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A robust optimized active disturbance rejection control (ADRC) based grid voltage sensorless current controller is developed for an LCL -filtered grid-connected inverter (GCI) via a predictive control approach under various sources of disturbance, including the model uncertainties, the LCL inherent resonance phenomenon, and nonideal grid environment. Aiming to improve the sinusoidal reference tracking performance as well as to reject lumped sinusoidal disturbances in the control practice, a resonant extended state observer is integrated into the ADRC structure, which guarantees a rigorous stable operation of inverter for bounded filter parameter uncertainties and adverse grid voltage conditions. The grid frequency adaptability is thoroughly considered in the controller design process and synchronization technique, offering an extra capability for GCI to operate under different grid voltage frequency levels or even the frequency deviation caused by grid fault events. Robustness against parameter uncertainty and system stability is analyzed through the discrete-time frequency analysis and pole-zero map approaches. The simulation and hardware experiments are conducted for GCI with LCL filter parameters designed for two typical regions (i.e., the resonance frequency is less and greater than one-sixth of switching frequency) to validate the theoretical analysis and the effectiveness of the proposed control method.

Discontinuous conduction mode (DCM) and critical conduction mode (CRM) have held dominant positions in low-power boost power factor correction (PFC) control. Typically, two-level DCM and CRM suffer from high peak current and variable frequency at several-hundred kilohertz, respectively. The two factors can cause secondary effects such as high electromagnetic interference, current stresses, input filter design difficulty, and switching losses. Only to take merits of DCM and CRM that are fixed frequency and lower peak current, this article utilizes three-level boost (TLB) and its three-level current-slope options. This article proposes a hybrid conduction mode (HCM) control, which adopts adaptive single-switch on -time to achieve fixed-frequency quadrangular CRM operation. At lower input voltage, low inductor current enters DCM so that TLB PFC experiences hybrid operation modes depending on varying ac input. Analyses and experiment results in this article confirm that the proposed HCM control can achieve huge reductions of switching losses and peak current stress, improving efficiency in wide-range input voltage and load at the cost of acceptable PF drop. Moreover, a simple voltage-balancing scheme is implemented to the proposed control and verified effective to resolve the inherent topology issue.

In this article, we present a bridgeless hybrid-mode Zeta inverter for distributed energy systems. We integrate the secondary diode of the conventional unfolding-type Zeta inverter into one of diagonal pairs of the secondary-side switches in a bridgeless Zeta inverter. This structure decreases the number of active power components and provides naturally well distributed loss at the body diodes of the secondary-side switches over one cycle of grid voltage, and as a result, increases both output power transfer and reliability. To attain medium-high power capacity with appropriate size of magnetic components, the bridgeless Zeta inverter operates in both discontinuous conduction mode (DCM) and continuous conduction mode (CCM). However, control of the proposed inverter is difficult because of the distinct system dynamics caused by the operations in DCM and CCM. To deal with this control problem, we first identify the mode boundaries and, corresponding to each mode, develop a dynamic model to design a controller. Then, we propose to use a feedback controller plus a feedforward controller supplemented with a repetitive controller that uses a phase-lead compensator. Experimental results using a 300-W prototype demonstrate the feasibility and effectiveness of the proposed modeling and control approach.

This article proposes an average periodic delay-based repetitive controller for power factor correction (PFC) converters undergoing grid frequency variations. When the grid frequency is changed, a conventional repetitive controller requires an interpolation method or additional memory to achieve frequency adaptability. Furthermore, frequency adaptability can also be realized using a variable sampling frequency; however, this requires additional computation effort and a multirate digital control system. In this article, with a fixed sampling frequency and memory length, the proposed repetitive controller, which generates the average periodic delay unit, maintains its harmonic compensation performance under grid frequency variations. In the proposed method, the control variables are updated depending on the phase of the grid voltage to reflect the grid frequency deviations in the average period delay unit. Because no complicated software modification is necessary, the proposed scheme can be easily plugged in the existing control algorithm. The theoretical analysis and stability issues of the proposed scheme have been discussed in detail to ensure stable operation and disturbance rejection. Both the simulations and the experiments on a 1.5-kW PFC converter have been carried out to verify the effectiveness of the proposed repetitive control.

Discontinuous conduction mode controlled boosting power-factor-correction (PFC) converters suffer from high differential-mode electromagnetic interference (EMI) and require a large input filter due to high inductor current ripples at a single switching frequency. To remedy the shortcomings, a downward spread-spectrum frequency modulation (SSFM) is proposed in this letter. The SSFM can be used by boosting PFC converters for reduced EMI and improved efficiency. On top of the frequency modulation, a novel adaptive three-level current (ATC) scheme is devised with three-level boost PFC using multilevel capability to obtain further reductions of peak current and EMI amplitude. Analyses and experiments validate that the proposed SSFM and ATC scheme can bring improvements of EMI and efficiency at the cost of increased output-voltage ripple.

This article presents a single-sourced hybrid binary-cascaded multilevel inverter (BCMLI) with front-end high-frequency-link (HFL) converters. The purpose of HFL converters is to boost a dc-source voltage to three bus voltages in a binary ratio. In this article, one of the HFL converters is removed from the system and the voltage of the floating capacitor caused by the removal is controlled by a newly proposed floating-capacitor-voltage (FCV) control. Compared with the conventional BCMLI, the system size and cost can be significantly reduced while maintaining promising features, including 15-level output, high voltage gain, galvanic isolation, and reactive-power capability remain unchanged. The proposed FCV control utilizes one of the switching angles at the voltage-level transitions as the control variable in a single-loop design, which facilitates control simplicity in the proposed system. The hybrid BCMLI with the proposed FCV control shows relative advantages of voltage gain, isolation capability, reactive supply, and simple control system over the state-of-the-art hybrid inverter topologies with FCV control methods. The details of the hybrid BCMLI, modulation schemes, proposed FCV control design, and comparative assessment are included in this article. In order to validate the proposed system and FCV control, a set of experiments with a 1-kW BCMLI prototype has been conducted for wide-range load levels and power factors.

