). With an explicitly designed IMN, the maximum efficiency drops are reduced from 18% to 6% under the changing final load and 21% to 3.9% under the changing coil coupling (0–40%), respectively. An actual application of wireless drone charging is also included to demonstrate the practicability of the proposed IMN design

Design and Multiobjective Optimization of an Auxiliary Wireless Power Transfer Converter in Medium-Voltage Modular Conversion Systems

This article proposes an optimized design for a wireless power transfer converter serving as an auxiliary power supply in a medium-voltage, high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{d}v/\mathrm{d}t$</tex-math></inline-formula> modular conversion system. A <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLLC-CL</i> circuit topology is implemented to generate a load-independent output voltage with a coupling-coefficient-independent resonant frequency. The output voltage of the circuit can be tuned by changing one pair of resonant <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> parameters, which decouples the circuit gain from the coil design. Essential parameters of coil and magnet are extracted analytically or numerically to avoid time-consuming 3-D finite element analysis simulations for the subsequent optimization. After the design of the circuit and coil, a multiobjective optimization is carried out with objectives being efficiency, isolation capacitance, and insulation rating of the converter. Finally, experiments demonstrate a 48- to 48-V dc–dc converter with 100 W output power, 92.78% efficiency, 2.78 pF isolation capacitance, and 27 kV insulation rating to validate the optimization result.

Output characteristics modeling and experimental verification of secondary-uncompensated inductive power delivery link operating without feedback

Series-none compensated induction wireless power delivery link (SN-IPDL) was recently shown to be capable of achieving minimal complexity receiver with reduced losses in case of strong coupling, typical for applications such as power delivery into enclosed compartments such as industrial glove boxes and close-range inductive heating. Common practical applications often feature power delivery to varying loads which require a stable operational voltage. Operation at LIVO frequency allows establishing potentially feedback-free wireless power delivery with constant output voltage for varying loads. The paper reveals that due to nonzero equivalent series resistances and invalidity of the first-harmonic-approximation (FHA) based equivalent circuit, the output voltage remains affected by the load despite operation at LIVO frequency, residing within a certain range of values, lower and upper bounds of which correspond to rated and zero loading, respectively. This must be considered during SN-IPDL system design in order to assure that output voltage stays within allowed bounds for relevant loading conditions. This paper derives analytical expressions for output voltage bounds of a power-loaded series-to SN-IPDL, functioning at load-independent-voltage-output (LIVO) frequency. Time domain differential equations (DE)-based analysis is applied to derive the two bounds and allows (as a by-product) establishing an alternative FHA-based equivalent circuit of SN-IPDL operating in continuous conduction mode. Simulations and experiments based on 380V, 1.2 kW-rated prototype demonstrate excellent matching with analytical outcomes, validating the proposed analysis.

Design of High-Efficiency Inductive Charging System With Load-Independent Output Voltage and Current Tolerant of Varying Coupling Condition

Maintaining constant-current (CC) and constant-voltage (CV) outputs for meeting the charging profile of lithium-ion batteries while realizing zero-voltage-switching on the primary bridge over wide ranges of load and coupling variations are rather challenging due to the presence of leakage and magnetizing inductances of a loosely coupled transformer. Although the challenges can be greatly relaxed by properly designing a compensation network with load-independent output characteristics, the desirable characteristics rendered by fixed compensation networks will be lost and cannot be fully restored by existing control methods once the designed compensation is deviated from the nominal coupling condition. In this article, a dynamic series/series-parallel compensation network, which originally aims to deliver constant output voltage only, is further investigated as a feasible topology for CC-CV battery charger adaptive to different sizes of air gap/misalignment of the coils. As benefited from the extra controllability offered by the parallel compensation capacitance as compared to the series/series compensation counterpart, this article provides alternative design criterion for the effective parallel compensation capacitance for designing and maintaining desirable charging currents in the CC stage while maximizing the efficiency in the CV stage under varying coupling condition. An experimental prototype with the air gap ranging from 10 to 16 cm is built to verify the idea.

