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Accurate estimation of the switching loss for power MOSFETs is critical to predict the device junction temperature and the efficiency in power converters. In recent power electronics design, wide bandgap (WBG) devices are adopted with zero-voltage switching (ZVS) to achieve higher efficiency. For devices with ZVS, the only switching loss will be channel turn-off loss, and on the contrary to hard switching, the energy stored in $C_{\text {oss}}$ will be recycled. The models for the estimation of total turn-off energy have been discussed in the literature, whereas the model of channel turn-off energy is absent. In this article, a behavioral model of channel turn-off energy using a simple $\alpha $ – $\beta $ equation for curve fitting is proposed, which can be applied to an entire current and voltage range. The proposed model is verified by different GaN HEMTs and SiC MOSFETs with a double-pulse test circuit in simulation and experiment.

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 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.

This brief presents a unified voltage balancing feedforward (UVBFF) for three-level boost (TLB) power-factor-correction (PFC) circuit in discontinuous and critical conduction modes. In general, the topology in both modes puts emphasis on low-cost system. Thus, electrolytic capacitors with wide-tolerance are commonly used at outputs and voltage unbalance is inevitable even under symmetric operations of two boost cells. In this brief, three-level switching capability of TLB PFC is introduced and asymmetric on-time feedforward for main switches is proposed. The proposed UVBFF can be realized by a single equation and harmonized with conventional feedback voltage-loop so that easy implementation is the key merit. The effectiveness of UVBFF is proven by analyses and experimental results.

Traditionally, standalone closed-loop drain–source sensed synchronous rectification (SR) implementation has been limited by the voltage limits of commercial IC controllers. Commercially available SR controllers have low maximum voltage limits, necessitating a high-voltage blocking mechanism for use in higher voltage rectifiers. Therefore, in this article, a board-level blocking method to protect drain–source voltage sensed SR controllers from high voltage and currents at the board level is presented. This method allows for the high-voltage operation of drain–source SR controllers. This method is self-biasing and allows for SR controller operation far beyond the datasheet voltage limits (kV+). The mechanisms of operation are analyzed, and design criteria created for selecting an optimal clamp is proposed. The proposed solution is simple and low in cost and component count, making closed-loop, cyclically adaptive SR a practical solution for high-voltage rectifiers. The blocking circuit is presented, analyzed, and tested for use with commercially available drain–source SR controller integrated circuits (ICs). Hardware SR testing is completed on two LLC-DCX (dc-transformer) modules with 300 V, and over 1-kV output voltages to demonstrate the voltage blocking capabilities.

In a magnetic bearing system, the levitation coils or electromagnets must be driven by an amplifier in order to control their currents for levitation stability. This amplifier typically drives every coil individually with hard-switching H-bridges. However, the inherent high dv/dt of hard-switching in conjunction with the large parasitic capacitance of magnetic bearing coils typically results in high current spikes and severe ringing in the switching process, which may interfere with position sensors and system communication. Furthermore, driving each coil individually requires a large number of switches. In this brief, a switch sharing zero-voltage-switching (ZVS) multichannel switching amplifier topology is proposed, which not only reduces the number of switches needed when driving multiple coils, but also eliminates current spikes and reduces the switching loss of the magnetic bearing amplifier. The proposed topology is experimentally tested to validate expected benefits.

Magnetic characteristics of a switched reluctance motor (SRM) determine the modeling accuracy, the performance of a position sensorless control, and the torque ripple reduction control. Due to double saliency structure and core saturation, the magnetic characteristics of SRM is dependent on both the rotor position and the winding current amplitude, which makes it difficult to measure the actual incremental inductance or flux linkage. This article presents the magnetic characteristics identification method. A quad current sampling technique is proposed to capture the derivative of the winding current at a specific rotor position and current amplitude. The flux linkage, co-energy, and torque are calculated from the obtained incremental inductance profile. The proposed method can be easily implemented on digital signal processor–based SRM drive without any extra circuit. The LCR meter and torque sensor measurements are provided to validate the proposed incremental inductance measurement method.

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%.

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.

