Load-independent Voltage Research Articles

Extending the lower bound of attainable load-independent voltage gain values range in contactless, feedbackless and sensorless power delivery links

It is well-known that series(capacitive)-series(capacitive) compensated inductive wireless power transfer links (SS(CC)-IWPTL) operating at fixed frequency with constant coupling coefficient may be designed to attain arbitrary load independent voltage gain (LIVG) value, residing within a region defined only by loosely coupled transformer (LCT) parameters. Such a characteristic allows designing the system to be sensorless and operate without feedback, which is an extremely useful feature in hostile environment applications. Unfortunately, LCT parameters are not selected arbitrarily in practice; hence, the desired LIVG value may reside outside the attainable region, calling for an additional power conversion stage. In particular, region of attainable LIVG values is especially narrow when LCT inductances ratio is imposed by the application and thus cannot be freely selected. One of existing challenges is regulating the system output to low DC voltage while it is being fed from a much higher valued voltage source, i.e. calling for an extremely low LIVG value. The paper demonstrates that adopting an inductor rather than a capacitor as primary series compensation element allows to extend the lower bound of attainable LIVG values region for given LCT and operating frequency, potentially eliminating the need for an additional step-down power converter. Resulting coil-to-coil efficiency is quantified and guidelines for sizing corresponding compensation elements pair are established as well. Simulations and experiments accurately verify the proposed methodology.

Coupled-Inductor-Based Bidirectional Resonant Converter With Reduced Component Count and Load-Independent Voltage Gain

The bidirectional DC/DC converter (BDC) is a critical part to interface the DC-bus with energy storage system (ESS) in DC microgrid. This paper proposes a coupled-inductor based bidirectional resonant converter with center-aligned pulse width modulation (PWM) modulation method and reduced component count. The structure of dual half-bridges with coupled inductor makes the current analysis more complex than the full-bridge PWM resonant converter. Hence, a method of performance analysis based on the symmetrical pi model is proposed and the corresponding parameter design process is introduced. In addition, fixed switching frequency operation, load-independent voltage gain, full-load range zero-voltage-switching (ZVS), automatic bidirectional power transfer and bidirectional power limitation can be realized in the proposed PWM converter. An experimental prototype with rated power of 1000W is built to demonstrate the operation and performance of the proposed converter.

Double Coupling In-Wheel IPT System for Electric Vehicles

The electric automotive industry revolutionizes the transportation sector with new disruptive technologies like wireless charging systems and the tire industry is no exception. New airless and sustainable tire designs offer new possibilities to embody wireless charging technologies. This paper presents a novel in-Wheel Inductive Power Transfer (inWIPT) batteries charger design for electric vehicles (EV). The proposed system uses two magnetic couplers (MC) to transfer the energy: one from the off-board side to the wheel and another from the wheel to the on-board side. The proposed configuration keeps the air gap between the off-board transmitter pad and the wheel receiver to a minimum and almost independent of the vehicle type (Sedan, Suv, Truck). The two MCs are connected by a new resonant tank which modifies the voltage transfer characteristics. A step by step methodology that identifies the operational range of the mutual inductances, based on certain design specifications and constraints, is also presented. Experimental results validate the proposed inWIPT in a quasi load independent voltage operation in different coupling charging scenarios and a maximum output power of 2.6 kW with an efficiency of 89.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> .

A Scalable Electronic-Embedded Transformer, a New Concept Toward Ultra-High-Frequency High-Power Transformer in DC–DC Converters

The demand for high-power and high-density isolated dc-dc converter is driven by the rapid development of energy storage system, data center power supplies and transportation electrification. However, designing a high-power and high-frequency transformer presents significant challenges due to the trade-off between thermal management, leakage inductance minimization, and insulation requirements. In order to overcome this trade-off between power rating and operation frequency, a scalable electronic-embedded transformer (EET) is proposed with a low voltage bridge integrated into the transformer windings. The EET addresses the challenge through its simple open-loop control and natural current sharing, enabling easy parallel connection and scaling to different power ratings. Based on this concept, a bi-directional EET-based dc transformer (EET-DCX) is proposed to solve the transformer-level paralleling and resonant point shift issues in traditional LLC-DCX designs. By employing the embedded full bridge, the EET-DCX effectively cancels out the impedance of the leakage inductance, ensuring optimal operation at any frequency. Additionally, the EET-DCX retains the inherent advantages of the LLC-DCX, such as load-independent voltage gain, simple open-loop control, full load range zero voltage switching (ZVS), and low circulating current. Leveraging these advantages, the proposed EET-DCX solution has the potential to push the boundaries of transformer performance to the MHz operation frequency range with hundreds of kilowatts power capability. To validate these merits, a 12 kW 500 kHz EET-DCX with four planar EET units was built. This article is accompanied by two videos demonstrating the dynamic load changing and frequency changing test.

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.

