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This document discusses the increasing demand for high-efficient isolated DC/DC converters in renewable energy applications. It presents seventeen papers that cover various topics such as reducing switch count and current balancing, addressing voltage ringing issues, solving current unbalance problems, and optimizing converter design. The papers provide experimental results and propose innovative methods to improve the efficiency and performance of DC/DC converters in renewable energy applications. The document also includes three research papers related to power converters, including one on a prototype for NanoGrid applications, one on a new DC-DC bipolar resonance converter, and one on enhancing the stability of a DCMG cluster. The text also includes acknowledgments and biographies of the guest editors. [Extracted from the article]

The single-stage dual active bridge (DAB) AC-DC converter has the advantages of high power density, low cost, and simple control; it has a broad potential for application in the field of onboard chargers (OBC). However, the lack of fast and accurate quantitative parameter optimization design methods in single-stage DAB AC-DC converters limits the overall efficiency of the converter. Based on the above problem, in order to improve the overall operating efficiency of the converter by optimizing the parameter transformer ratio and power inductance, this paper proposes a parameter design method considering a multi-timescale strategy by combining the steady-state analysis model of the converter in the line cycle and switching cycle and step-by-step reducing its design space through the constraints on the parameters. The first step is to obtain a safe design space for the parameters under the converter's transmitted power and current stress constraints. The second step obtains the optimization design space of the parameters under the optimization of conduction loss and switching loss of the converter. Finally, the optimal parameters are determined by the loss analysis model. The proposed parameter optimization method entirely takes into account the steady-state characteristics of the DAB AC-DC converter during the line cycle, and the step-by-step constraints greatly accelerate the parameter design process. In addition, the proposed parameter optimization design method applies to all types of single-stage DAB AC-DC converters, which can be well applied to engineering practice. [ABSTRACT FROM AUTHOR]

The LLC resonant converter, which is extensively utilized across various industrial fields, significantly depends on its parameters for performance optimization. This paper establishes a time-domain analytical model for the LLC resonant converter under Pulse Frequency Modulation (PFM) and proposes a multi-objective parameter optimization design method with stepwise constraints. The proposed method limits the resonant capacitor voltage while ensuring that the converter meets the voltage gain requirement and realizes Zero-Voltage Switching (ZVS). The converter's performance is then optimized with the objective of minimizing the switching frequency range, the resonant inductor current, and the RMS value of the switching current on the secondary side. Compared with the existing methods, the proposed method has the advantages of comprehensive consideration and wide application scenarios. Finally, a 1200 W experimental prototype was fabricated, with experimental results verifying the feasibility of the proposed optimization design method and demonstrating that the prototype's maximum efficiency reaches 96.54%. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN) devices are generally more prone to severe switching oscillations than Si mosfets due to their fast switching speeds. Different from Si and SiC mosfets, GaN devices typically have a lower maximum gate voltage rating. Taking the Efficient Power Conversion Corporation series as an example, the gate drive voltage is usually 5 V, but the maximum voltage rating is 6 V. Therefore, GaN devices are more susceptible to damage from gate-source voltage oscillation. In this letter, a gate drive circuit with a clamping function is proposed to suppress gate-source voltage oscillation. The adopted method can not only clamp the gate voltage of the GaN device near the drive voltage to protect the device, but also does not affect the switching speed of the device. The proposed method mainly uses some passive components that make the circuit design simpler compared with other active gate driver methods. Finally, the effectiveness of the proposed method is verified by experiments. [ABSTRACT FROM AUTHOR]

With the widespread application of lithium-ion battery packs, it calls for efficient balancing methods to improve the performance of these battery systems. The relevant research has focused on the design of equalization circuits and control strategies while neglecting quantitative analysis and comparison of battery active equalization structures from the systematic level. In this paper, we propose an improved system-theoretic modeling approach for active equalization structures that takes into account the battery's constraints, including equalization current rate and SOC operation range. Accordingly, the mathematical models of various active equalization structures are derived that can describe their equalization dynamic behavior. Then, the analytical algorithms are developed to estimate the balancing time of active equalization structures online. Finally, extensive numerical simulations are conducted to evaluate and compare the average performance of these equalization systems in term of equalization time. It then comes up with a comprehensive comparison among them. Consequently, the Multicell-to-Multicell (MC2MC) balancing structure benefits the fast balancing speed and the lowest sensitive to initial batteries’ SOCs. The global structure has the lowest implementation cost and the series structure benefits the simplest control. Moreover, the module-based structures have faster balancing speed when each module contains about 4–6 batteries. [ABSTRACT FROM AUTHOR]