The increasing usage of LLC-type dc-dc converters in utility, automotive, and power distribution applications has led to a push for a further increase in the converter's operating load range and efficiency. The secondary-side rectifier remains one of the lossiest areas in the converter, alluring designers to synchronous rectification (SR). One method is drain-source SR for cyclically adaptive, closed loop SR. However, when utilized, a severe current oscillation can be observed. An increase in SR duty cycle results in an increase in conduction time of the SR channel over the body diode. This issue becomes increasingly prevalent due to the usage of wide band-gap MOSFETs with high reverse drops and low sensed signal strength. This results in a current oscillation effect, leading to inconsistent SR operation, output ripple, and high EMI. In this paper, the issue is root caused analyzed. A method of improving drain-source SR for light-load SR operation is proposed. The method is prototyped on an FPGA to alleviate the issue on a $600\text{-}V_{in}/ 340\text{-}V_{out}$ 2.5-kW LLC-DCX (DC transformer) power converter.

This paper presents modeling, control, and system design of three-level boost (TLB) rectifier based medium-voltage (MV) solid-state transformer (SST) for dc fast charger application. In order to operate at MV with 1.2-kV SiC devices, the target system is configured by input-series output-parallel (ISOP) multimodule structure. Each module adopts TLB to front-end rectifier and stacked half-bridge LLCs to back-end isolated dc–dc converters. Operation principles of TLB rectifier and LLC converters are analyzed to derive an equivalent model of the entire system. Based on the detailed derivation, this paper aims to achieve three performance criteria, including bus voltage regulation, input current regulation, and voltage balancing. Basic feedback control loops are designed to control TLB’s output voltage and input current in an average manner, based on the derived system model. Then, ideal TLB duty-ratios in both continuous/discontinuous conduction modes are analyzed and duty-ratio feedforward control is proposed to improve the low quality of input current due to inherent feedback control limitations. For voltage balancing, system design approaches are used in the SST prototype. With the proposed control and system design, a prototype of four-module-stacked TLB SST was built and tested up to 3.8-kVrms and 16-kW conditions.

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.

The inverter that requires isolation typically consists of an isolated dc-dc and a pulse-width-modulated dc-ac stages. However, the component counts, control complexity, efficiency and power density tend to be suffered with such a two-stage configuration. In this paper, a single-stage soft-switching isolated inverter based on MHz LLC converter is introduced with a novel hybrid modulation method, which operates under full-bridge and half-bridge variable frequency conditions for high- and low-line outputs, respectively. A rectified sine wave can be generated at the output of LLC converter and isolated through a high-frequency transformer for additional voltage gains. A lossless line frequency unfolder circuit is added afterwards to convert the rectified sine wave to ac sine output. The optimization method for the resonant tank is also proposed to achieve even lower voltage gains at high frequency to improve the output ac waveform quality. Compared to the traditional two-stage method, this single-stage solution makes the circuit simpler, and the bulky dc bus capacitor can be saved. To verify the circuit design and the proposed hybrid modulation method, a GaN based 380 V dc – 600 V ac , 1.2 kW prototype is built and tested. Under the full load condition, the measured total harmonic distortion of output voltage is 2.67% with 98.30% efficiency for the overall inverter.

This paper introduces a hybrid binary cascaded multilevel inverter (BCMLI) that allows the supply voltage of the lowest level to be a floating capacitor and thus reducing the need of multiple separated sources. Multiple such BCMLIs can be cascaded to increase the number of levels. This paper extends the configuration with two of 3-cell hybrid BCMLI modules cascading to achieve 29 voltage levels. A utility-scale hybrid BCMLI was designed and built using a single-source as the input and LLC converters to produce the 2-kV, 11.2-kW step-like output with 15 voltage levels. Two of such units were further cascaded to produce the 4-kV, 22.4-kW step-like output with 29 voltage levels. The circuit and control have been designed and simulated, and the hardware prototype has been built and tested. The total harmonic distortion was measured at 3.5% under the full-load condition. The LLC stage is a gallium nitride based converter operating at 1 MHz for an ultra-compact design. As the only loss is on the LLC stage, and high-voltage side is nearly lossless, the overall system efficiency exceeds 98.8%.