The low-voltage ride through condition (LVRT) attracts wider attention with the increased power of central grid-connected photovoltaic (PV) systems. The conventional double-loop-based control strategy designed for the nominal voltage is slow and potentially unstable under the LVRT operation. In this article, a novel sliding-mode control-based feedback linearization is proposed to improve response speed of the PV system under LVRT condition. To increase the robustness of the control to parameter variations, sliding-mode control is combined with the feedback linearization in this article. Further, the complete linearization of the PV system can be achieved over the whole operating range. Moreover, the dc-link voltage operating point of the PV panel can be dynamically adjusted based on the drop depth of the grid voltage, which will reduce the load changes during the LVRT and subsequently ensure the ac current levels to stay within the design limits in any fault situation and enhance the LVRT capability of the central grid-connected PV system. Finally, a 100-kW PV inverter is built to validate the feasibility and effectiveness of the proposed strategy for LVRT.

In inductive power transfer systems, impedance matching is critical in obtaining high efficiency. However, since the load impedance is subject to variations in most applications, especially wireless charging, maintaining impedance matching is a challenge. One valid solution is to maintain the equivalent load impedance at the optimal value against load variations by controlling a load-side power converter. Nonetheless, the system overall cost, size, and complexity can be increased. In this article, based on a proper selection between full-bridge and half-bridge modes of the inverter and rectifier, an operation mode selection method is proposed that can achieve semi-impedance-matching with large load variations. In general, with the proposed method, high overall efficiency can be obtained while neither additional power converters nor complicated control is required. A 1-kW prototype has been developed and the experimental results have verified the proposed method.

Conventional critical conduction mode (CRM) control for many boost-derived power factor correction (PFC) circuits forms triangular inductor current. For three-level boost (TLB) converter, the conventional method is unattractive because losses increase but waveform quality remains unchanged, compared to other topologies. In order to obtain higher efficiency and improved waveform qualities, this article focuses on the distinctive structure of TLB and its inherent degree-of-freedom (DOF) in current-slope shaping. Based on the DOF, a new CRM control for TLB PFC is proposed and analyzed. Detailed design and digital implementation method are also provided. Different from conventional methods, the proposed CRM control divides each switching cycle into three parts including common on -time of two switches, additional on -time of one switch and common off -time. Accordingly, TLB inductor current is synthesized in quadrangular forms. The analyses and experimental results confirm that switching frequency and peak input current of TLB can be reduced by the proposed control. Due to the reductions, efficiency, total harmonic distortion, and quality of input current are improved with practically unchanged power factor.

In this article, an optimal design procedure of shunt RC damped LCL filter is discussed. The proposed design procedure finds the minimum damping resistance to guarantee the stability of the grid-current controller without affecting the effectiveness of the filter. This minimum value is derived using the factorized filter input admittance and filter forward transadmittance. The proposed factorization procedure allows us to have an analytical expression of system poles and zeros. Moreover, the minimum damping resistor is determined upon the desired crossover frequency and the desired phase margin of the grid-current controller. After that, an objective function is assumed, and an optimization problem is formulated to minimize the physical size of the filter by minimizing the total capacitance and inductance and by minimizing the filter ripple content as well. Accordingly, the proposed objective function minimizes the filter physical size and filter power losses and, hence, enhances filter's efficiency. A prototype demonstrates and validates the proposed filter design and analysis experimentally.

Pseudo-dc-link inverters are promising due to the single-stage configuration. The inverters can be isolated by high-frequency transformers, which also provide the freedom of step-up/down through winding turns ratio. Conventionally, a pseudo-dc-link inverter is based upon a pulsewidth modulation converter like flyback converter, but the accompanying switching losses can degrade efficiency and overall system performance. Alternatively, resonant converters (e.g., series resonant converter) seem to fit the need as they typically display soft-switching characteristics. However, the relevant research results are scarce because of two main reasons: high resonant current and limited soft-switching range. To overcome the difficulties, a hybrid modulation method is proposed in this article for a pseudo-dc-link inverter based on series resonant converter. In high-line region, the inverter operates with variable-frequency modulation, which leads to zero-voltage switching and relatively low-resonant current. In low-line region, the inverter operates with a short-pulse density modulation so that switching losses are greatly curtailed without excessive resonant current. The merits in both regions together contribute toward the high efficiency of the inverter. This article presents the operational principle of the proposed modulation as well as a comparison with other modulation methods. In addition, a 400 V–230 V ac, 2-kW prototype was built to verify the proposed modulation and the peak efficiency reaches 98.0%.

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%.

This paper presents a bridgeless Cuk-derived single power conversion inverter with reduced number of power components, which has reactive-power capability. The secondary diode of the conventional unfolding-type Cuk inverter is integrated into one of the secondary-side switches in the bridgeless Cuk-derived configuration, which reduces the number of active-power components. A modulation technique is developed to provide a current path during reverse power flow; this technique enables operations under unity and nonunity power factors. A repetitive controller coupled with a linear feedback controller and a feed-forward controller are used to achieve accurate output current tracking under the harsh control environment. Driving the diode-integrated switch in synchronous rectification mode provides higher power conversion efficiency than that under diode-based secondary side rectification. Circuit operation and design guidelines of the bridgeless Cuk inverter are presented in detail. A 500-VA prototype inverter was fabricated to demonstrate the desirable performance of the bridgeless Cuk-derived inverter.