This letter proposes an indirect power dissipation level short circuit protection (IPDL-SCP) method for silicon carbide (SiC) metal-oxide semiconductor field-effect transistors (mosfets). Due to differences between normal conducting and short-circuit (SC) conditions, this method is triggered by the voltage oscillation vss at the parasitic inductance (Lss) of the source and directly monitoring vds of SiC mosfet. For verification, the short circuit protection circuit is tested and proved functional on the SC test bench. Experimental results show that the protection circuit can respond to both hard switching fault (HSF) and fault under load (FUL), while the fastest response delays are 75 ns for HSF and 170 ns for FUL. Comparisons with other commonly used SC protection methods have also been made, which show that the proposed IPDL-SCP method has better performance in shorter protection delays. [ABSTRACT FROM AUTHOR]

This article performs a comprehensive steady-state performance optimization for the isolated triple active bridge (TAB) dc–dc converter adopting the five-variable modulation (FVM) technique. First of all, a universal analytical model suitable for all the hardware conditions of the TAB converter is established using frequency-domain analysis method. Second, considering the influence of port voltages and the energy stored in parasitic capacitances, a simplified and accurate zero-voltage switching (ZVS) constraint is derived without complicated condition judgments. Based on this, aiming at reducing the overall system losses, a comprehensive efficiency optimization modulation scheme with FVM is proposed in this article. Distinct from the previous optimization methods, the proposed scheme is not only a multiobjective optimization with minimized circulating current level and full-ZVS operation. It is also a generic optimization scheme covering all the five operation modes, and there is no individual optimization for each mode, which greatly simplifies difficulty of implementation. Afterward, to overcome the cumbersome optimization process, an advanced particle swarm optimization algorithm is used to seek the global optimal operating point in the entire power flowing range. Extensive simulation is carried out using MATLAB/Simulink. And the steady-state characteristics of the proposed scheme are fully compared with the traditional phase shift modulation method. Finally, a 1.2 kW laboratory platform is built. A significant efficiency improvement under light-load condition up to 21% is achieved, which substantiates the feasibility of the proposed optimization scheme. [ABSTRACT FROM AUTHOR]

Gate-oxide degradation has been one of the critical reliability concerns of silicon carbide (SiC) metal–oxide–semiconductor-field-effect transistors (mosfets), which could be monitored through aging-sensitive parameters. In this article, an online gate-oxide degradation monitoring method for planar SiC mosfets is proposed by extracting gate charge time at a specific range of gate voltage. It is based on the findings that the input capacitances of planar SiC mosfets change significantly over gate-oxide degradation, which is theoretically analyzed and experimentally verified. The capacitance variations are converted into the gate charge time as the new aging-sensitive parameter. The new parameter measurement circuit is proposed and integrated into the gate driver module. The article results indicate that the new parameter varies noticeably with gate-oxide degradation and the difference of this parameter caused by junction temperature is much smaller than that caused by degradation. Besides, the parameter is immune to package degradation and load current. The condition monitoring method can be implemented online since the parameter is extracted during the off-state of SiC mosfet devices, which does not affect normal operation. The confirmatory experiment is carried out to verify the correctness of the proposed method. [ABSTRACT FROM AUTHOR]

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In order to further improve the efficiency of dual-active-bridge (DAB) converter, a hybrid five-variable modulation scheme with minimal root mean square (rms) current is proposed in this article, which combines the traditional phase-shift and pulsewidth modulation schemes. By adopting the proposed optimal scheme, a wider zero-voltage switching (ZVS) operation range and lower rms current level are achieved simultaneously when compared with other modulation schemes, which significantly improves the efficiency of the DAB converter. Moreover, for the complexity of the proposed modulation scheme with five variables, the Fourier decomposition based analytical expressions are derived for the converter and a generalized performance analysis results are presented. Besides, in order to avoid the analysis error, especially at light-load condition, the optimization strategy takes the rms current as the optimal objective, which includes the eight harmonic component of the inductor current rather than just considers the fundamental component. And an offline particle swarm optimization algorithm is introduced to acquire the optimal operating points for the whole power range. To substantiate the effectiveness of the proposed modulation scheme, a comprehensive comparison among related works in terms of the rms value and ZVS range were carried out under various operation condition. Finally, a 1.2 kW experimental platform was built and the highest experimental efficiency under the proposed modulation scheme reaches 96.8%. [ABSTRACT FROM AUTHOR]

Light-emitting diodes (LEDs) are widely used in street lighting, landscape lighting, liquid crystal display (LCD) backlighting, healthcare, etc. As an important part of LED lighting products, LED driver plays a vital role in maintenance costs reduction, energy saving and lifetime prolongation. In this paper, a single-stage LED driver is proposed, which is integrated by a totem-pole bridgeless power factor correction (PFC) unit and several modular LCL-T resonant rectifiers. The totem-pole bridgeless PFC unit and the LCL-T resonant rectifiers are integrated by sharing switches, which can simplify the circuit and reduce the system cost. The output current can be regulated by applying the variable inductor control. Constant duty cycle and fixed switching frequency operation can be achieved as well. The operating principle and the performance of the proposed converter are analyzed in this paper. In addition, a precise parameter design method is proposed by considering the high-order harmonics. At last, an experimental prototype is established to validate the proposed parameter design and variable inductor control method. [ABSTRACT FROM AUTHOR]