With the availability of wide bandgap devices, the power converters can now potentially operate at megahertz (MHz) or higher frequencies while achieving ultrahigh efficiency. Such a high-frequency switching is particularly important for isolated converters to reduce the size of the passive components, achieve high power-density, and reduce costs. However, unlike the conventional kilohertz switching, in MHz switching, the parasitic components will significantly affect the circuit operation. These effects are compounded by high-ratio step-up or step-down isolated converters because the parasitic capacitances/inductances are squared times the turns’ ratio of the transformer when reflecting from high-voltage (HV) to low-voltage (LV) side to LV/HV side. In this paper, a high-step-up series resonant converter is used as a design example to explore the effects of parasitic inductances induced from the LV side and parasitic capacitances induced from the HV side on the circuit operation under MHz switching conditions. Afterward, the printed circuit board (PCB) layout and the planar transformer are optimized through the finite-element method to minimize the parasitic effects. Finally, a 1-MHz, 38-V/380-V, 300-W resonant converter prototypes are built and compared to verify the design optimization.

LLC resonant converter is one of the promising converter topologies due to its simple structure and high efficiency. However, when LLC resonant converter is designed considering a hold-up time requirement, it has definite drawbacks such as large conduction and turn-off losses due to large circulating current. To solve this problem, various resonant converters with boost capability have been investigated. Although these methods improved overall performance, they still suffer from a large number of switching components. In this paper, a new LLC resonant converter with boost capability is introduced and verified. The advantage of the proposed converter is a small number of components while achieving the hold-up time capability. Furthermore, it achieves zero average transformer current despite its asymmetric operation. A prototype has been built and tested with 400–250-V input and 200-V/200-W output specification.

A 12-pulse converter has been commonly used for a large-scale power generating system. The arrangement of a delta–delta (Δ–Δ) and a delta–wye (Δ–Y) phase-shift transformer pair in series with the voltage source eliminates undesirable harmonics from nonlinear loads. Recently, a novel front-end for a multi-generator power system was proposed, which utilizes a single Y–Δ transformer to shunt between the two 30° phase-shifted generator outputs and results in the comparable harmonic elimination performance to the conventional 12-pulse rectifier. The shunt-type Y–Δ transformer front-end achieves more than 75% transformer size reduction, making it extremely attractive to large-scale industrial and shipboard applications. However, the exact 30° phase shift between the two rotating generators can drift in a practical system, especially under dynamic conditions. The harmonic performance under the non-ideal 30° phase-shift condition can deteriorate with the Y–Δ shunt-type front-end. To understand the impact of non-ideal phase-shifted generator sources, this paper derives the mathematical models through the use of an equivalent circuit, a square-wave analysis, and the phasor representation to show the harmonic cancellation principle and verifies the results with the computer simulation.

Since the last decade day by day, there is a revolution in an existing power grid network due to an increase in demand for the convenient electric power supply required for the smart component in the power grid network. So this is a time for the power grid to become smart in accordance with all components which are actively participated in his network which is nothing but Smart Grid network. A different component in a Smart grid network required variable AC/DC power supply at the different amplitudes and different frequencies. Power electronics converters are required to meet all these requirements. Solid State Transformer (SST) is one of the key components in a smart grid network. This paper presents the Bidirectional Soft Switched Solid-State Transformer for a Smart Grid Network. A proposed circuit presents in this paper used a LCLC type resonant network to achieved soft switching action. An analysis of the resonant network and it's effect on the performance indices are presents in this paper. A front end bidirectional boost rectifier is used in a proposed circuit to achieved bidirectional power flow feature and desired voltage gain at the output. A proposed circuit is designed for power level 10KW and is simulated in a PSIM/Simulink co-environment. The results were obtained at an input voltage of Three-phase 440V, 50 Hz, and provide the output of 800V, 12.5A and is verified for desired features in the proposed converter.

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.

High frequency PCB windings magnetic is one of the key components in high power density and high efficiency MHz power converter. High frequency PCB windings magnetic losses are the main limitation for the whole converter efficiency and power density, while the PCB windings losses are the main part of the magnetic losses. Currently, the Dowell’s model is used for the layout of PCB windings. However, the Dowell’s model has its restrictions, namely, magnetic field is assumed to be distributed evenly in one direction. But the magnetic field around the windings are three-dimensional and unevenly. Therefore, further arrangement of PCB windings is proposed to minimize the PCB windings losses. Besides, the magnetic circuit is analogous to electrical circuit, and the magnetic core structure shape the footprint of the PCB winding, so different magnetic core structure can be proposed for the trade-off design among the high power density, high efficiency, and low temperature stress. Therefore, both PCB winding layout and magnetic core structure design are important for the optimization of high frequency magnetic components.