Constant Current/Voltage Charging for Primary-Side Controlled Wireless Charging System Without Using Dual-Side Communication

To realize the misalignment insensitivity and stable constant current/voltage (CC/CV) charging without using the extra dc-dc converter, composite compensation topology, and wireless communication modules, a primary-side control method based on the phase-shift full-bridge inverter and the laminated magnetic coupler (LMC) is proposed for the wireless charging system. First, the load-independent output current and voltage characteristics for the series-series compensation are analyzed. The equations for estimating the charging current and voltage are deduced by only using the primary-side parameters. Second, the excellent antimisalignment capability of the LMC with the primary-side integrated auxiliary coil is verified. As a result, the influence of horizontal misalignment on the estimation precision is reduced; the controllability of the charging current is ensured. Third, the dual-loop cascade PI controller is designed, and CC/CV charging is realized by adjusting the phase-shift angle. Finally, both the simulation and experimental results show that the primary-side control method realizes CC/CV charging with high precision within the reasonable horizontal misalignment range. Compared to the conventional primary-side control methods, the proposed one avoids both the load resistance estimation and mutual inductance identification. Meanwhile, the compact and lightweight receiver module is ensured.

Output characteristics of none-series compensated inductive wireless power transfer link operating at load-independent-voltage-output frequency

The most important performance merits of inductive wireless power transfer links (IWPTL) operating at load-independent-voltage-output (LIVO) frequency are efficiency and output characteristics. It was recently demonstrated that none-series (NS) compensated IWPTL operating at LIVO frequency outperforms its series-series (SS) and series-none (SN) compensated counterparts in terms of efficiency. In order to complete the comparison, this paper presents output characteristics modeling and analysis (namely, relation between output voltage and load power) of a practical NS-compensated IWPTL operated at LIVO frequency. It is demonstrated that even though the system is operated at LIVO regime, its output voltage remains load-dependent due to practical issues. The lower bound of the output voltage range corresponds to operation under rated loading and is derived using phasor domain equivalent circuit, assuming continuous conduction mode (CCM) of the receiving-side diode rectifier without neglecting equivalent series resistances. On the other hand, upper bound of the output voltage range corresponds to operation under zero loading (inevitably imposing discontinuous conduction mode (DCM) of the receiving-side diode rectifier) and is obtained using time-domain solution of corresponding differential equations system due to the fact that classical phasor analysis fails to accurately describe DCM operating NS-compensated IWPTL. The proposed method is applied to a 1 kW, 400 V NS-compensated IWPTL operated at LIVO frequency and is well-verified by simulations and experiments. Comparison with output voltage characteristics of SS and SN-compensated IWPTLs assert the advantage of NS-compensated topology.

Output Voltage and Resistance Assessment of Load-Independent-Voltage-Output Frequency Operating Inductive Wireless Power Transfer Link Utilizing Input DC-Side Measurements Only

The paper puts forward a method for predicting output voltage and resistance of a series-series (SS) compensated inductive wireless power transfer (IWPT) link operating at load-independent-voltage-output (LIVO) frequency. The link is a part of the static system (reported by the authors in earlier works), wirelessly delivering power into an enclosed compartment without any secondary-to-primary feedback. The proposed algorithm employs input DC-side quantities (which are slow-varying and nearly noise-free, thus measured utilizing low-cost, low-bandwidth sensors) only to monitor output DC-side quantities, required for protection and/or control. It is shown that high estimation accuracy is retained as long as system parameter values are known and the phasor-domain equivalent circuit is valid (i.e., upon continuous-conduction mode (CCM) of the diode rectifier, where the proposed methodology utilizes the recently revealed modified diode rectifier equivalent model for enhanced accuracy). Under light loading (i.e., in discontinuous conduction mode (DCM)), a nonlinear correction is combined with the proposed technique to retain accuracy. The proposed methodology is well-verified by application to a 400 V to 400 V, 1 kW static IWPT link by simulations and experiments.