Although LLC converters are attractive in industrial applications due to its advantages of soft switching, high efficiency, and simple control, in wide voltage gain range applications, large switching frequency operating range is required, which may cause the loss of soft switching operation and existence of control instability issue. In this article, a novel LLC converter with two LLC resonant tanks is proposed. The proposed converter has three operation modes, namely, dual full-bridge operation mode, full-bridge operation mode, and half-bridge operation mode, which can cover the high voltage gain range, medium voltage gain range, and low voltage gain range operations, respectively. The operational principles of the proposed converter are presented, and brief design considerations are discussed. A 250 W experimental prototype with six times input voltage range (50–300 V) was built to validate the theoretical analysis of the proposed converter. Comparisons among different LLC topologies for wide voltage gain range applications are made. [ABSTRACT FROM AUTHOR]

In this article, an online condition monitoring (CM) method is proposed for the bond wire degradation of insulated-gate bipolar transistor (IGBT) modules in a single-phase or three-phase three-level neutral-point-clamped (3LNPC) dc–ac, ac–dc converter. The proposed method has the ability to detect the whole IGBTs in a 3LNPC converter, based on finding the idle on-state IGBTs under the specific load current direction and switching sequence of a leg. The detection time is short (0.2 ms) and the influence of online CM on load current can be completely neglected as long as the insertion areas for CM test are chosen reasonably. Meanwhile, an advanced bond wire CM module is designed to conduct the CM test. This module can generate and inject the current pulse trains into IGBT modules, with the high current for CM test and low current for temperature-sensitive electrical parameter measurement, and sample the collector–emitter on-state voltage (VCE ON) stably and precisely. Finally, the experimental work was carried out to verify the effectiveness of the proposed method and the practicality of the developed CM module. [ABSTRACT FROM AUTHOR]

Hold-up time operation is a crucial requirement for distributed power systems and server power supplies. LLC resonant converter is widely used in these applications due to its high efficiency and power density characteristics. Conventional pulse frequency modulation (PFM)-controlled inductor-inductor-capacitor (LLC) resonant converter requires a wide switching frequency operation range to satisfy the hold-up requirement. In this article, a novel control scheme is proposed for stacked structure LLC resonant converter. During the normal operation, the frequency doubler modulation strategy is adopted to reduce the switching loss and driving loss, and the output is regulated by PFM. During the hold-up time operation, the pulsewidth modulation is adopted to increase the system voltage gain, where the switching frequency is fixed during this operation stage. For the proposed converter, the resonant tank parameters are designed at the nominal operation point, and the only design consideration for the magnetizing inductor is the zero-voltage switching for primary switches. Therefore, compared with PFM-LLC, both the switching frequency operating range and the circulating current are reduced, and high efficiency feature is obtained. Finally, a 100-W experimental prototype with input voltage range of 160–340 V was built to validate the advantages of the proposed converter. [ABSTRACT FROM AUTHOR]

Gallium nitride (GaN) devices are widely used due to their excellent performance when integrated into power electronics applications. However, owing to their low parasitic parameters and fast switching speed, they are more prone to instability than the silicon devices. The sustained oscillation discussed in this article is one of the instability problems, which may result in overshoot or even more serious device breakdown; thus, it is pretty necessary to suppress it effectively in practical applications. It is the first time to adopt an RC snubber to suppress the sustained oscillation in this article, which is very simple and cheap but effective. First, the sustained oscillation modeling based on a double pulse circuit is carried out. Then, the RC region is established quantitatively according to the root locus analysis results, which makes up for the defects of these qualitative methods before. Furthermore, it is first found that the sustained oscillation can be fully suppressed due to the existence of dipoles. Finally, the oscillation suppression effect within the RC region is compared with that outside the region by the simulation and experimental results, which are consistent with the theoretical analysis. Additionally, the possible negative influence of the added RC snubber is also discussed in this article, which shows that it has a little impact on the switching speed and energy loss, while the sustained oscillation is well suppressed. [ABSTRACT FROM AUTHOR]

Unregulated LLC resonant converters operating at resonant frequency have been adopted in numerous applications due to its high efficiency characteristic. However, in practice, resonant frequency operation cannot always be guaranteed due to the tolerances and variations of circuit parameters. Therefore, in this brief, a simple resonant frequency tracking method based on the transformer secondary voltage is proposed. When LLC converter operating in the below resonant frequency region, the ratio of the average transformer secondary voltage and output voltage is less than unity, while the ratio is unity at resonant frequency. By adopting this characteristic, the resonant frequency tracking is achieved by comparing the calculated voltage ratio and unity, then the switching frequency is adjusted to track the resonant frequency. To reduce the sampling circuit complexity, the rectified transformer secondary voltage is sensed, and the voltage ratio becomes 0.5 at resonant frequency. The effectiveness of the proposed resonant frequency tracking method has been experimentally validated under different scenarios. [ABSTRACT FROM AUTHOR]

