Analysis of LLC converter with series-parallel connection

In this study, a parallel inductor-inductor-capacitor (LLC) resonant converter series-connected on the primary side and parallel-connected on the secondary side is presented for server power supply systems. Based on series resonant behaviour, the power metal-oxide-semiconductor field-effect transistors are turned on at zero voltage switching and the rectifier diodes are turned off at zero current switching. Thus, the switching losses on the power semiconductors are reduced. In the proposed converter, the primary windings of the two LLC converters are connected in series. Thus, the two converters have the same primary currents to ensure that they can supply the balance load current. On the output side, two LLC converters are connected in parallel to share the load current and to reduce the current stress on the secondary windings and the rectifier diodes. In this article, the principle of operation, steady-state analysis and design considerations of the proposed converter are provided and discussed. Experiments with a laboratory prototype with a 24 V/21 A output for server power supply were performed to verify the effectiveness of the proposed converter.

A new zero-voltage switching (ZVS) converter with the series half-bridge legs for high DC bus application is presented. Two circuit modules with the interleaved pulse-width modulation are adopted to share the load current. In each circuit module, a series resonant converter with two transformers that are series-connected in primary side and parallel-connected in secondary side is used to achieve load current sharing and reduce the current stresses of rectified diodes and transformer secondary windings. Based on the series resonant behaviour, power metal-oxide semiconductor field-effect transistors (MOSFETs) are turned on at ZVS and rectifier diodes can be turned off at zero current switching. Therefore the switching losses of power semiconductors are reduced. Two half-bridge legs and two split capacitors are connected in series such that the voltage stress of each active switch is clamped to one-half of DC bus voltage. Thus, the low voltage stress of MOSFETs can be used in high DC bus application. The principle of operation, steady-state analysis and design consideration of the proposed converter are provided and discussed. Experiments with a laboratory prototype with 24 V/40 A output are provided to verify the effectiveness of the proposed converter.

The paper describes a method how to measure the high-power, high-voltage transformer that is used in LLC converter based on GaN transistors. These transistors allow high frequency switching which reduces the power losses and minimizes the form factor. On the other hand, this high frequency is challenging due to the parasitic parameters of the transformer. To achieve soft switching with Zero Voltage Switching (ZVS) on the primary side and Zero Current Switching (ZCS) on the secondary side it must have predictable properties and it must be considered in the controller design. This paper presents measurement of real parameters of given transformer and how to consider its properties in the converter’s design using the SPICE simulations. Key words. LLC converter, Real transformer, SPICE simulation, GaN transistors, Electric Vehicle on-board charger

An interleaved series resonant converter with fixed frequency pulse-width modulation (PWM) is presented to achieve load current sharing, ripple current cancelation, zero-voltage switching (ZVS) for power switches and zero-current switching (ZCS) for rectifier diodes. Two three-level DC converters with clamped diodes and flying capacitor are adopted to share load current. The voltage stress of power switches is clamped at one-half of DC bus voltage. The interleaved PWM scheme is used to control two converters in order to reduce the output current ripple and size of output capacitor. The series resonant tank in three-level PWM converter is adopted to realise ZVS turn-on for all power switches andZCS for rectifier diodes. Thus, the switching losses of active switches are reduced and the reverse recovery losses of rectifier diodes are eliminated. The fixed frequency PWM operation is adopted to regulate output voltage, so that the drawback of a wide range of switching frequency in the conventional series resonant converters is overcome. Finally, experiments based on a scale-down prototype are provided to verify the effectiveness of the proposed converter.

Hard-switching-induced switching loss can contribute significantly to the power loss of an isolated bidirectional dual-active-bridge (DAB) dc–dc converter operating at high frequency. An $LC$ -type series resonant DAB converter based on a switch-controlled-inductor (SCI) is proposed to mitigate the loss arising from hard switching under wide-range variations in output voltage and current. Zero-voltage switching is achieved at the primary side (high voltage), while at the secondary side (low voltage), zero-current switching is preferred to reduce excessive ringing due to circulating current and switching loss. In order to achieve reduced conduction loss, a nominal operating point is chosen where the root-mean-square resonant tank current is the minimum. To validate the proposed topology and modulation scheme, an $LC$ -type series resonant DAB converter based on SCI operating at 100 kHz is designed to interface a 400-V dc bus to a supercapacitor-based energy storage. Simulation and experimental results validate the effectiveness of the proposed topology for charging/discharging a supercapacitor with an output voltage variation of between 10 and 48 V and a maximum rated power of 480 W. A maximum efficiency of 94.6% is achieved using the proposed topology and modulation scheme.