LLC resonant converter is widely used due to its high efficiency and high power density capability. The design methodology for it has been addressed in many literatures. However, most of them focus on the step-down LLC converter due to its high-output current applications such as data center. In this paper, the optimal design methodology for high frequency GaN based step-up LLC converter for solid-state transformer application is discussed. An accurate model for deadtime operation is derived. From the model, the approach for device selection for both primary and secondary sides and the magnetizing inductance design to have zero-voltage-switching (ZVS) are presented. To verify the design methodology, LTSPICE simulation is performed. Besides, a 1 MHz 1.8 kW step-up LLC converter prototype is designed and tested. With the proposed design methods, the efficiency can achieve 98.75%.

Due to high efficiency and simple structure, LLC resonant converter is a promising topology for developing solid-state transformers. The heart of an LLC converter is the high frequency transformer that provides necessary insulation and step-up/down between primary and secondary sides. In addition, the efficiency of the transformer is a critical factor as it processes the entire power. This becomes even more challenging when circuit switching frequency is raised for shrinking passive component sizes, as skin effect and other non-ideal effects start to kick in. In solid-state transformer applications, size, efficiency and insulation have stringent specifications and meeting all requirements at once is not a simple task. Therefore, this paper presents multiple perspectives in terms of transformer design in an LLC converter and demonstrates with a 3-kW transformer capable of more than 15 kV insulation. Testing results show peak converter efficiency of 98.9% with transformer power density of 480 W/in3.

A trend for low-power conversion system is to integrate every component to a single chip. Magnetic component has been the key to determine the power density of the integrated switching power converter. In this paper, a hybrid resonant buck-type converter with an ultracompact custom-designed coupled inductor is presented for low-power integrated on-chip applications. This converter achieves zero-voltage switching for the high-side switch and near zero-voltage zero-current switching for low-side and auxiliary switches. The soft-switching condition allows the circuit operating at high switching frequencies. An ultracompact planar coupled inductor with a dimension of 5.25 mm × 1.95 mm × 3.86 mm is designed and customized for future integrated on-chip applications. The high-power density of this magnetics component is achieved by high switching frequency and optimized printed circuit board (PCB) windings design. A hardware prototype with 24 to 60-V input and 3.3 to 5-V, 3-A output operating at 2–3 MHz is presented to verify the circuit operation. The prototype achieves a peak efficiency of 90.7% using silicon devices at 2-MHz switching frequency.

A microconverter serves as a front-end dc–dc stage of a microinverter to convert the power from a photovoltaic module to a dc bus. These front-end microconverters require isolation, high-boost ratio, wide-input voltage regulation, and high efficiency. This paper introduces an isolated resonant converter with hybrid modes of operation to achieve wide-input regulation while still maintaining high efficiency. The proposed converter is designed as a series resonant converter with nominal-input voltage and operates under two additional modes: a boost converter with low-input voltage and a buck converter with high-input voltage. Unlike conventional resonant converters, this converter operates at discontinues conduction mode with a fixed frequency, simplifying the design and control. In addition, this converter can achieve zero-voltage switching (ZVS) and/or zero-current switching (ZCS) of the primary-side MOSFETs, ZVS and/or ZCS of the secondary-side MOSFETs, and ZCS of output diodes under all operating conditions. Experimental results using a 300-W prototype achieve a peak efficiency of 98.1% and a California Energy Commission efficiency of 97.6% including all auxiliary and control power at nominal-input voltage.

A front-end for multi-generator power system was recently proposed, which utilizes a single wye-delta (Y-Δ) transformer to shunt between the two 30° phase-shifted generator outputs. The shunt type phase-shift transformer front-end provides comparable 5th and 7th harmonic elimination performance to the conventional 12-pulse rectifier with more than 75% transformer size reduction. In this paper, a hybrid front end, which consists of the shunt Y-Δ transformer and an active power filter is proposed to further improve the power quality. In this configuration, 11th and 13th harmonics from nonlinear load are actively compensated by the active power filter and 5th and 7th harmonics are trapped by Y-Δ transformer. Theoretically, the performance of the proposed front end is equivalent to a traditional 24-pulse rectifier with a significant reduction in the overall size and weight, which makes it attractive to large-scale industrial and shipboard applications. The harmonic cancellation principle of the proposed front end is derived and verified by simulation and experimental results.