In the no-load condition, LLC converter usually fails to regulate its output voltage although it operates at a high switching frequency. Till now, it is hard to obtain the exact relationship between design parameters and the maximum switching frequency for no-load regulation capability. In this paper, a specific criterion for no-load regulation of LLC converter is provided, without using active components or other modulation schemes. By analyzing the macroscopic switching period and microscopic switching transition in the no-load condition, it is shown that not only the peaking resonant current during the switching transition, but also the resonant tank design affect the no-load regulation of the LLC converter, which affects the no-load regulation capability. Furthermore, the relationship among design parameters is analyzed and the design guideline is also provided to achieve no-load regulation at the specified maximum switching frequency. To verify the effectiveness of the proposed design, 400 V input and 50 V/200 W output prototype is built and tested.

In photovoltaic applications, many previous research works have focused on pulsewidth modulation (PWM) resonant converters in order to achieve a high efficiency with a wide input voltage range. Conventional approaches utilized symmetric boosting modulation at the secondary side rectifier to obtain a symmetric operation, and they utilized two boosting modes in a switching period. Among various rectifier structures, the voltage doubler structure has a strong advantage due to a small number of components. However, it suffers from serious hard switching losses in the secondary side rectifier. In this paper, a new converter with a novel asymmetrical modulation is proposed and verified. The strong point of the proposed converter is that it eliminates hard switching turn- on losses from the rectifier, while maintaining the minimized number of components. Although the proposed converter adopts an asymmetric modulation, the offset current on the transformer becomes zero inherently. Furthermore, a “forced half resonance” operation of the proposed converter keeps rms current stresses at the same level as conventional converter although it has a higher peak current. Accordingly, the proposed converter achieves a superior efficiency with the minimum number of components at 35–25 V input and 380 V/300 W output specification.

LLC resonant converter can achieve zero voltage switching (ZVS) for primary-side devices and zero current switching (ZCS) for secondary-side rectifiers. However, the reverse recovery and junction capacitance ( Cj ) of secondary-side diode critically affect the ZVS condition of primary-side switches. The effect of Cj has been discussed in literature, but not the reverse recovery. In this paper, the reverse recovery charge ( Q rr) is converted to an equivalent capacitance $(C_{{\rm rr}\_{\rm eq}})$ for the study of primary-side ZVS performance. An accurate model during deadtime is derived and further applied to characterize ZVS performance with different reverse recovery charges in different regions. The concept of establishing parameter C total to consider both Cj and $C_{{\rm rr}\_{\rm eq}}$ is proposed to evaluate the effect of the secondary-side rectifiers. This concept provides the guideline for diode and synchronous rectification mosfet selection to ensure ZVS condition for LLC converters. To verify the concept and the derived model, a 200/400 V 400 W LLC resonant converter prototype operating from 200 to 700 kHz is built and its ZVS performances with different diodes are compared. Two issues caused by Q rr effect, including $V_{\rm ds}$ reverse charging and asymmetrical waveform during deadtime, are explained thoroughly as well.

A conventional asymmetrical half-bridge (AHB) converter is one of the most promising topologies in low-power applications because of the zero-voltage switching (ZVS) of all switches and the small number of components. However, when it operates with an asymmetrical duty ratio, it has a large dc-offset current in the transformer, which increases the size and core loss of the transformer. Moreover, because the dc-offset current decreases ZVS energy for one of the half-bridge switches, it has low efficiency in a light load condition. In addition, because most of the output current is concentrated in high-voltage-rating diodes that have large forward voltage drops, large conduction loss occurs in the secondary rectifier. To solve these problems, a new AHB converter with a coupled inductor rectifier (CIR) is proposed in this paper. By adopting the CIR structure in the AHB converter, the proposed converter not only eliminates the dc-offset current in the transformer, but it also has equalized current stress in the rectifier diodes. As a result, the proposed converter achieves high efficiency in entire load condition, compared to an AHB converter with a full-bridge rectifier. The effectiveness and feasibility are verified with a 250–400 V input and 100 V/200 W output prototype.

In this paper, a highly efficient isolated resonant converter with a novel modulation method is proposed for delivering power from photovoltaic (PV) modules to the dc microgrid. The proposed modulation method allows the converter to boost low input voltages and regulate a wide input voltage range. The converter design is based on a series resonant converter (SRC) that operates at the resonant frequency to achieve highest efficiency under nominal input voltage condition. Under shadowed or low irradiance conditions of PV panels, the converter will operate with the proposed “double-pulse duty cycle” modulation method to step up the voltage for the dc microgrid connection. With the proposed modulation method, the output switches serve for both synchronous rectification and voltage boost function. This method enables a higher voltage boost ratio than the SRC without adding additional switches while operating at the resonant frequency. A 300-W hardware prototype with gallium-nitride devices is built to verify the performance of the proposed converter and modulation method. The converter achieved a peak efficiency of 98.9% and a California Energy Commission weighted efficiency of 98.7% under nominal input voltage condition.

This paper presents a dual-mode Cuk inverter for photovoltaic/thermoelectric power applications. A dual-mode Cuk inverter operates in both discontinuous conduction mode (DCM) and continuous conduction mode (CCM), and has the advantages of low ripples of voltage and current at the input and output, medium power density, and step-up/step-down ability, but is difficult to control because DCM and CCM have distinct system dynamics. To overcome this control problem, we propose to use a repetitive controller (RC) with a multiple phase-lead compensator for the dual-mode Cuk inverter. If the RC is applied by itself, the distinct system dynamics may severely degrade its system performance. Thus, in the proposed RC, we mainly use a multiple phase-lead compensator to compensate for the different phase lags of the dual-mode Cuk inverter. To reduce the burden from the RC, we use the dual-mode nominal duty ratio as feedforward control input. We also analyze the boundary of operation modes in the dual-mode Cuk inverter, then provide detailed and practical guidelines to design the control parameters. Experimental results obtained on a 500-W digitally controlled module integrated converter prototype confirmed the effectiveness of the control approach.