Asymmetric duty modulation (ADM), as a recently proposed modulation scheme, provides new chance to improve the performance of dual active bridge (DAB) converter. On this basis, an optimal ADM (OADM) scheme is proposed in this article to minimize the inductor peak-to-peak current over the whole voltage and power range. First, the DAB converter adopting ADM scheme is analyzed in frequency-domain and time-domain, and four operating modes are established. Then, the minimum peak-to-peak inductor current for each operating mode is obtained under Karush–Kuhn–Tucker condition, and the corresponding optimal closed-loop control with low calculation complexity is derived. Then, the key values and features of the OADM scheme are compared with recent state of the art, mainly including the inductor root mean square current and soft-switching performance. Finally, an experimental prototype was built to validate the efficiency enhancement of the converter by adopting the proposed OADM scheme. [ABSTRACT FROM AUTHOR]

In this article, an efficiency-oriented hybrid duty modulation (HDM) scheme is proposed for the dual active bridge (DAB) converter that can reduce the inductor root-mean-square (rms) current and extend the switch soft-switching range. First, a complete description of the combination forms of different modulation schemes is presented considering the allocation of the zero-voltage portion. A unified description for these combination forms is obtained in the frequency domain. Second, the symmetric–symmetric duty modulation (SSDM) scheme and asymmetric–symmetric duty modulation (A2SDM) scheme are established to minimize the inductor rms current by considering its fundamental component. Third, the characteristics of SSDM, A2SDM, and the traditional phase shift modulation schemes are compared from several aspects, mainly including the inductor rms current and the soft-switching performance. Furthermore, the loss breakdown analysis results for the three modulation schemes are presented and compared. According to these results, the HDM scheme is proposed to improve the overall efficiency of the DAB converter. Finally, an experimental prototype was built with a peak efficiency of 97%, which validates the effectiveness of the proposed modulation scheme. [ABSTRACT FROM AUTHOR]

LLC resonant converters have been widely used in many different industrial applications. Analysis and design methodologies have great effect on the converter performance. Accordingly, a complete step-by-step optimal design methodology based on time-domain analysis has been proposed for an LLC resonant converter in this article. The proposed design methodology is implemented under the worst operation condition, and the following considerations are included to obtain the suitable design area: operation mode; voltage stress for resonant capacitor; zero voltage switching operation for primary switches; and resonant tank root-mean-square current. Then, by finding all possible design candidates and comparing them based on the power loss model, the optimized design candidate can be found. Compared with the existing design methodologies, the proposed one has the advantages of high accuracy and small computation requirement, which makes it application in industry possible. Finally, a 192-W experimental prototype was built to validate the effectiveness of the proposed design methodology. In addition, a MATLAB graphical user interference program was built based on the proposed design methodology to visualize and facilitate the design process for engineers. [ABSTRACT FROM AUTHOR]

Inverter‐side current feedback control (ICFC) has been extensively adopted in distributed generation systems because of its simple implementation and better consistency with actual operating conditions. At the same time, when there are a large number of background harmonics in the grid, capacitor voltage feedforward (CVF) is usually added to the system for its suppression. This method could make the system keep the first‐order characteristics under the analogue control. However, under digital control, due to the digital control delay, a reverse resonance peak will be generated in the loop gain, which makes the system unstable in the weak grid. In order to solve the aforementioned problems, this study proposes a feedforward phase compensation method of LCL grid‐connected inverter based on the all‐pass filter (AF). By introducing AF into the CVF channel, the phase lag in the range of reverse resonance peak frequency is compensated, so as to enhance the robustness of the system in the weak grid. At the same time, this study gives the detailed design process of the proposed method. Experimental results on a 3‐kW prototype are provided, and the effectiveness of the proposed control method is verified.. [ABSTRACT FROM AUTHOR]

A conventional pulse frequency modulation (PFM) controlled LLC resonant converter requires a wide switching frequency operating range to regulate the output voltage. There are some disadvantages of PFM LLC resonant converters: (i) the design and optimisation of magnetic components and gate driver circuitry are challenging; (ii) due to the variable switching frequency operating range, electro‐magnetic interference performance of the converter is degraded; (iii) efficiency degradation; (iv) inability to achieve independent control in multiple‐output applications. To address the above‐mentioned problems and achieve fixed switching frequency operation, a variable magnetising inductance control (VMIC) strategy is proposed. In the proposed VMIC control strategy, the operating switching frequency of the primary switch is fixed and the duty cycle remains constant at 0.5. At the resonant frequency operating point, the output voltage is independent of both the magnetising inductance and load. In addition, the zero current turn‐off operation of the secondary rectifier requires that the switching frequency be designed below the resonant frequency. The operational principles and design considerations of the LLC resonant converter with VMIC are presented. Experimental results from a 200 W prototype are presented to validate the theoretical analysis and the effectiveness of the proposed VMIC control strategy. [ABSTRACT FROM AUTHOR]