Isolated Boost Type Resonant Converter With Fixed Frequency PWM and Load-Independent Voltage Gain

This article proposes an isolated boost type resonant converter (IBTRC) with fixed frequency pulsewidth modulation (PWM), which has load-independent gain characteristics. Only one magnetic component and two active switches are used in the proposed IBTRC. Therefore, compared with the two-stage converter and the current-fed resonant converter (CFRC), the number of magnetic components and switches of the proposed IBTRC is reduced. Moreover, with the proposed modulation, the converter obtains the benefits of load-independent voltage gain characteristics. Under wide load and voltage range, all switches can realize zero-voltage switching (ZVS) and all diodes can realize zero-current switching (ZCS). The operation mode, characteristics, and parameters design process of the proposed solution are also described in detail. Finally, a 600-W prototype is built to verify the validity and the advantages of the proposed scheme.

Constant frequency resonant inductive wireless power transfer link with load-independent voltage output - a generalized approach

A closed-form analytical expression is derived in the paper, linking five parameters (coil inductances, compensating capacitor values and coupling coefficient) of a typical series-series compensated resonant inductive wireless power transfer link (WPTL) with given operating frequency so that the output voltage remains load independent. It is shown that for a given operating frequency, infinite amount of compensation capacitor pairs exists, yielding load independent voltage gain. Feasible solutions range is determined and it is demonstrated that two commonly used operation points are particular cases of the proposed generalized design expression. Intuitive explanation based on generalized loosely coupled transformer model is also provided.

Load Independent Constant Current and Constant Voltage Control of LCC-Series Compensated Wireless EV Charger

In this article, load-independent constant current/voltage control operations of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -Series compensated wireless electrical vehicle (EV) chargers are presented. The parameters of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -Series compensation topology are designed based on load-independent zero phase angle and soft switching operation in the constant current charging operation. Then, the load independent soft switching conditions are achieved for constant voltage charging operation with a semibridgeless active rectifier using pulse density modulation control. Thus, high efficiency power transfer to the load is performed during the entire charging period. The wireless communication is not needed to achieve the output voltage regulation in the proposed EV charging system. The soft switching operations of the switches on the primary and the secondary side are achieved in a wide load range. The performance of the proposed charging system is validated by a prototype. At the full load condition, the output voltage of the system is regulated at 210 V while the output current is 5 A. The maximum efficiency is measured 93.8% at 1 kW output power with 150 mm air gap. The efficiency performance of the system is also evaluated based on the misalignment conditions.

Coupled Inductor Based Bidirectional Resonant Converter With Sine Wave Modulation in Wide Voltage Range

This letter proposes a coupled inductor based bidirectional series resonant converter (CIBSRC). In addition, a sine-wave modulation (SWM) with fixed frequency is also proposed to achieve load independent voltage gain for wide voltage range. The proposed CIBSRC integrates a boost stage and a resonant stage by a coupled inductor which replaces boost inductor, resonant inductor, and high frequency transformer. Compared with the bidirectional integrated resonant topologies, the number of magnetic components is less. Under proposed SWM, only one set of pulsewidth modulation signal is needed and zero voltage switching (ZVS) of all switches could be realized. Besides, the resonant current is a sine-wave which can simplify ZVS analysis and reduce current harmonics. A 500-W prototype with 100–200 V input voltage and 360 V output voltage was built and the results proved the reliability and validity.

Switching Transition Analysis and Optimization for Bidirectional CLLC Resonant DC Transformer

The demand for isolated bidirectional dc transformer (DCX) is driven by the fast development of energy storage system, data center power supply and transportation electrification. Due to zero voltage switching (ZVS) and small rms current, the open-loop CLLC resonant converter operating at the resonant frequency is considered a promising candidate for DCX with a constant voltage transfer ratio. To solve unsmooth bidirectional power flow and current distortion in traditional CLLC-DCX with synchronization rectification (SR) modulation, a dual-active-synchronization (DAS) modulation is adopted with identical driving signals on both sides. Firstly, the switching transition of this modulation is fully analyzed with the consideration of large device output capacitances. After comparison of different transitions, a so-called Sync-ZVS transition is found more desirable with ZVS, no deadtime conduction loss, and load-independent voltage gain. In order to achieve this switching transition, an Axis and Center Symmetric (ACS) method is proposed. Based on this method, an overall design procedure of CLLC-DCX with DAS modulation is also proposed. Finally, Sync-ZVS transition and the proposed ACS method are both verified by three 18 kW 500 kHz prototypes with a 98.8% peak efficiency. This article is accompanied by one video demonstrating the load changing test.

A Graphical Design Methodology Based on Ideal Gyrator and Transformer for Compensation Topology with Load-Independent Output in Inductive Power Transfer System

Compensation is crucial in the inductive power transfer system to achieve load-independent constant voltage or constant current output, near-zero reactive power, higher design freedom, and zero-voltage switching of the driver circuit. This article proposes a simple, comprehensive, and innovative graphic design methodology for compensation topology to realize load-independent output at zero-phase-angle frequencies. Four types of graphical models of the loosely coupled transformer that utilize the ideal transformer and gyrator are presented. The combination of four types of models with the source-side/load-side conversion model can realize the load-independent output from the source to load. Instead of previous design methods of solving the equations derived from the circuits, the load-independent frequency, zero-phase angle (ZPA) conditions, and source-to-load voltage/current gain of the compensation topology can be intuitively obtained using the circuit model given in this paper. In addition, not limited to only research of the existing compensation topology, based on the design methodology in this paper, 12 novel compensation topologies that are free from the constraints of transformer parameters and independent of load variations are stated and verified by simulations. In addition, a novel prototype of primary-series inductor–capacitance–capacitance (S/LCC) topology is constructed to demonstrate the proposed design approach. The simulation and experimental results are consistent with the theory, indicating the correctness of the design method.