This paper presents small-signal modeling of three-level boost (TLB) converter for active front-end power-factor-correction in medium-voltage (MV) solid state transformer (SST). The analysis has been conducted for continuous conduction mode (CCM) in two different voltage modes of TLB to derive system dynamics and to confirm that transfer functions in the voltage modes are identical to one another. Based on the derived models, this paper introduces MV-SST system design guidelines including controller and input filter, considering limited selection width of off-the-shelf high-voltage-rating capacitors and nontrivial source impedances. Digital feedback controller and input filters are designed based on the derivations and implemented to a 4-module-stacked SST system prototype of which TLB plays a role of active front-end PFC. In order to verify the effectiveness of small-signal analysis and system design, experimental verifications of the prototype have been conducted for single phase 3.8-kV rms input voltage to 400-V de bus voltage conversion and 15-kW full load condition in the existence of 65-mH source impedance.

The LLC series resonant converter is widely used in many applications due to its high power density and efficiency. However, under applications with relatively small magnetizing current and resonant inductance such as DC transformer (DCX) for telecom power systems and PV optimizers, the resonance between the leakage inductance and the secondary parasitic capacitance during dead-time is unneglectable. It will affect the zero-voltage switching (ZVS) transition and the resonant current during the power delivery period. Therefore, in this paper, two different scenarios of the ZVS condition for an LLC series resonant converter with secondary parasitic capacitance considered will be presented. Moreover, the impact of the resonant to steady state operation under different ZVS timing will be discussed. Experimental results of a 140kHz prototype are provided for verification.

This paper analyzes CCM/DCM operations of three-level boost (TLB) converter and derives ideal duty-ratios for feedforward control in power factor correction (PFC) applications. The analyses are conducted for two different TLB operation modes characterized by input voltage range. Since duty-ratio feedforward is a widely used technique to improve PFC transient responses and mitigate the burden of feedback control, accurate duty-ratio computations for all possible operation modes and conduction modes are necessary. By comparing the derived duty-ratios for CCM and DCM regions, current conduction modes of TLB can be detected in real-time and proper duty-ratio feedforward with minimum duty-ratio selection can be provided. The experimental results by a 500 W prototype of TLB PFC rectifier are provided in order to verify the effectiveness of the derivations and feedforward technique.

Nowadays, high power density is one of main pursuits for power converters. High switching frequency can greatly reduce the size of passive components. With the availability of wide bandgap devices, the converters have opportunities to operate at megahertz switching frequency with higher efficiency and power density. However, unlike the conventional kilohertz switching, parasitic inductances induced from the printed circuit board (PCB) could significantly affect the circuit operation under megahertz switching. In particularly, the parasitic effects will be enlarged for high step-up or step-down isolated converters, since the parasitic inductances are square times the turns ratio of the transformer when reflecting from low voltage side to high voltage side. This paper takes LLC converter as an example to discuss the optimization of PCB layout to reduce the parasitic effects with finite element method. In the experiments, a couple of 1-MHz 38V/380V 300-W LLC resonant converters are built and compared to verify the design optimization.

A high step-up constant frequency resonant converter with PWM controlled active boost quadrupler rectifier (ABQR) is proposed in this paper. The proposed converter operates at the resonant frequency throughout the whole operation region to achieve the highest efficiency. Under low input voltage conditions, the quadrupler based active boost rectifier will operate with the proposed PWM modulation scheme to step up the voltage. With the proposed ABQR, the output switches serve both for synchronous rectification and voltage boost function. It enables a higher voltage boost ratio than series resonant converter. Moreover, the voltage stress on secondary power devices are reduced to half of the high output voltage and the transformer turns ratio can be lower. A 300-W hardware prototype with gallium-nitride devices is built to verify the performance of the proposed converter and modulation method. It can achieve a peak efficiency of 98.9% and a California Energy Commission weighted efficiency of 98.7% under nominal input voltage condition.