The data centers of the telecommunication infrastructure are recently consuming more electric power than before. This increasing electrical burden is requiring a small but high-power isolated power converter with a high step-down ratio. Conventionally, the LLC converter is regarded to be a promising candidate, although this converter needs vacant space for galvanic isolation due to extremely high voltage caused by the resonance in the primary side, i.e. high voltage side, of the isolation transformer, which hinders the compact implementation inside the server computers in high power design. The purpose of this paper is to propose a novel LLC converter topology to reduce this extremely high voltage by locating the LC resonator on the secondary side. The performance analysis revealed that the proposed converter exhibits a similar frequency-voltage characteristic enabling the boost and buck operation modes as well as the small rectifier conduction loss similar to the full-wave rectifier with a center-tapped transformer. The simulation confirmed the operating principles and reduction of the resonance voltage amplitude of the proposed LLC converter in comparison with the conventional LLC converter. Furthermore, the experiment also supported the operating principles, which suggests the usefulness of the proposed converter for a high voltage dc power supply system.

In future dc distributed power systems, high performance high voltage dc-dc converters with redundancy ability are welcome. However, most existing high voltage dc-dc converters do not have redundancy ability. To solve this problem, a wide load range zero-voltage switching (ZVS) three-level (TL) dc-dc converter is proposed, which has some definitely good features. The primary switches have reduced voltage stress, which is only Vin/2. Moreover, no extra clamping component is needed, which results simple primary structure. Redundancy ability can be obtained by both primary and secondary sides, which means high system reliability. With proper designing of magnetizing inductance, all primary switches can obtain ZVS down to 0 output current, and in addition, the added conduction loss can be neglected. TL voltage waveform before the output inductor is obtained, which leads small volume of the output filter. Four secondary MOSFETs can be switched in zero-current switching (ZCS) condition over wide load range. Finally, both the primary and secondary power stages are modular architecture, which permits realizing any given system specifications by low voltage, standardized power modules. The operation principle, soft switching characteristics are presented in this paper, and the experimental results from a 1 kW prototype are also provided to validate the proposed converter.

A new series resonant converter composed of two resonant circuits with the same converter leg is presented to achieve the zero-voltage switching (ZVS) turn-on of power switches at a wide range of input voltage and load conditions and zero-current switching (ZCS) turn-off of rectifier diodes in a low input voltage case. Thus, the switching losses of power switches and reverse recovery current on rectifier diodes are improved. If the minimum dc voltage gain of the proposed converter is properly selected, the output voltage can be regulated at no-load condition. Two resonant converters using the same power switches are operated by the phase shift half-switching cycle. Thus, the current ripple at the input terminal capacitor can be reduced. In order to reduce the current stresses of output diodes, the output sides of two resonant converters are connected in parallel. The root mean square currents on transformer secondary windings, transformer copper losses and thermal losses can be further reduced. The output voltage doubler topology is adopted in the output side such that the voltage stress of rectifier diodes is equal to the output voltage instead of two times the output voltage in the conventional centre-tapped rectifier topology. Based on the resonant behaviour, all switches are turned on at ZVS and rectifier diodes are turned off at ZCS if the switching frequency is less than the series resonant frequency. The laboratory experiments with a 1000 W prototype are provided to verify the effectiveness of the proposed converter.

This paper proposes a novel interleaved single-stage LLC resonant ac-dc converter. Two derivative buck-boost typed power-factor-correctors (PFCs) with interleaved operation are integrated with an LLC resonant converter to form the presented circuit. The proposed single-stage converter has the features of simple circuit structure, balance switch currents, low dc-bus voltage, low input current harmonics, and high system reliability. Both the active switches can turn on with zero voltage switching (ZVS), and both the output rectifier diodes can turn off with zero current switching (ZCS), which results in lower switching losses. Circuit derivations, operation principles, and circuit analysis of the proposed converter are addressed. Finally, a laboratory prototype with an 110V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> utility-line voltage is implemented correspondingly. The experimental results have verified the validity of the theoretical predictions and the feasibility of the proposed circuit.