Current control loop response in voltage source inverters impacts the quality of output current and output voltage waveforms. Parabolic current control provides a fast transient response with approximate-constant switching frequency, solving the frequency variation problem of hysteresis current control. This makes it a good candidate for the current control loop of voltage source inverters to achieve a good system performance. Yet, parabolic current control is often implemented with digital-to-analog converters, analog comparators, and field-programmable gate array circuits where increasing switching frequency pushes update speed and bandwidth requirements. Concurrently, even if the transient response of parabolic current control is fast, it can still take up several switching cycles converging to steady state. In order to solve both problems, a new current control strategy, motivated by the convergence analysis of parabolic current control but with a convergence process that takes just one switching operation, is proposed: single-step current control. Since single-step current control samples two data points in a switching cycle instead of using continuous parabolic carriers, it can be easily implemented in a digital microcontroller then high switching frequency can be achieved. The small signal model, dead-time compensation, and stability analysis are also studied in this paper. The convergence speed of the algorithm is verified with experimental hardware prototype and the transient performance of the designed voltage source inverter meets expectation.

This paper proposes an iterative learning control (ILC) scheme for a dual-mode flyback inverter operating in both discontinuous conduction mode (DCM) and continuous conduction mode (CCM). In dealing with more than 200-W power applications, the dual-mode flyback inverter takes advantage of both DCM and CCM operations. However, it is difficult to control the output current because the system gain is quite low in DCM and the system transfer function has a right-half plane zero in CCM. To overcome the problems occurring in DCM and CCM operations, we propose an ILC scheme with multiple phase-lead compensation. The ILC is proposed to achieve accurate reference tracking and to reject periodic disturbances. The multiple phase-lead compensation technique is then employed to compensate for the effect of different system dynamics. As a theoretical result, we derive the asymptotic stability of the closed-loop system. We also performed the numerical simulations and experimental tests to validate the proposed control approach.

The magnetic component is considered one of the major factors that dominate the power density of power converters. In this paper, magnetic integration of inductors of a three-phase LCL filter on delta-yoke composite core is explored and investigated. Using the conventional rectangular core as the baseline design reference, the proposed delta-yoke core achieved a weight reduction of close to 10% without affecting the filter's performance. The inductances based on both shapes were calculated and verified through magnetic circuit analysis and three dimensional finite element analysis simulation. Hardware prototypes of the integrated inductor with both the conventional rectangular and the proposed delta-yoke cores were fabricated and tested with a three-phase inverter circuit to verify the design validity.

A multiscale electrothermal simulation approach is presented to optimize the design of a hybrid switch soft-switching inverter using a library of dynamic electrothermal component models parameterized in terms of electrical, structural, and material properties. Individual device area, snubber capacitor, and gate drive timing are used to minimize the total loss of the soft-switching inverter module subject to the design constraints including total device area and minimum on-time consideration. The proposed multiscale electrothermal simulation approach allows for a large number of parametric studies involving multiple design variables to be considered, drastically reducing simulation time. The optimized design is then compared and contrasted with an already existing design from the Virginia Tech Freedom Car Project using the generation II module. It will be shown that the proposed approach improves the baseline design by 16% in loss and reduces the cooling requirements by 42%. Validation of the electrical and thermal device models against measured data is also provided.

This article proposes an improved odd-harmonic repetitive controller (IORC) for a Cuk-derived inverter. The IORC is composed of a modified structure of an odd-harmonic repetitive controller (ORC) combined with a proportional-integral (PI) controller and a feedforward controller to achieve a zero steady-state error in the Cuk-derived inverter. The IORC features much larger gains at odd-harmonic frequencies by modification of an existing ORC, so both its convergence rate and transient response become much faster. The added benefit of using the IORC is that it has wider bandwidth at odd-harmonic frequencies, so it is more insensitive to the grid frequency fluctuation. The IORC also adopts a linear phase-lead compensator to compensate for the phase lag of the Cuk-derived inverter. Hence, the IORC can effectively eliminate the odd-harmonic components in the output current of the grid-connected Cuk-derived inverter while using less memory space. However, the distinctive structure of the IORC makes it difficult to guarantee the stability of the overall control system. Thus, in this article, a new stability analysis method is developed for the IORC system, and then a practical design guideline is also provided to obtain the optimal control parameters. Experimental results are presented to validate the proposed controller in the 300-W Cuk-derived inverter prototype.

An inductive power transfer (IPT) system usually consists of four parts: an ac–dc power factor correction (PFC) converter, a high-frequency dc–ac inverter, a compensation network comprising a loosely coupled transformer (LCT) and the resonant capacitors, and a rectification output circuit. Due to the relatively large air gap, the magnetic coupling coefficient of the IPT system is significantly lower than that with tightly coupled transformer. As a result, the efficiency of the IPT system is always a main concern for applications with possible gap variation or misalignment condition. To ensure high power transfer efficiency, these IPT systems should have high tolerance for different gap variation and horizontal misalignment conditions. In this paper, the effect of coupling coefficient deviation to compensation network efficiency is analyzed, and design considerations to reduce gap and misalignment effects for the IPT system are proposed. By using finite-element analysis simulation method, the performance of different transmitter and receiver coil dimensions is compared. In order to validate the performance of the proposed design considerations, a 100-W hardware prototype with two sets of LCT is built and the corresponding experiments are carried out. As compared to the symmetrical LCT architecture, the proposed asymmetrical LCT prototype improves the coupling coefficient reduction from 68% to 28% when the gap varies from 6 to 20 mm and from 89% to 31% when the misalignment ranges from 0 to 50 mm. Therefore, the efficiency deviation for the asymmetrical LCT is maintained within 3.5% over the entire tested gap variation and misalignment ranges.