High-Order Network Based General Modeling Method for Improved Transfer Performance of the WPT System

In wireless power transfer system, zero phase angle (ZPA), zero voltage switching (ZVS), load-independent output voltage, and current are key to improve transfer performance, like efficiency and output power over load change. More concerned, it is hard to realize those expected electrical characteristics in a topology simultaneously. To solve the issue, this article proposes a general modeling method based on the high-order resonant network. The equivalent impedance of passive components in each circuit branch is replaced by jX and then, according to the analysis of Kirchhoff principle, the conditions to realize load-independent output and simplified calculation of input impedance are given out. Hence, the proper passive components can be chosen to get expected electrical characteristics. A specific prototype, using the proposed modeling method for parameter design, is implemented to verify the validity of proposed modeling method. In the prototype, load-independent constant voltage and constant current modes can be switched by restructuring the passive network; meanwhile, ZPA or ZVS could also be realized by regulating only one component. The experimental results fit well with the theoretical analysis.

Accurate first-harmonic-approximation-based model of the diode rectifier in series-series compensated inductive wireless power transfer link at load-independent-voltage-output frequency

The brief proposes a modification of classical pure resistive first harmonic approximation (FHA) based equivalent of diode rectifier in series-series compensated inductive wireless power transfer link (SS-IWPTL), operating at load-independent-voltage-output (LIVO) frequency. It is shown by means of time-domain analysis that a load-independent inductance should be added in parallel with the typically utilized load-dependent resistance to accurately represent input rectifier impedance at operating frequency. It is also shown that the worst case (minimal) inductance value is obtained when equivalent series resistances (ESRs) are ignored. In practical systems, non-zero ESRs are typically present and are typically desired to be as low as possible to increase the power transfer efficiency. It is shown that increasing the ESRs leads to escalation of equivalent resistance and inductance values (i.e. reducing the influence of the latter), without altering the load-independence characteristics of equivalent inductance. Simulations and experiments based on a 400 V, 1 kW strongly-coupled SS-IWPTL operating at LIVO frequency validate the proposed analysis.

Analysis and design of inductive wireless power transfer link for feedback-less power delivery to enclosed compartment

The paper presents analytical investigation and hands-on design of inductive wireless power transfer link for a through-glass AC power delivery system into enclosed compartment such as industrial glove box or vehicle cabin. The system is fed from AC grid and creates a standard AC outlet within the enclosed compartment, capable of supplying loads of up to 1 kW. The system consists of three (namely, AC/DC, DC/DC and DC/AC) conversion stages, with the former and the latter realized by standard off-the-shelf units. The intermediate DC-DC power conversion stage, which is the paper focus, is realized by a from-scratch-built inductive wireless power transfer link, operating in load-independent voltage output regime without any feedback. It was recently shown that the output voltage of such a system, even when operating at load-independent frequency, remains influenced by the load due to practical issues. As a result, the output voltage resides within a certain range rather than remains constant, obtaining minimum/maximum values under rated load/no-load conditions, respectively. Since coils separating glass width typically possesses some finite uncertainty, values of both coupling coefficient and coils self-inductances may be different than nominal (design) ones and hence the output voltage would drift from its nominal range. Hence, due to the fact that upper and lower bounds of an inductive wireless power transfer link output voltage range are limited by DC link capacitor and DC/AC power stage constraints, tolerated glass width uncertainty is obtained in the paper. Moreover, compensating capacitors pair symmetrizing uncertainty tolerance of the inductive wireless power transfer link is also revealed. Simulations and experiments based on 400 V, 1 kW-rated inductive wireless power transfer link are presented to validate the analysis.

A Single-Stage Dynamically Compensated IPT Converter With Unity Power Factor and Constant Output Voltage Under Varying Coupling Condition

Leakage and magnetizing inductances of a loosely coupled transformer are constantly changing due to the relative positions of the primary and secondary transformer windings in inductive power transfer systems. In order to counteract the nonnegligible inductances and extract desirable characteristics from the system, various compensation networks have been proposed to obtain unity input power factor and constant output voltage for minimum reactive power and ease of control for battery charging or other power electronic applications. However, most of the existing control methods with fixed compensation networks, which are usually designed for a specific coupling coefficient, have failed to fulfill both requirements simultaneously due to the mismatch between the resonant frequencies of the networks after the deviation of the designed coupling condition. In this article, a dynamic series/series–parallel compensation network based on a switch-controlled capacitor is proposed to rematch the series- and parallel-resonant frequencies of the network such that the magnetizing inductance can be adaptively compensated for different sizes of air gap/misalignment. By doing so, the requirements of load-independent output voltage and unity input power factor can be simultaneously fulfilled, while zero-voltage switching on the primary bridge is still maintained under different coupling conditions by a single-stage converter. An experimental prototype with the air gap ranging from 10–16 cm is built to verify the idea.