In this paper, the low voltage stress of switches, high voltage gain DC-DC converter is proposed. The operating principle of the converter is similar to that of the conventional buck converter. Therefore, the proposed converter can be operated in the discontinuous conduction mode, continuous conduction mode, depending on the design. Moreover, the controller is also similar to that of the traditional buck converter, results in the simple controller. Furthermore, the voltage stress of every switch is reduced significantly compared with the conventional converter. Therefore, the use of the low voltage rating switches reduces the losses significantly. A 200W converter prototype is successfully built and tested to verify the feasibility of the proposed converter.

A 12-pulse rectifier has been widely used for harmonic mitigation in a large-scale power system. Recently, a shunt phase-shift transformer rectifier was proposed and already has been utilized in a marine transportation. For a power system equipped with multiple generators, the shunt phase-shift transformer front-end selectively eliminates 5th and 7th harmonic component with more than 75% size reduction over the conventional 12-pulse rectifier. However, a shunt phase-shift transformer rectifier requires additional source inductors to eliminate the undesired harmonics and the relationship between the source impedance and harmonic elimination has not been verified. This paper evaluates the effect of the line inductor on a harmonic cancellation performance of the shunt phase-shift transformer. An equivalent circuit analysis and a mathematical derivation of the shunt phase-shift transformer rectifier are provided and the relationship between the source impedance and harmonic cancellation is mathematically derived. The computer simulation and experimental test results from proto type hardware verify the equivalent model and the mathematical analysis.

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.

LLC converter is widely used in various applications due to its simple structure and zero voltage switching capability. To achieve high efficiency, LLC converter is usually designed to operate near the resonant frequency over the entire load conditions. However, a constant core loss of the main transformer due to the constant frequency operation has a bad effect on the efficiency in a light load condition where the core loss dominates the total loss. Therefore, it is essential to reduce transformer core loss in order to improve light load efficiency. Adapting asymmetric pulse-width modulation (APWM) in a half-bridge LLC resonant converter with voltage doubler rectifier for improving light load efficiency has been proposed. By adapting APWM, the core loss can be reduced dramatically in the light load condition, because both the switching frequency and the flux swing of the transformer are reduced. Furthermore, the proposed method does not affect the design of the converter. The experimental results with 400–250 V input and 100 V/2 A output prototype converter support the effectiveness of the proposed method.

This paper investigates the effects of low frequency harmonics and source impedance on active front-end rectifier in modular solid-state transformer. The topology for analysis was selected to three-level boost (TLB) converter which is widely used for power factor correction. Harmonic contents of input source cause periodic distortions in input current, degrading THD factor. In the presence of finite source impedance, performance of properly designed current controller can be deteriorated in terms of phase margin and system instability. In this paper, those external effects are studied with equivalent single module TLB. Repetitive controller and input filter designs are presented to compensate periodic distortion and overlapped impedance interaction in frequency domain, respectively. Simulation and experimental results are provided to verify the analysis.

High switching frequency (HF) operation of solid-state transformers (SSTs) reduces the weight and volume of magnetic parts. Yet in medium-voltage (MV) SSTs, electrical insulation becomes a great concern and demands extra spacing. To resolve the issue, resistive shielding technique is considered in this paper to mitigate the threat of partial discharge. In addition, geometric shaping is applied to alleviate local electrical field. On top of the insulation and size requirements, HF transformer design should also consider skin effect and other effects induced by ac electromagnetic field. This paper is meant to develop a planar transformer for a MV SST, taking all critical factors into account. 2-d finite element analysis is adopted as main research and conceptual verification method.

A position sensorless control method for switched reluctance motors from standstill to running without pre-stored magnetic characteristics is proposed. For startup, diagnosing pulses are injected into all three phases, and the current magnitudes of all phases are examined to estimate the rotor position. Additionally, a phase current vector method is applied to eliminate the risk of locked-rotor and startup hesitations. In running condition, diagnosing pulses are injected to two non-torque productive phases to estimate the position and commutation instant. An angle tracking observer estimates the continuous angle from the acquired discrete position information. The proposed method does not require any prior knowledge of the switched reluctance motor except the pole configuration. Therefore, memory storage for magnetization curves of a switched reluctance motor is not needed. The experimental results show a hesitation-free startup, a good speed reference tracking, and an accurate position estimation performance under different load conditions.