Hysteresis current control is an attractive nonlinear current-control method for voltage source inverters when a fast system response is required. A well-known disadvantage of hysteresis current control is that the system has to operate over a wide switching frequency range. This causes an increase in the switching losses of the system and increases the difficulty in designing the output filter. The recently proposed parabolic current control solves this problem by employing a pair of parabolic carriers as the control band. Through the use of parabolic current control, constant switching frequency can be achieved. In the implementation of parabolic current control, dead time is employed to prevent shoot through of the inverter leg. The employment of dead time impacts the current-tracking precision and the stability range of the parabolic current-control method. Another side effect of using dead time is that the switching frequency deviates from the desired value. In this paper, the effects of dead time on parabolic current control are analyzed, and a compensation method is proposed for voltage source inverters that use parabolic current control. Using the output current direction of the voltage source inverter, a new pair of improved parabolic carriers is derived. As a result, the current error can be well controlled and the effects of dead time can be eliminated. The improvement in the current tracking of the system comes with an added benefit where the duty cycle range is extended. The effectiveness of the proposed dead-time compensation method is experimentally verified by the use of a full-bridge voltage source inverter.

This paper presents for the first time a full three-dimensional (3-D), multilayer, and multichip thermal component model, based on finite differences, with asymmetrical power distributions for dynamic electrothermal simulation. Finite difference methods (FDMs) are used to solve the heat conduction equation in three dimensions. The thermal component model is parameterized in terms of structural and material properties so it can be readily used to develop a library of component models for any available power module. The FDM model is validated with a full analytical Fourier series-based model in two dimensions. Finally, the FDM thermal model is compared against measured data acquired from a newly developed high-speed transient coupling measurement technique. By using the device threshold voltage as a time-dependent temperature-sensitive parameter (TSP), the thermal transient of a single device, along with the thermal coupling effect among nearby devices sharing common direct bond copper (DBC) substrates, can be studied under a variety of pulsed power conditions.

This paper presents a nonisolated, high boost ratio dc-dc converter with the application for photovoltaic (PV) modules. The proposed converter utilizes a hybrid transformer to incorporate the resonant operation mode into a traditional high boost ratio active-clamp coupled-inductor pulse-width-modulation dc-dc converter, achieving zero-voltage-switching (ZVS) turn-on of active switches and zero-current-switching turn-off of diodes. As a result of the inductive and capacitive energy being transferred simultaneously within the whole switching period, power device utilization (PDU) is improved and magnetic utilization (MU) is optimized. The improved PDU allows reduction of the silicon area required to realize the power devices of the converter. The optimized MU reduces the dc-bias of magnetizing current in the magnetic core, leading to smaller sized magnetics. Since the magnetizing current has low dc-bias, the ripple magnetizing current can be utilized to assist ZVS of main switch, while maintaining low root-mean-square (RMS) conduction loss. The voltage stresses on the active switches and diodes are maintained at a low level and are independent of the wide changing PV voltages as a result of the resonant capacitor in series in the energy transfer loop. The experimental results based on 250 W prototype circuit show 97.7% peak efficiency and system CEC efficiencies greater than 96.7% over 20 to 45 V input voltages. Due to the high efficiency over wide power range, the ability to operate with a wide variable input voltage and compact size, the proposed converter is an attractive design for PV module applications.

Modular photovoltaic (PV) power conditioning systems (PCSs) require a high-efficiency dc-dc converter stage capable of regulation over a wide input voltage range for maximum power point tracking. In order to mitigate ground leakage currents and to be able to use a high-efficiency, nonisolated grid-tied inverter, it is also desirable for this microconverter to provide galvanic isolation between the PV module and the inverter. This paper presents a novel, isolated topology that is able to meet the high efficiency over a wide input voltage range requirement. This topology yields high efficiency through low circulating currents, zero-voltage switching (ZVS) and low-current switching of the primary side devices, ZCS of the output diodes, and direct power transfer to the load for the majority of switching cycle. This topology is also able to provide voltage regulation through basic fixed-frequency pulsewidth modulated (PWM) control. These features are able to be achieved with the simple addition of a secondary-side bidirectional ac switch to the isolated series resonant converter. Detailed analysis of the operation of this converter is discussed along with a detailed design procedure. Experimental results of a 300-W prototype are given. The prototype reached a peak power stage efficiency of 98.3% and a California Energy Commission (CEC) weighted power stage efficiency of 98.0% at the nominal input voltage.

Photovoltaic (PV) source simulators serve as a convenient tool for the dynamic evaluation of solar power conditioning systems and maximum power point tracking algorithms. High efficiency and fast transient response time are essential features of any PV source simulator. This paper proposes a new type of PV source simulator that incorporates the advantages of both analog and digital-based simulators. The proposed system includes a three-phase ac-dc dual boost rectifier cascaded with a three-phase dc-dc interleaved buck converter. The selected power stage topology is highly reliable and efficient. Moreover, the multiphase converter helps improve system transient response though producing low output ripple which makes it adequate for PV source simulators. The simulator circuitry emulates precisely the static and the dynamic characteristics of actual PV generators under different load and environmental conditions. Additionally, the system allows the creation of the partial shading and bypass diodes effect on PV characteristics. The paper investigates the dynamic performance of a commercial solar power inverter using the proposed PV source simulator in steady-state and transient conditions. Closed-loop output impedance of the proposed PV source simulator has been measured and verified at different operating regions. The impedance profile--magnitude and phase--matches the output impedance of actual PV generators. Mathematical modeling and experimental validation of the proposed system is thoroughly presented based on a 2.0 kW hardware prototype. The proposed simulator efficiency, including the active-front-end rectifier and the converter stages, peaks at 96.7%.