A Novel S–S–LCLCC Compensation for Three-Coil WPT to Improve Misalignment and Energy Efficiency Stiffness of Wireless Charging System

Recent studies have proven that a three-coil wireless power transfer (WPT) design shows better performance and has higher efficiency compared to the two-coil designs, especially when the source resistance and transmission distance between the primary and receiver coil increases. The three-coil WPT system, similar to a two-coil system, is capable of achieving constant output current (CC) and constant output voltage (CV) with ZPA. However, in the CV mode of the conventional three-coil design, the efficiency of the light-load system dramatically decreases as the load becomes smaller. In this article, a new series-series-LCLCC (S-S-LCLCC) compensation design for the three-coil WPT system with load-independent output voltage, which is capable of realizing ZPA characteristics during the entire process of the charging process, is proposed. The new design is capable of significantly improving the energy efficiency stiffness against the load variation, misalignment, increasing the flexibility to optimize the system efficiency, reducing the voltage stress, and increasing the power delivery to load compared to the conventional topology. The experimental design shows that the overall trend of efficiency of the proposed design is higher than the conventional design as the load decreases and the new system has approximately 10% higher efficiency when the battery equivalent load resistance reaches 222 Ω.

Load-Independent Operative Regime for an Inductive Resonant WPT Link in Parallel Configuration

In this article, the possibility of obtaining a load-independent regime for a wireless power transfer link consisting of two magnetically coupled resonators in parallel configuration is discussed. Analytical formulas and experimental data to achieve a load-independent output voltage or current are presented. With regard to the input side, the cases of a current and a voltage generator are considered. It is shown that when the source is a current generator, either a constant output voltage or a current can be achieved by suitably choosing the operating frequency. As per the case of a voltage generator, by acting on the operating frequency, only a constant output current can be obtained.

On the Minimal Loading of Sensorless Series-Series Compensated Inductive WPT Link Operating at Load Independent Voltage Output Frequency Without Feedback

It is well-known that continuous conduction mode (CCM) operation at certain frequency yields nearly load-independent voltage gain in practical series-series compensated inductive wireless power transfer links (SS-IWPTL). Such an operation is usually referred to as “load independent voltage output” (LIVO). Residual load dependence is due to the presence of non-zero equivalent series resistances in both transmitter and receiver. On the other hand, it has been shown that voltage gain remains load-dependent at the same operating frequency in discontinuous conduction mode (DCM) irrespectively of equivalent series resistances values. Consequently, the output voltage of SS-IWPTL operated at LIVO frequency resides within a range with lower bound corresponding to rated load and upper bound corresponding to zero loading for a given input voltage. The upper bound expression (typically attained in CCM) is derived using the first harmonic approximation (FHA) based equivalent circuit and is well-established by previous works. On the contrary, derivation of the lower bound (achieved in DCM) requires differential equation (DE) based analysis and was barely treated in the literature. The paper demonstrates that lumped capacitance of receiving side coil and input capacitance of receiving side rectifier must be taken into account during DE-based analysis since they impose significant receiving side voltage oscillations, which may give rise to safety issues. In order to damp these oscillations and keep the output DC voltage below a predetermined limit, minimal loading should be applied at the SS-IWPTL output, creating an RCD-snubber equivalent. Guidelines for the minimal load sizing are derived in the paper analytically. Simulations and experiments based on a 400V, 1kW SS-IWPTL operating at LIVO frequency demonstrate excellent matching, validating the proposed analysis.