Wide bandgap (WBG) devices offer the advantages of high frequency, high efficiency, and high power density to power converters due to their excellent performance. However, their low parasitic capacitance and fast switching speed also make them more susceptible to switching oscillations. The switching oscillations can cause voltage and current overshoots, shoot-through, electromagnetic interference, additional power loss, and even device damage, which can seriously affect the performance of power converters and systems. However, a comprehensive and in-depth overview is lacking on this topic. This article reviews the types, the causes and negative effects, the effects of parasitic parameters and suppression methods of these switching oscillations, which is helpful for practical engineering. First, the switching oscillations are divided into different types, and their causes and negative effects are reviewed. Then, the effects of different parasitic parameters on the switching oscillations are overviewed. It is found that due to the different physical structures of silicon carbide metal-oxide-semiconductor field-effect transistors, enhancement-mode gallium nitride high-electron mobility transistors (eGaN HEMTs), and cascode GaN HEMTs, the effects are also different. Finally, the main methods of suppressing the switching oscillations are summarized, and the advantages and disadvantages of these methods are presented. Furthermore, future research works on this topic and the conclusion of this paper are drawn, which will help readers deepen their understanding of the switching oscillations of WBG devices, and inspire readers to better use WBG devices for high-frequency and high-efficient power conversion. [ABSTRACT FROM AUTHOR]

In wide voltage range applications, traditional LLC converters suffer from the following issues: (i) to cover a wide voltage gain operating range, the required switching frequency range is also large, which challenges the design and optimisation of magnetic components, gate driver circuitry, and electro-magnetic interference filter; (ii) a small magnetising inductance is preferred in wide voltage gain range application, which leads to the increase of circuit circulating current and corresponding conduction loss; (iii) passive elements are determined based on the minimum switching frequency, so bulky passive elements are required to satisfy the required operation range. To solve these problems, in this study, a full-bridge LLC (FBLLC) resonant converter with two operation modes, namely full-bridge operation and frequency doubler operation, is proposed for wide voltage gain range application. The FBLLC converter discussed in this study can be designed in half of the input voltage range when compared with the conventional method, so smaller switching frequency operating range and higher efficiency operation are guaranteed. In addition, the smooth mode transition is achieved by introducing switching frequency feedforward control and small output voltage overshoot or undershoot is observed. A 150 W experimental prototype was built to validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

In wide voltage range applications, traditional LLC converters suffer from the following issues: (i) to cover a wide voltage gain operating range, the required switching frequency range is also large, which challenges the design and optimisation of magnetic components, gate driver circuitry, and electro‐magnetic interference filter; (ii) a small magnetising inductance is preferred in wide voltage gain range application, which leads to the increase of circuit circulating current and corresponding conduction loss; (iii) passive elements are determined based on the minimum switching frequency, so bulky passive elements are required to satisfy the required operation range. To solve these problems, in this study, a full‐bridge LLC (FBLLC) resonant converter with two operation modes, namely full‐bridge operation and frequency doubler operation, is proposed for wide voltage gain range application. The FBLLC converter discussed in this study can be designed in half of the input voltage range when compared with the conventional method, so smaller switching frequency operating range and higher efficiency operation are guaranteed. In addition, the smooth mode transition is achieved by introducing switching frequency feedforward control and small output voltage overshoot or undershoot is observed. A 150 W experimental prototype was built to validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

In order to realize power flow control among photovoltaic (PV) panel port, battery port, and load port in the distributed photovoltaic power generation system, integrated three-port dc–dc converters (TPCs) are widely adopted. The objective of this article is to provide a comprehensive, comparative, and insightful analysis and overview of these TPCs. First, based on the power transmission and control requirements of the system, five topological structures of three-port dc–dc conversion system (TPCS) composed of two or three two-port dc–dc converters are presented. And the five structures are comparatively analyzed. Second, based on these topological structures, and combing with the basic two-port dc–dc converters, three groups of typical TPCs with different galvanic isolation requirements are derived, respectively, by sharing switches, combining branches, etc., And comprehensive analysis, comparisons, and reviews of these TPCs with different structures of each isolation type are presented with tables and figures. Finally, the comparison results of the three isolation groups are summarized, which leads to a guidance for the selection of TPCs in different applications, and then some suggestions for the future research on TPCs are given. [ABSTRACT FROM AUTHOR]

In this article, based on the LLC resonant converter fundamental harmonic analysis model, the modulation strategies for LLC resonant converter can be categorized into the following three groups: 1) input voltage fundamental harmonic modulation; 2) resonant tank elements modulation; and 3) secondary equivalent impedance modulation. Operational principles and control diagrams for different modulation strategies are given. Comprehensive comparisons between these modulation strategies in terms of topology complexity, control complexity, and voltage gain range are presented with respect to the same system specifications. The advantages and disadvantages for each modulation strategy are summarized to provide guidance for engineers when analyzing and designing an LLC resonant converter. The hybrid modulation strategies are categorized into different groups based on the specific applications. Brief introductions of these hybrid modulation strategies are presented. Future trends regarding the modulation strategies of LLC resonant converters are presented. [ABSTRACT FROM AUTHOR]