This paper presents a zero-voltage-switching (ZVS) full-bridge dc-dc converter combing resonant and pulse-width-modulation (PWM) power conversions for electric vehicle battery chargers. In the proposed converter, a half-bridge LLC resonant circuit shares the lagging leg with a phase-shift full-bridge (PSFB) dc-dc circuit to guarantee ZVS of the lagging-leg switches from zero to full load. A secondary-side hybrid-switching circuit, which is formed by the leakage inductance, output inductor of the PSFB dc-dc circuit, a small additional resonant capacitor, and two additional diodes, is integrated at the secondary side of the PSFB dc-dc circuit. With the clamp path of a hybrid-switching circuit, the voltage overshoots that arise during the turn off of the rectifier diodes are eliminated and the voltage of bridge rectifier is clamped to the minimal achievable value, which is equal to secondary-reflected input voltage of the transformer. The sum of the output voltage of LLC resonant circuit and the resonant capacitor voltage of the hybrid-switching circuit is applied between the bridge rectifier and the output inductor of the PSFB dc-dc circuit during the freewheeling phases. As a result, the primary-side circulating current of the PSFB dc-dc circuit is instantly reset to zero, achieving minimized circulating losses. The effectiveness of the proposed converter was experimentally verified using a 4-kW prototype circuit. The experimental results show 98.6% peak efficiency and high efficiency over wide load and output voltage ranges.

This paper presents a high-reliability single-phase transformerless grid-connected inverter that utilizes superjunction MOSFETs to achieve high efficiency for photovoltaic applications. The proposed converter utilizes two split ac-coupled inductors that operate separately for positive and negative half grid cycles. This eliminates the shoot-through issue that is encountered by traditional voltage source inverters, leading to enhanced system reliability. Dead time is not required at both the high-frequency pulsewidth modulation switching commutation and the grid zero-crossing instants, improving the quality of the output ac-current and increasing the converter efficiency. The split structure of the proposed inverter does not lead itself to the reverse-recovery issues for the main power switches and as such superjunction MOSFETs can be utilized without any reliability or efficiency penalties. Since MOSFETs are utilized in the proposed converter high efficiency can be achieved even at light load operations achieving a high California energy commission (CEC) or European union efficiency of the converter system. It also has the ability to operate at higher switching frequencies while maintaining high efficiency. The higher operating frequencies with high efficiency enables reduced cooling requirements and results in system cost savings by shrinking passive components. With two additional ac-side switches conducting the currents during the freewheeling phases, the photovoltaic array is decoupled from the grid. This reduces the high-frequency common-mode voltage leading to minimized ground loop leakage current. The operation principle, common-mode characteristic and design considerations of the proposed transformerless inverter are illustrated. The total losses of the power semiconductor devices of several existing transformerless inverters which utilize MOSFETs as main switches are evaluated and compared. The experimental results with a 5 kW prototype circuit show 99.0% CEC efficiency and 99.3% peak efficiency with a 20 kHz switching frequency. The high reliability and efficiency of the proposed converter makes it very attractive for single-phase transformerless photovoltaic inverter applications.

This paper presents a nonisolated, high boost ratio hybrid transformer dc–dc converter with applications for low-voltage renewable energy sources. The proposed converter utilizes a hybrid transformer to transfer the inductive and capacitive energy simultaneously, achieving a high boost ratio with a smaller sized magnetic component. As a result of incorporating the resonant operation mode into the traditional high boost ratio pulsewidth modulation converter, the turn-off loss of the switch is reduced, increasing the efficiency of the converter under all load conditions. The input current ripple and conduction losses are also reduced because of the hybrid linear-sinusoidal input current waveforms. The voltage stresses on the active switch and diodes are maintained at a low level and are independent of the changing input voltage over a wide range as a result of the resonant capacitor transferring energy to the output of the converter. The effectiveness of the proposed converter was experimentally verified using a 220-W prototype circuit. Utilizing an input voltage ranging from 20 to 45 V and a load range of 30–220 W, the experimental results show system of efficiencies greater than 96% with a peak efficiency of 97.4% at 35-V input, 160-W output. Due to the high system efficiency and the ability to operate with a wide variable input voltage, the proposed converter is an attractive design for alternative low dc voltage energy sources, such as solar photovoltaic modules and fuel cells.

Effective photovoltaic power conditioning requires efficient power conversion and accurate maximum power point tracking to counteract the effects of panel mismatch, shading, and general variance in power output during a daily cycle. In this paper, the authors propose an integrated boost resonant converter with low component count, galvanic isolation, simple control, as well as high efficiency across a wide input and load range. Provided is a discussion of the converter synthesis, key operational features, converter design procedure, and loss analysis, as well as experimental verification by way of a 250-W prototype with a California Energy Commission efficiency of 96.8%.

This paper first presents a hybrid-switching step-down dc-dc converter, and then, by introducing transformer isolation, a novel hybrid-switching phase-shift full-bridge dc-dc converter is derived for electric vehicle battery chargers. The proposed converter provides wide zero-voltage-switching range in the leading-leg switches, achieves zero-current-switching for lagging-leg switches, and uses a hybrid-switching method to avoid freewheeling circulating losses in the primary side. Because the resonant capacitor voltage of the hybrid-switching circuit is applied between the bridge rectifier and the output inductor for the duration of the freewheeling intervals, a smaller sized output inductor can be utilized. With the current rectifier diode of the hybrid-switching circuit providing a clamping path, the voltage overshoots that arise during the turn-off of the rectifier diodes are eliminated and the voltage stress of bridge rectifier is clamped to the minimal achievable value, which is equal to secondary-reflected input voltage of the transformer. The inductive energy stored in the output inductor and the capacitive energy stored in the resonant capacitor of the hybrid-switching circuit are transferred to the output simultaneously during the freewheeling intervals with only one diode in series in the current path, achieving more effective and efficient energy transfer. The effectiveness of the proposed converter was experimentally verified using a 3.6-kW prototype circuit designed for electric vehicle onboard chargers. Experimental results of the hardware prototype show that the converter achieves a peak efficiency of 98.1% and high system efficiencies over wide output voltage and power ranges.