General Pathways to Higher Order Compensation Circuits for IPT Converters via Sensitivity Analysis

This article first presents a systematic extension of second-order compensated inductive power transfer (IPT) converters, designed for achieving load-independent current (LIC) or load-independent voltage (LIV) output, to third-order compensated IPT converters through adding an inductor or capacitor at the input or output side. Conditions on the parameters to achieve the required output are given. Then, the system sensitivity to various parameter fluctuation is analyzed. Results from sensitivity analysis provide a convenient design guide for selecting parameters to achieve the required LIV and LIC operation for higher order IPT systems with fewer design constraints. Moreover, the analysis effectively reveals the roles of extra input-side or output-side inductors and capacitors in making the whole system less sensitive, and hence provides a fast understanding of the choice of higher order compensation circuits for applications addressing wide ranges of input variations, transformer coupling, and compensation network changes.

Analysis and Design of an LCL–T Resonant DC–DC Converter for Underwater Power Supply

Constant dc current distribution is preferred over dc voltage distribution in under-sea power distribution networks due to ruggedness over long distance cable impedance and faults. DC--DC converters with constant current input have different design constraints from their constant voltage input counterparts. Considering these constraints, an LCL-T resonant network based isolated dc-dc converter is analyzed in this article to operate as a load independent constant voltage source when supplied with constant dc current at the input. It is shown that use of passive rectification for this converter can limit converter operating load range, which is overcome through use of an active rectifier. A three-angle modulation strategy for a dual-active bridge (DAB) based LCL-T resonant converter is presented to achieve minimum VA rating for tank elements, transformer and the bridges. Detailed analysis and design method are provided for a converter regulating its output voltage at 150 V fed from a 1 A source, operating with 250 kHz switching frequency, over the load range of 50 to 500 W. Experimental results from prototype hardware are presented to show the accuracy of the 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.

Compensation Capacitors Sizing for Achieving Arbitrary Load-Independent Voltage Gain in Series-Series Inductive WPT Link Operating at Fixed Frequency

The letter reveals that for a given operating frequency, infinite amount of compensation capacitor pairs exists, yielding load independent voltage gain of a typical series-series compensated resonant inductive wireless power transfer link (WPTL). Closed-form analytical expression is derived, linking the values of compensating capacitors with the desired load independent voltage gain based on given coil inductances, coupling coefficient and operating frequency. Feasible range of achievable voltage gains is shown to be bounded by the value of coupling coefficient based on a novel 4-parameter generalized loosely coupled transformer model. Experimental results are given to validate the presented findings.

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.

Cost-Effective Compensation Design for Output Customization and Efficiency Optimization in Series/Series-Parallel Inductive Power Transfer Converter

Load-independent output with zero-phase-angle (ZPA) input is desirable in wireless inductive power transfer (IPT) converters for effective power delivery, but it usually greatly relies on the parameters of the loosely coupled transformer, normally fixed or constrained by space. Thus, customizable outputs cannot be readily achieved unless a new transformer is redesigned. In this article, we elaborate the rather complex relationships among compensation parameters, customizable load-independent-voltage (LIV) outputs with ZPA input, power efficiency, and overall compensation capacitance cost of the series/series-parallel (S/SP) IPT converter. We present a cost-effective compensation design to free the customization of LIV outputs from a parameter-constrained loosely coupled transformer, with optimization between efficiency enhancement and overall compensation capacitance cost. We conducted the proposed design supported by experimental results in S/SP IPT converters with an identical loosely coupled transformer and various sets of compensation capacitors. Compared with a conventional design, the proposed design provides custom ranges of LIV outputs in both a weak and a relatively strong coupling condition, with over 5.9% and 5% efficiency improvement, respectively. The overall compensation capacitance can also be reduced by up to 37% and 21.5%, respectively.

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.

A Three-Bridge IPT System for Different Power Levels Conversion under CC/CV Transmission Mode

This paper proposes an inductive power transfer (IPT) system with three-bridge switching compensation topology. With the proposed IPT topology, the equivalent circuit and the resonant condition are analyzed to achieve the load-independent constant current (CC) and load-independent constant voltage (CV) outputs. On this basis, multiple power levels can be achieved under CC/CV conditions by bridge arm switching, which makes it possible to complete charging tasks for multiple power level electric vehicles (EV) without switching the IPT system. A circuit simulation was built to verify the different power level switching effects of the structure. A 3.3 kW IPT system was designed to verify the proposed structure. At the rated output power, the experimental efficiency was up to 92.04% and 91.21% in CC and CV output modes, respectively.

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.