In this letter, a magnetically controlled single-stage ac–dc converter is proposed for low-power application which has the following features. First, single-stage operation is achieved by sharing switches between LLC resonant converter and totem-pole bridgeless power factor corrector (PFC) converter. Second, the power factor correction function is inherently achieved by designing the PFC converter to operate in discontinuous conduction mode. Third, the dc-link voltage is almost constant. Fourth, fixed switching frequency and duty cycle can be implemented for the switches, which simplifies the magnetic component and driver circuit design. Fifth, the LLC resonant converter is designed to operate at series resonant frequency, the highest efficiency operating point, so the converter can always achieve the maximum efficiency operation. Meanwhile, at this operating point, the converter output voltage is independent of the load so that it is functioning as a dc transformer; and finally, soft switching for all semiconductors is achieved. A 75-W experimental prototype is built to validate the proposed magnetically controlled single-stage ac–dc converter [ABSTRACT FROM AUTHOR]

Grid-connected current quality is seriously affected by the low-order background harmonics in the gird-inverter dynamic system. The point of common coupling voltage feedforward impedance reshaping method of the grid-connected inverter is generally applied to solve this problem, however about 90° phase lag of the inverter output impedance is brought in, which would deteriorate the adaptability of the inverter to the weak grid. To solve this problem, this article proposes a novel frequency-division virtual impedance shaping method, which shapes the output impedance of the grid-connected inverter with different coefficients in different frequency bands, respectively. The basic principles, parameters design, and application analysis are studied in detail. With the proposed scheme, the grid-connected inverter can be equipped with strong adaptability to a wide range of grid impedance and suppression ability to the background voltage harmonics simultaneously. Theoretical analysis and experimental results confirm the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The LLC resonant converters commonly adopt frequency modulation (FM) or combination of FM with phase-shift modulation to regulate its output voltage. However, in these control schemes, a variable switching frequency range is required, which makes the magnetic components design complicated. Therefore, in this article, magnetic control (or variable inductor control) is adopted to make the converter operating at constant switching frequency and constant duty cycle. The fundamental harmonic analysis is commonly used because of its characteristic of simplicity. However, the accuracy of this method is reduced and considerable errors occur when the switching frequency or output power changes. Therefore, an optimal design methodology based on time-domain analysis of the LLC resonant converter with magnetic control is proposed in this article. The proposed methodology can assure that the converter will operate in PO or OPO modes within the whole operating range, and zero voltage switching operation for primary switches and zero current switching operation for secondary rectifier will be guaranteed. In addition, by limiting the resonant tank root-mean-square current, the system efficiency is improved. A 200-W experimental prototype is built and the effectiveness of the proposed optimal design methodology is verified. [ABSTRACT FROM AUTHOR]

LLC resonant converter has been widely used in many applications due to its characteristics of high efficiency and high frequency characteristics. Analysis method plays great role in converter design and operation performance. For LLC resonant converter, the most frequently used analysis methods are frequency-domain analysis (FDA) and time-domain analysis (TDA). FDA assumes that only fundamental harmonics can deliver energy in the system, while TDA is obtained by solving circuit state equations. In this study, comprehensive and complete comparisons between FDA and TDA in terms of analysis methodology, parameter design, analysis accuracy, conduction loss, other power losses, and system efficiency are implemented. The analysis and comparison results show that under the same system specifications, by adopting TDA, a larger magnetising inductance value can be used when compared with FDA, which will reduce the resonant tank circulating current and increase the system efficiency. The comprehensive analysis can provide guidance for engineers when analysing and designing LLC resonant converter. Finally, a 200 W experimental prototype was built and tested to verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

Wide bandgap devices such as gallium nitride (GaN) devices are being widely used due to their low on-resistance and parasitic parameters that can improve system performance compared to Si mosfets. However, these advantages mentioned above can sometimes lead to unexpected behavior in practical systems, such as false triggering oscillation. False triggering oscillation may cause overshoot, electromagnetic interference, shoot-through, and even device damage, which seriously affects the performance of the system. In this paper, an RC snubber circuit is proposed to suppress false triggering oscillation in a half-bridge circuit, and the design method of the RC parameters is proposed. The proposed oscillation suppression method is simpler, easier to implement, and more effective than the active gate driver. In addition, it can provide guidance for oscillation suppression design in high-order systems. First, the double pulse circuit is used as an example to analyze false triggering oscillation and its high-frequency equivalent circuit is obtained. Then, the RC region is established by scrutinizing the characteristic equations via a root locus method. The RC snubber circuit has better oscillation suppression effect when the RC parameters are within the region rather than outside the region. Finally, the RC region designed by the proposed method is verified by simulation and experiment results, and the proposed method indicates a substantial improvement of the switching characteristics of the controlled device at turn-off. [ABSTRACT FROM AUTHOR]