In this paper, a high-efficiency solar optimizer with a new modulation method is proposed for wide range regulation. The converter is designed as a series resonant converter under nominal-input condition where the voltage step-up ratio is entirely determined by transformer. A new modulation method with fixed switching frequency is proposed to gain a higher voltage step-up ratio for low-input conditions. The proposed modulation method does not add any auxiliary power switches compared with previous works while it yields wide range regulation. A 300-W hardware prototype using Gallium-Nitride devices is built to verify the performance of the proposed solar optimizer.

A 12-pulse rectifier with the combination of $\Delta-\Delta$ and $\Delta-$ Y type phase-shift transformers has been commonly used for harmonic cancellation in a large-scale power system. Recently a shut phase-shift transformer was suggested to have the same harmonic cancellation effect with more than 75% size reduction over the traditional phase-shift transformers provided that the power generation system equips with multi-generator that can be regulated with 30° phase difference between the two sources. Theoretically, the harmonic trapping performance of a shunt configuration is equivalent to 12-pulse rectifier with a significantly reduced transformer rating and size. However, the working condition and the performance of a shunt phase-shift transformer rectifier under the practical condition have not been verified. This paper presents a detailed analysis of a harmonic elimination principle of a shunt phase-shift transformer through an equivalent circuit model and a mathematical derivation. The analysis also describes the working condition for a shunt phase-shift transformer rectifier and the effect of phase difference of generators on the harmonic cancellation performance. The derived model is verified with computer simulation under the ideal and non-ideal 30° phase-differential configurations.

A conventional asymmetrical half-bridge (AHB) converter is a good candidate for low power applications such as TV and LED driver. It has small number of components and all switching devices have soft swtiching capability. However, when the AHB converter is designed with wide input voltage range, it has a large offset current in transformer which increases size of the transformer and core loss. Also, DC-offset current worsens ZVS condition of one of the half-bridge switches resulting in low efficiency in light load condition. To overcome above problems, a new asymmetrical half-bridge converter with coupled inductor rectifier (CIR) is proposed in this paper. Since two capacitors of the new rectifier equalize an average current flowing through secondary rectifier, the proposed converter doesn’t have DC-offset current. Therefore, the proposed converter reduces size of the transformer and can increase efficiency. Experiment is implemented with a 250V-400V input voltage variation and 50V/200W output specifications.

This paper discusses the details of the control scheme and EMI filter design for a modular front-end solid-state transformer (SST). The multi-cell configuration is achieved by connecting the active-front-end (AFE) cells in series, with the voltage from the MV evenly distributed among the cells. The isolated outputs of the dc-dc converters are tied in parallel. The EMI filter is designed to reduce line impedance and switching noise. Simulations showed the EMI filter reduced noise below FCC Class A standards. The SST controller consists of a voltage controller, current controller, and a phase lock loop (PLL) with an ideal duty feedforward. The implementation of these subsystems is explained and its integration in the system is shown. The results show that at low voltage level, the input current waveform is nearly sinusoidal.

Synchronous rectification technique can reduce secondary-side conduction loss of the LLC resonant converter. Typically, the control of synchronous rectifier (SR) relies on either voltage or current information; however, the detection circuit is sensitive to parasitic effects and high frequency noises. Since reliability is among top priorities in solid-state transformer application, open-loop controlled scheme becomes advantageous. Unfortunately, secondary-side current of LLC converter reaches zero-crossing-point (ZCP) earlier at light-load condition and the SR signal could turn off after ZCP. In that case, high circulating current appears in the secondary side and dramatically deteriorate efficiency. Therefore, a tuning method utilizing external primary-side output capacitor and dead-time extension is proposed to avoid late turn-off issue of open-loop controlled scheme. In this paper, the cause of ZCP shifting and late turn-off issue are explained first. Then a model for dead-time transient of LLC converter is derived as the theoretical basis of proposed tuning method. Finally, hardware testing results of a 4-kW LLC converter module are presented. With the proposed tuning method, the open-loop controlled synchronous rectification can improve the efficiency of the LLC converter module even at light-load condition.