A novel zero‐voltage switching (ZVS) DC/DC converter with series connected transformers for high input voltage application is presented. Two series half‐bridge legs and two split capacitors are adopted in order to limit the voltage stress of active switches at Vin/2. Two resonant converter modules are used to reduce current stress on active switches and rectifier diodes. Based on the series resonant tank, the active switches are turned on at ZVS and rectifier diodes are turned off at zero‐current switching. Thus, the switching loss of active switches and reverse recovery problem of rectifier diodes are reduced. The secondary windings are connected in series to ensure that the primary winding currents are balanced. Finally, experiments are provided to verify the effectiveness of the proposed converter.

Hard switching specifies the stressful switching behavior of the controlled switches. During the turn-off and turn-on processes, the power electronic device has to withstand high current and voltage simultaneously, resulting in high stress and switching losses. The switching loss is directly proportional to the switch frequency, thus reducing the maximum switch frequency of the power electronic converter. The concept was to incorporate resonant tanks in the converters to create oscillatory (usually sinusoidal) voltage and/or current waveforms, so the zero current switching (ZCS) or zero voltage switching (ZVS) conditions can be achieved for the power control switches. The Soft-switched power converters are generally utilizing the resonance condition. Resonance condition is generally occurred just during the turn-off and turn-on processes, so as to create ZCS and ZVS across each switch. The Regulated three and five multiple-output dc-dc converter under zero-voltage switching (ZVS) condition is proposed. The converter is consists of three outputs altogether. With the help of two asymmetric half bridge converters, the first and second outputs are controlled. Based on the phase shift between two asymmetric half bridge converters, the third output is controlled. At high switching frequency, these multiple-output dc–dc converters can give higher efficiency. The various stages of operation, soft switching condition and controlling schemes are also explained. A closed loop and open loop control techniques of the three multiple output converter is explained.

본 논문에서는 영전압 스위칭(ZVS)과 전류불연속 모드(DCM)에 의한 새로운 고효율의 DC-DC 컨버터에 대해 연구된다. 일반적으로 고효율의 컨버터를 만들기 위해서는 전력변환기내에 사용된 반도체 스위칭 소자의 손실을 최소화하여 이루어진다. 제안한 컨버터는 DCM에 의하여 스위치의 턴-온 동작을 영전류 스위칭(ZCS)으로 만들고, 또한 새로운 유사공진 회로를 접목하여 컨버터의 고효율을 실현시킨다. 제안한 컨버터에 사용된 제어용 스위칭 소자들은 유사공진 기법에 의해 소프트 스위칭, 즉 ZVS와 ZCS으로 동작시키고, 이에 따른 제어용 스위칭 소자들은 전압과 전류의 스트레스 없이 동작한다. 그 결과 제안한 컨버터는 스위칭 손실의 저감에 의해 고효율로 구동된다. 제안한 DCM-ZVS 컨버터의 소프트 스위칭 동작과 시스템 효율은 디지털 시뮬레이션과 실험결과를 통해 그 타당성이 입증된다. This paper is study on a new high efficiency DC-DC converter of discontinuous conduction mode (DCM) with zero voltage switching (ZVS). The converters of high efficiency are generally made that the power loss of the used semiconductor switching devices is minimized. The proposed converter is accomplished that the turn-on operation of switches is on zero current switching (ZCS) by DCM. The converter is also applicable to a new quasi-resonant circuit to achieve high efficiency converter. The control switches using in the converter are operated with soft switching, that is, ZVS and ZCS by quasi-resonant method. The control switches are operated without increasing their voltage and current stresses by the soft switching technology. The result is that the switching loss is very low and the efficiency of the converter is high. The soft switching operation and the system efficiency of the proposed DCM-ZVS converter are verified by digital simulation and experimental results.