Low-voltage enhancement-mode gallium nitride high electron mobility transistors (eGaN HEMTs) are a kind of promising devices to realize high-efficiency and high-power-density conversion because of their fast switching speed, low conduction, and switching losses. Since the reverse conduction loss of eGaN HEMTs accounts for a large part of switching losses, it is of great importance to improve the traditional switching analytical model to predict the reverse conduction loss. In this paper, a complete and accurate switching analytical model of low-voltage eGaN HEMTs is presented, which considers the effects of low-parasitic inductances, nonlinear junction capacitances and nonlinear transconductances. The turn-on and turn-off switching processes are described in detail, and the resulting equations are solved by using mathematical software such as MATLAB to predict the switching waveforms during the switching periods. And an accurate loss calculation method based on the proposed model is proposed including the reverse conduction loss. The accuracy of the proposed model is validated by comparing the predicted switching waveforms and the efficiency of a synchronous buck converter with the experimental results, respectively. Moreover, the proposed model is compared with the traditional model, which proves the advantages of the proposed model. In the end, according to the proposed complete model, the effect of the top switch (TS) gate resistance, the TS drain-source capacitance, power loop inductance, current ripple, the bottom switch (BS) gate resistance, the BS drain-source capacitance, and dead time on the loss breakdown of the synchronous buck converter is explored and some qualitative suggestions are given to improve the conversion efficiency based on these analyzing results. [ABSTRACT FROM AUTHOR]

In this paper, a dual half-bridge LLC resonant converter with magnetic control is proposed for the battery charger application. The primary switches are shared by two LLC resonant networks, and their outputs are connected in series. One of the LLC resonant converters is designed to operate at the series resonant frequency, which is also the highest efficiency operating point, and the constant output voltage characteristic is achieved at this operating point. The second LLC resonant converter adopts magnetic control to regulate the total output current and voltage during both constant current charge mode and constant voltage charge mode. Meanwhile, the function decoupling idea is adopted to further improve the system efficiency. The significant amount of the power is handled by the LLC resonant converter operating at the series resonant frequency, whereas the second LLC resonant converter fulfills the responsibility to achieve closed-loop control. By carefully designing the resonant networks, the zero-voltage switching for primary switches and zero-current switching for secondary diodes can be achieved for whole operation range. A 320-W experimental prototype is built to verify the theoretical analysis, and the maximum efficiency is measured about 95.5%. [ABSTRACT FROM AUTHOR]

High step-up boost converters with coupled inductor have attracted much attention in the fuel cell or photovoltaic grid-connected generation system, however, there are few literatures elaborated on the construction ideas and derivation methods of them. Accordingly, in order to obtain a clear roadmap on the derivation and inner connection of these converters, a comprehensive review and analysis are presented in this paper. First, the basic boost converter with coupled inductor is regarded as the basic topology, and its merits and demerits are analyzed in detail. Then, in order to address these demerits, various step-up techniques are introduced, such as the rectifier circuit, the active-clamped circuit, the multi-winding coupled inductor, and the voltage doubler rectifier; and numerous new topologies are continuously proposed by combinations and equivalent simplifications. In addition to a detailed synthesis of each topology, a comparative and quantitative analysis among some important converters is presented, and the optimal one is chosen to build a 250 W prototype. Finally, based on comparisons and analysis, the main characteristics and inner connections of these high step-up boost converters with coupled inductor are identified and clarified. [ABSTRACT FROM AUTHOR]

In the applications such as mixed-color light emitting diode (LED) lighting system, LED drivers are always required to provide constant current for each LED string and each current should be controlled independently. For this purpose, a single-stage series-type LED driver with independent control of N-channel output currents is proposed in this paper. The function of power factor correction can be realized inherently to obtain high power factor. N active output units, each one made up of a power switch, a diode, and an output filter capacitor, are series-connected to form the series-type N-channel outputs structure. According to different current reference values, N-channel output currents can be independently regulated just by sampling each output current to adjust the duty cycle of each secondary output-switch automatically. The operating principle and independent closed-loop control strategy of the proposed LED driver with N = 3 are analyzed as an example in detail. Finally, the quantitative parameter design method for a 120 W prototype is elaborated. And, the experimental results are presented to verify the effectiveness of the LED driver and the correctness of parameter design method. [ABSTRACT FROM AUTHOR]

In this paper, we propose to monitor the thermal parameters of insulated gate bipolar transistor (IGBT) module using the case temperature. This method works during the shutoff period of IGBT module where the case temperature is in the cooling phase By establishing the relationship between the Cauer-type thermal RC parameters and the time constants of case temperature cooling curves, thermal parameters of the IGBT module can be identified. The proposed method is able to eliminate the influence of thermal interface material (TIM) degradation on the monitored results, which is quite challenging for the current case temperature monitoring methods. Moreover, it enables the detection of thermal resistance and thermal capacitance of the TIM and that of the IGBT module at the same time. Experimental tests are performed to validate the accuracy and feasibility of the proposed method. Moreover, impacts of the junction temperature, case temperature sensing position, and thermal coupling on the monitored results are analyzed. [ABSTRACT FROM AUTHOR]