A novel soft-switching hybrid converter combining the phase-shift full-bridge (FB) and half-bridge (HB) LLC resonant converters' configuration with shared zero-voltage switching (ZVS) lagging leg is proposed to ensure the switches in the lagging leg operating at fully ZVS condition. The dual outputs of the proposed hybrid FB-HB converter are connected in series and the whole dc-output voltage can be regulated by the PWM phase-shift control within the desired voltage range. A resonant circuit is used in the secondary side of the FB converter to reset the primary current during the freewheeling period, as well as to transfer more input energy and clamp secondary rectifier voltage. The proposed converter is attractive for hybrid electric vehicle/electric vehicle on-board charger applications. The principle of operation, the validity, and performance are illustrated and verified on a 3.7-kW experimental circuit. Experimental results show that the proposed converter can get good efficiency curves at different operation points, and the maximum efficiency is 98.30%.

This paper presents a unique modulation method for extending the input range of pulse-width modulation (PWM)-integrated resonant converters, such as the isolated boost resonant converter, while maintaining high conversion efficiency. The technique includes primarily the hybridizing of constant-on, constant-off, and fixed-frequency control depending only on the required duty cycle. The modulation scheme reduces core loss and conduction loss dramatically by decreasing the applied volt-seconds at the transformer and improving the switching period utilization. With hybrid-frequency control, the circuit also maintains zero current switching for the output diodes, minimizes switching loss, and eliminates circulating energy at the transformer across the entire operating range. It also allows for a predictable voltage gain, dependent only on duty cycle and transformer turns ratio. A detailed loss analysis is provided and verified against a 180 W experimental prototype, with an input range of 12-48 V and a switching frequency range of 30-70 kHz. Implementation issues are also handled with a variety of solutions for realizing the modulation scheme. Experimental results show greater than 4% weighted efficiency improvement in the prototype using the proposed method.

This paper investigates a plug-in repetitive control scheme for bridgeless power factor correction (PFC) converters to mitigate input current distortions under continuous conduction mode and discontinuous conduction mode operating conditions. From the PFC converter model and the fact that a type-II compensator is used, a design methodology to maximize the bandwidth of the feedback controller is suggested. After that, the error transfer function including the feedback controller is derived, and the stability of the repetitive control scheme is evaluated using the error transfer function. The implementation of the digital repetitive controller is also discussed. The simulation and experimental results show that the input current THD is significantly improved by using the proposed control scheme for a 1-kW single-phase bridgeless PFC converter prototype.

A novel soft-switching converter combining resonant half-bridge and phase-shifted pulsewidth modulation (PWM) full-bridge configuration is proposed to ensure the switches in the leading-leg operating at zero-voltage switching from true zero-load to full-load, and the switches in the lagging leg working at zero-current switching with minimum duty cycle loss and circulating conduction loss by significantly reducing leakage or series inductance. Experimental results of a 3.4-kW hardware prototype show that the circuit achieves true full-range soft switching with 98% peak efficiency. The hybrid resonant and PWM converter is attractive for electrical vehicle battery charger application.

The high-brightness white-light-emitting diode (LED) has attracted a lot of attention for its high efficacy, simple to drive, environmentally friendly, long lifespan, and compact size. The power supply for LED also requires long life, while maintaining high efficiency, high power factor, and low cost. However, a typical power supply design employs an electrolytic capacitor as the storage capacitor, which is not only bulky, but also with a short lifespan, thus hampering performance improvement of the entire LED lighting system. In this paper, a novel power factor correction (PFC) topology is proposed by inserting the valley-fill circuit in the single-ended primary inductance converter (SEPIC)-derived converter, which can reduce the voltage stress of the storage capacitor and output diode under the same power factor condition. This valley-fill SEPIC-derived topology is, then, proposed for LED lighting applications. By allowing a relatively large voltage ripple in the PFC design and operating in the discontinuous conduction mode (DCM), the proposed PFC topology is able to eliminate the electrolytic capacitor, while maintaining high power factor and high efficiency. Under the electrolytic capacitor-less condition, the proposed PFC circuit can reduce the capacitance of the storage capacitor to half for the same power factor and output voltage ripple as comparing to its original circuit. To further increase the efficiency of LED driver proposal, a twin-bus buck converter is introduced and employed as the second-stage current regulator with the PWM dimming function. The basic operating principle and analysis will be described in detail. A 50-W prototype has been built and tested in the laboratory, and the experimental results under universal input-voltage operation are presented to verify the effectiveness and advantages of the proposal.

This paper presents a new type of cascade inverter based on dual buck topology and phase-shift control scheme. The proposed cascade dual buck inverter with phase-shift control inherits all the merits of dual buck type inverters and overcomes some of their drawbacks. Compared to traditional cascade inverters, it has much enhanced system reliability thanks to no shoot-through problems and lower switching loss with the help of using power MOSFETs. With phase-shift control, it theoretically eliminates the inherent current zero-crossing distortion of the single-unit dual buck type inverter. In addition, phase-shift control and cascade topology can greatly reduce the ripple current or cut down the size of passive components by increasing the equivalent switching frequency. A cascade dual buck inverter has been designed and tested to demonstrate the feasibility and advantages of the system by comparing single-unit dual buck inverter, 2-unit and 3-unit cascade dual buck inverters at the same 1 kW, 120 V ac output conditions.

This paper presents a new type of three-phase voltage source inverter (VSI), called three-phase dual-buck inverter. The proposed inverter does not need dead time, and thus avoids the shoot-through problems of traditional VSIs, and leads to greatly enhanced system reliability. Though it is still a hard-switching inverter, the topology allows the use of power MOSFETs as the active devices instead of IGBTs typically employed by traditional hard-switching VSIs. As a result, the inverter has the benefit of lower switching loss, and it can be designed at higher switching frequency to reduce current ripple and the size of passive components. A unified pulsewidth modulation (PWM) is introduced to reduce computational burden in real-time implementation. Different PWM methods were applied to a three-phase dual-buck inverter, including sinusoidal PWM (SPWM), space vector PWM (SVPWM) and discontinuous space vector PWM (DSVPWM). A 2.5 kW prototype of a three-phase dual-buck inverter and its control system has been designed and tested under different dc bus voltage and modulation index conditions to verify the feasibility of the circuit, the effectiveness of the controller, and to compare the features of different PWMs. Efficiency measurement of different PWMs has been conducted, and the inverter sees peak efficiency of 98.8% with DSVPWM.