The inductor inductor capacitor (LLC) resonant topology has become more popular for deployment in high power density and high-efficiency power converter applications due to its ability to maintain zero voltage switching (ZVS) over a wider input voltage range. Due to their ease of operation and acceptable accuracy, frequency domain-related analytical methods using fundamental harmonic approximation (FHA) have been frequently utilized for resonant converters. However, when the switching frequency is far from the resonant frequency, the circuit currents contain a large number of harmonics, which cannot be ignored. Therefore, the FHA is incapable of guiding the design when the LLC converter is used to operate in a wide input voltage range applications due to its inaccuracy. As a result, a precise LLC converter model is needed. Time domain analysis is a precise analytical approach for obtaining converter attributes, which supports in the optimal sizing of LLC converters. This work strives to give a precise and an approximation-free time domain analysis for the exact modeling of high-frequency resonant converters. A complete mathematical analysis for an LLC resonant converter operating in discontinuous conduction mode (DCM)—i.e., the boost mode of operation below resonance—is presented in this paper. The proposed technique can confirm that the converter operates in PO mode throughout its working range; in addition, for primary MOSFET switches, it guarantees the ZVS and zero current switching (ZCS) for the secondary rectifier. As a function of frequency, load, and other circuit parameters, closed-form solutions are developed for the converter’s tank root mean square (RMS) current, peak stress, tank capacitor voltage, voltage gain, and zero voltage switching angle. Finally, an 8 KW LLC resonant converter is built in the hardware-in-loop (HIL) testing method on RT-LAB OP-5700 to endorse the theoretical study.

In this paper, a novel soft switching push-pull LC resonant DC-DC converter for energy sources is presented. In a high step-up converter, the input of primary side possesses low voltage and high current, so the losses caused by the current account for most of the total power loss. At the same time, the high-voltage stress of the high-voltage output components on the secondary side is also a major problem. Therefore, a high-gain isolated push-pull converter with a secondary-side resonant circuit is proposed, so that the primary-side switches have zero voltage switching (ZVS) and the secondary-side diodes have zero current switching (ZCS). The push-pull structure can reduce the number of active switches, so that the total power loss on the primary side can be reduced. The converter has a resonant tank circuit arranged between the secondary side of isolation transformer and the high-voltage output rectification module. The high-voltage output rectifier module adopts a full-bridge architecture suitable for high-voltage coupling connection. The low-side power switching module adopts a push-pull architecture suitable for low-voltage and high-current applications. The resonant tank circuit uses an inductor–capacitor (LC) structure to improve the resonant tank circuit, which achieves soft switching during power transfer, increasing the efficiency of the converter and improving the electromagnetic compatibility. The main advantage of this technology is that the secondary-side leakage inductance of transformer and the resonant capacitance are connected in series to achieve ZVS for switches and ZCS for diodes. Finally, a prototype of a high-gain push-pull resonant converter was established. The converter was operated at a fixed switching frequency of 135 kHz and a duty cycle of approximately 0.5. The efficiency of the converter can reach 97.1% under experimental tests at an output voltage of 400 V and a rated output power of 500 W.

Power electronic converters play a vital role in modern power systems. To meet the surging demand for more efficient and compact devices for these applications, high frequency and high efficiency have become two main areas of development in power electronics. Increasing switching frequency in power electronics allows for a significant size reduction in passive components, thus enhancing the power density and reducing overall system size. However, high-frequency operation also causes higher switching losses and poses several design and efficiency challenges. These are further compounded by constraints arising from the harsh operating environment, which dictates the need to pursue new and better materials and circuit approaches to maximize converter efficiency. This study has explored the application of latest wide-bandgap semiconductor materials, including gallium nitride (GaN) and silicon carbide (SiC), as well as ultra-wide-bandgap semiconductor materials such as Ga2O3 and AlN. They present new opportunities for the development of high-frequency and high-efficiency power electronic converters that significantly allow for higher frequency operation, reducing most of the switching losses to achieve overall efficiency. Additionally, vital advancements in soft-switching techniques such as zero-voltage switching (ZVS) and zero-current switching (ZCS), also the development of new circuit topologies like LLC and DAB converters, are evaluated. Further, various gate driver designs, and their comparison are considered key strategies toward minimal energy losses at high frequencies. The review is based on a novel perspective of the bottlenecks left unchallenged so far in power conversion systems, considering some critical technological deficiencies and showing pathways to more efficient, compact, and reliable future solutions.