A single-stage high-frequency resonant ac/ac converter converts the ac-line input voltage into the high-frequency sinusoidal output voltage, which consists of two power conversion units, i.e., a power factor correction (PFC) unit and a high-frequency resonant dc/ac unit, and it can be used to derive a modular multichannel constant current LED driver. By adopting a totem-pole bridgeless PFC boost rectifier to reduce conduction losses, a single-stage bridgeless high-frequency resonant ac/ac converter is proposed in this paper. The operating principle is described, and the performance analysis is presented in detail. Moreover, the quantitative parameter design method of the main circuit is presented, which is suitable for other single-stage ac input converters, such as a single-stage PFC rectifier. Finally, a 130-W prototype has been built and tested to verify the correction of related theoretical analysis and parameter design. [ABSTRACT FROM AUTHOR]

In order to reduce the cost and volume of small-medium power ac–dc light-emitting diode (LED) lighting systems, single-stage LED drivers are often used to drive LEDs with constant current. In this paper, a single-stage isolated high-power factor ac–dc LED driver is proposed and analyzed in detail, which is composed of a buck–boost power factor correction unit operating in discontinuous conduction mode and an isolated dc/dc unit with a voltage doubler rectifier, and the two units are integrated together by sharing the same power switch. Very high input power factor is achieved since the PFC unit operates with almost fixed duty ratio during half period of the utility line voltage. Since a voltage doubler rectifier is included in the isolated dc/dc unit, magnetic core size of the transformer and voltage stress of the secondary side diodes are reduced remarkably. First, the operating principle and performance of the LED driver are analyzed. Then, the main parameters are quantitatively calculated out in detail. Finally, a comparative analysis is presented and a 120 W experimental prototype with 1 A output current is built according to the calculated parameters and the experimental results are presented to verify the correctness of the theoretical analysis and parameter design. [ABSTRACT FROM PUBLISHER]

Insulated-gate bipolar transistor (IGBT) modules and dc-link capacitors are important parts in the majority of power electronic converters which contribute to cost, size, and failure rate on a considerable scale. This paper presents an online condition monitoring method for both IGBT modules and dc-link capacitors of power converters based on short-circuit current of an IGBT module, which is a good condition indicator according to the theory analysis. The failure prediction of dc-Link capacitor is realized by equivalent series resistance, which can be calculated by short-circuit current and a step voltage, and the bond wires fatigue can also be identified by the short-circuit current. The proposed method is capable for detecting small changes in the failure indicators of IGBT modules and electrolytic capacitors, and its effectiveness is validated by a confirmatory experiment. The novelty of the proposed method is that the degradations of IGBT modules and capacitors can be identified simultaneously, and has the merits of low cost and circuit simplicity [ABSTRACT FROM PUBLISHER]

In many applications, such as large-scale LCD panel backlighting, street lighting, tunnel lighting, etc., an ac/dc light-emitting diode (LED) driver should realize the following three functions at least: power factor correction, multichannel constant current outputs, and galvanic isolation. A novel two-stage multichannel constant current ac/dc LED driver with a low cost and simple structure is proposed in this paper, which is composed of a high-frequency resonant ac/ac converter and as many passive LCL-T resonant rectifiers as the number of the output channels. The high frequency resonant ac/ac converter is focused on in this paper, which converts the ac-line input voltage into high frequency sinusoidal output voltage. First, topology derivation of the single-stage high-frequency resonant ac/ac converter is presented, also the operating principle of it. Then, the steady-state performance of it is completely analyzed, including its input power factor, voltage gain, total harmonic distortion of its output voltage, and its soft-switching condition. Finally, a 130 W prototype is built and the experimental results are given to verify the theoretical analysis. [ABSTRACT FROM PUBLISHER]

Active power factor correction converters are often introduced as the front stage of power electronic equipment to improve the power factor and eliminate higher harmonics. A Boost or Buck-Boost converter operating in discontinuous current mode is always adopted to achieve high power factor correction. In addition, the input current contains a large amount of higher harmonics, and a low-pass input filter is commonly adopted to filter it out. In this paper, a single-stage high-frequency AC/AC converter is taken as an example to demonstrate the design method of a passive low-pass filter. Firstly, the input side of the grid needs to meet the power factor and harmonic requirements. The preset parameters are set to a range to characterize the performance of the LC filter. The quantitative design method of input filter is proposed and summarized. Moreover, the sensitivity of the filter parameters is analyzed, providing a direction in practical applications. Preset parameters are all proved to conform to the preset range through PSIM simulation. Finally, a 130-W prototype is established to verify the correction of proposed design method. The power factor is around 0.935 and harmonic content in the input current is about 26.4%. All requirements can be satisfied. [ABSTRACT FROM AUTHOR]