A Hybrid Inductive Power Transfer System With Misalignment Tolerance Using Quadruple-D Quadrature Pads

Pad misalignments are almost inevitable in most inductive power transfer (IPT) systems. It tends to cause parameter variations and, thus, significantly affects the performance of the IPT system. In this article, a hybrid IPT system with misalignment tolerance using the quadruple-D quadrature pads (QDQPs) is proposed to tolerate the x, y, z, and diagonal misalignments with load-independent output voltage, simplifying or even canceling control schemes. Besides, the proposed approach can restrict the increase of the primary current when the secondary side moves out of the operating region. Moreover, a new parametric design method is presented according to the misalignment characteristics of QDQPs. The method can limit the output voltage fluctuation to a certain range, given a predetermined misalignment distance. A 3.5-kW prototype was built to verify the proposed hybrid IPT system. The primary and secondary coil sizes are 400 mm × 400 mm, and the air gap is 150 mm. Experimental results demonstrate that the proposed hybrid system can tolerate -150 to +150 mm x-misalignment, -150 to +150 mm y-misalignment, -20 to +35 mm z-misalignment, and -100 to +100 mm diagonal misalignment with load-independent output voltage. Within the predetermined misalignment range, the output voltage fluctuation is less than 5%.

Comments on “Analysis, Design, and Optimization of $LC$ /S Compensation Topology With Excellent Load-Independent Voltage Output for Inductive Power Transfer”

In the article “Analysis, design, and optimization of LC /S compensation topology with excellent load-independent voltage output for inductive power transfer,” by Yao et al. , a novel LC /S (primary series-inductor parallel-capacitor/secondary series-capacitor) compensation topology was proposed. A single set of three-compensating-elements was derived, allowing to simultaneously attain LIVO and zero-voltage switching (ZVS). However, the inverter was not able to operate at a zero-phase angle (ZPA), minimizing its volt-ampere rating, with the proposed compensating element set. Since the proposed topology is extremely promising, the purpose of this note is to complement the presented analysis by pointing out the following: 1) A group of infinite three-compensating-elements sets exists (to which the set derived in the article belongs to), allowing to simultaneously achieve LIVO and ZVS; and 2) A unique three-compensating-elements set exists, allowing to simultaneously achieve LIVO and ZPA.

Design for continuous‐current‐mode operation of inductive‐power‐transfer converters with load‐independent output

The control design of inductive power transfer (IPT) converters can be greatly simplified by exploring the property of load-independent output. However, IPT converters can easily enter a discontinuous current mode (DCM) operation due to the non-linearity of the output diode rectification circuit at some loading conditions. The switching between operations of continuous current mode (CCM) and DCM within a switching cycle is highly non-linear, making the converter behaviour difficult to predict and analyse. The well-known first harmonic approximation (FHA) analysis method and the load-independent output property are no longer applicable when the converter enters DCM operation. This paper presents a simple and yet effective harmonic analysis method to reveal the main reason for the converter to enter DCM operation. Subsequently, the load boundary between CCM and DCM operations is derived in detail as a design criterion. A solution by increasing the input impedances at some higher order harmonic frequencies is also suggested to extend the load range against DCM operation. Finally, IPT prototypes using two higher order compensation circuits, having load-independent voltage output and capacitor filter, are demonstrated to verify the theoretical analysis.

Load-Independent Voltage and Current Transfer Characteristics of High-Order Resonant Network in IPT System

Load-independent output characteristics of an inductive power transfer (IPT) system are of increasing interest in electric vehicle and LED lighting applications. All compensation networks in the IPT system are actually high-order resonant circuits. In a high-order resonant network, there are multiple resonant frequencies to get load-independent voltage output and current output. It is critical to analyze the resonant conditions to achieve high efficiency in both load-independent voltage output and current output modes. This paper proposed a general modeling method for arbitrary high-order resonant networks to get both the load-independent voltage and current transfer characteristics. A high-order circuit can be modeled as a combination of an LC network, a multistage T-circuit, and/or multistage $\Pi $ -circuit in series. The proposed method is verified by applying to voltage-fed double-sided inductor–capacitor–capacitor (LCC), series–series (SS), S-SP, LCC-S, and current-fed CLC-LC compensation networks in the IPT system. The MATLAB simulation and the experimental prototype of a constant voltage-fed double-sided LCC compensated IPT system with up to 3.3-kW power transfer are built. The efficiency of the double-sided LCC compensated IPT system is up to 92.9% and 90.6% when the IPT system operates at resonant frequencies that achieve constant current output and constant voltage output, respectively, which are compliance with the frequency requirement by SAE J2954 standard.