In this paper, a detailed design procedure is presented for a bidirectional CLLLC-type resonant converter for a battery charging application. This converter is similar to an LLC-type resonant converter with an extra inductor and capacitor in the secondary side. Soft-switching can be ensured in all switches without additional snubber or clamp circuitry. Because of soft-switching in all switches, very high-frequency operation is possible; thus, the size of the magnetics and the filter capacitors can be made small. To reduce the size and cost of the converter, a CLLC-type resonant network is derived from the original CLLLC-type resonant network. First, in this paper, an equivalent model for the bidirectional converter is derived for the steady-state analysis. Then, the design methodology is presented for the CLLLC-type resonant converter. Design of this converter includes determining the transformer turns ratio, design of the magnetizing inductance based on ZVS condition, design of the resonant inductances and capacitances. Then, the CLLC-type resonant network is derived from the CLLLC-type resonant network. To validate the design procedure, a 3.5-kW converter was designed following the guidelines in the proposed methodology. A prototype was built and tested in the laboratory. Experimental results verified the design procedure presented.

In this paper, a contactless charging system for an electric vehicle (EV) battery is proposed. The system consists of three parts: 1) a high-frequency power supply from a full-bridge inverter with frequency modulation; 2) a loosely coupled transformer that utilizes series resonant capacitors for both the primary and secondary windings; and 3) a rectification output circuit that uses a full-bridge diode rectifier. With carefully selected compensation network parameters, zero-voltage switching can be ensured for all the primary switches within the full range of an EV battery charging procedure, which allows the use of low ON-state resistance power MOSFETs to achieve high-frequency operation and system efficiency. The design of loosely coupled transformer is simulated and verified by finite element analysis software. For a 4-kW hardware prototype, the peak dc–dc efficiency reaches 98% and 96.6% under 4- and 8-cm air gap conditions, respectively. The prototype was tested with an electronic load and a home-modified EV to verify the performance of constant current and constant voltage control and their transitions.

This paper discloses the development of modular front-end medium-voltage solid-state transformer (SST) to show system level design, circuit topology, and prototype test results. Each multilevel front-end module consists of two stages: (1) ac to dc boost converter with power factor correction dc and (2) isolated dc-dc LLC converter. The proposed system has been designed, simulated, and fabricated with the rating of 7.2-kV input, 400-V output, and 25-kW power capability. With the adoption of silicon carbide devices, the peak efficiency of the entire front-end converter from high-voltage ac to low-voltage dc achieves 98.2% efficiency. The individual switch is switched at 10 kHz. With complementary switching for the 3-level boost converter, the equivalent switching frequency is 20 kHz for the individual module and 200 kHz for the combined ten-module power stage. The full-voltage, full-power testing with a near ideal source shows a clean input current waveform.

This paper presents a hybrid switch that parallels a power MOSFET and an insulated-gate bipolar transistor (IGBT) as the main switch of a zero-voltage switching (ZVS) inverter. The combination features the MOSFET conducting in the low-current region and the IGBT conducting in the high-current region, and the soft switching avoids the reverse recovery problem during the device turn-on. A custom hybrid switch module has been developed for a variable-timing-controlled coupled-magnetic-type ZVS inverter with a nominal input voltage of 325 V and the continuous output power of 30 kW for a traction motor drive. Experimental results of the hybrid-switch-based inverter with the total loss projected by temperature indicate that the inverter achieves 99% efficiency at the nominal condition and demonstrate ultrahigh-efficiency operation over a wide load range. At 375-V input, the maximum measured efficiency through temperature projection and loss separation analysis is 99.3%.

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.

Three dual-mode boost-buck-derived inverters with smooth transition between modes have been proposed: 1) boost-full bridge inverter; 2) boost-H5 inverter; 3) boost-dual buck inverter. For the same topologies, in conventional control method, the first stage is needed to boost input voltage to a constant high voltage, and the second stage is for sinusoidal inverting. However, in the proposed three inverters, either buck or boost mode works at a single time, thus, only one switch works at a high switching frequency. In this way, a lot switching loss can be reduced, and moreover, since the voltage across the middle capacitor is smaller than the conventional constant high voltage, the further switching loss mitigation is possible. In order to achieve smooth transition between the two modes, a double-carrier-based sinusoidal pulse width modulation (SPWM) is proposed. Other than this, three advanced modulation methods are proposed: 1) double-carrier with different frequencies; 2) double-carrier with different magnitudes; and 3) double-carrier with different frequencies and magnitudes. Following that, the loss distribution in every component is provided and the California Energy Commission efficiency of these inverters under different input voltage conditions is compared. Finally, the experimental results show the dual-mode double-carrier-based SPWM inverter can improve the efficiency by 2% than the traditional constant dc bus voltage solutions.