In this paper, a single-phase grid-connected transformerless photovoltaic inverter for residential application is presented. The inverter is derived from a boost cascaded with a buck converter along with a line frequency unfolding circuit. Due to its novel operating modes, high efficiency can be achieved because there is only one switch operating at high frequency at a time, and the converter allows the use of power MOSFET and ultrafast reverse recovery diode. It also features a robust structure because the phase leg does not have a shoot-through issue. This paper begins with theoretical analysis and modeling of this boost-buck converter-based inverter. And the model indicates that small boost inductance will lead to an increase in the resonant pole frequency and a decrease in the peak of Q, which results in easier control and greater stability. Thus, interleaved multiple phases structure is proposed to have small equivalent inductance; meanwhile, the ripple can be decreased, and the inductor size can be reduced as well. A two-phase interleaved inverter is then designed accordingly. Finally, the simulation and experiment results are shown to verify the concept and the tested efficiency under 1-kW power condition is up to 98.5%.

An improved twin-bus converter using n + 1 active switches is proposed for the n-string LEDs to be dimmable from true zero to the rated current level. The n-switches that perform the current-loop regulation can operate at very high frequency because of significantly reduced voltage stresses, while achieving high efficiency over a wide load range. The dimming switch, however, can operate at a low frequency to ensure high-efficiency operation without light flicker. The detailed power stage operating principle and control scheme of the individual string current for the presented converter are described. Experimental results of a 3-string 50-W LED driver hardware prototype show that the independent string current can be regulated from zero to the desired 350 mA for dimming control. A peak efficiency of 98.3% was achieved with 1.08-MHz switching frequency.

A novel, high-efficiency inverter using MOSFETs for all active switches is presented for photovoltaic, nonisolated, ac-module applications. The proposed H6-type configuration features high efficiency over a wide load range, low ground leakage current, no need for split capacitors, and low-output ac-current distortion. The detailed power stage operating principles, pulsewidth modulation scheme, associated multilevel bootstrap power supply, and integrated gate drivers for the proposed inverter are described. Experimental results of a 300 W hardware prototype show that not only are MOSFET body diode reverse-recovery and ground leakage current issues alleviated in the proposed inverter, but also that 98.3% maximum efficiency and 98.1% European Union efficiency of the dc-ac power train and the associated driver circuit are achieved.

Lithium-ion-based battery energy storage system has started to become the most popular form of energy storage system for its high charge and discharge efficiency and high energy density. This paper proposes a high-efficiency grid-tie lithium-ion-battery-based energy storage system, which consists of a LiFePO4-battery-based energy storage and a high-efficiency bidirectional ac-dc converter. The battery management system estimates the state of charge and state of health of each battery cell and applies active charge equalization to balance the charge of all the cells in the pack. The bidirectional ac-dc converter works as the interface between the battery pack and the ac grid. A highly efficient opposed-current half-bridge-type inverter along with an admittance-compensated quasi-proportional resonant controller is adopted to ensure high power quality and precision power flow control. A 1-kW prototype has been designed and implemented to validate the proposed architecture and system performance.

A novel soft-switching inverter using two small coupled magnetics in one resonant pole is proposed to ensure the main switches operating at zero-voltage switching from zero load to full load and the auxiliary switches at zero-current switching with load adaptability and small current stress. Since independent coupled magnetics structure avoids the unwanted magnetizing current freewheeling loop, the size of the coupled magnetics can be minimized with low magnetizing inductance, and the saturable inductor can be eliminated. Detailed circuit operation is described, and voltage-second balance condition of the magnetics is expressed mathematically. A 4-kW hardware prototype has been designed, fabricated, and tested to verify the validity of the novel circuit and the improved performance of the proposed soft-switching inverter. Experimental results show an excellent agreement with analytical results. Since the measured efficiency from 20% to 100% load consistently shows above 97.8% and peaks at 98.2%, the proposed inverter is very attractive for high-efficiency applications where energy saving is a major concern.

This paper first introduces the design of an ultrahigh efficiency 50-kW bidirectional dc-dc converter at zero-voltage-switching operation, and then, a high-precision efficiency measurement method using a regenerative approach. The ultrahigh efficiency bidirectional dc-dc converter is achieved with 1) the use of CoolMOS as the main switch under zero-voltage soft switching condition; 2) multiple-phase legs for current sharing to reduce the conduction loss; and 3) coupling inductors between each two-phase legs to reduce the core loss. Two identical hardware prototypes were designed, fabricated, and tested for performance evaluation. In order to precisely measure the converter efficiency, the two identical bidirectional dc-dc converters are tested with one as the device under test and the other as the regenerative unit. With the use of ± 0.5% current shunt and regenerative measurement, the relative efficiency error stays below ±0.025%. Measured efficiency with load from 20% to 100% consistently shows above 97.50%. At the 50 kW full-load condition, the efficiency is 99.05% with ±0.01% efficiency relative error.

In this paper, smooth mode transfers and accurate current sharing are performed in a multi-inverter-based microgrid system by the designed system level controls with control area network communication. Controllers of individual inverters within the microgrid in both grid-tie and islanding modes are also designed to ensure high-quality output waveforms. The mode transfer tests are conducted with an inverter-simulated grid to define the proper transfer procedures. Experimental results show that the inverters can provide stable outputs in different basic microgrid operation modes. With the designed current sharing scheme, the output current is equally shared among paralleled inverters without noticeable circulating current. Both the simulation and experimental results of mode transfer show that the multi-inverter-based microgrid system is able to smoothly switch between the grid-tie and islanding modes to guarantee an uninterrupted power supply to the critical loads within the microgrid.