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.

Overview of Modulation Strategies for LLC Resonant Converter

A family of two-phase interleaved LLC (iLLC) resonant converter with hybrid rectifier is proposed for wide output voltage range applications. The primary sides of the two LLC converters are in parallel, and the connection of the secondary windings in the two LLC converters can be regulated by the hybrid rectifier according to the output voltage. Variable frequency control is employed to regulate the output voltage and the secondary windings are in series when the output voltage is high. Fixed-frequency phase-shift control is adopted to regulate the configuration of the secondary windings as well as the output voltage when the output voltage is low. The output voltage range is extended by adaptively changing the configuration of the hybrid rectifier, which results in reduced switching frequency range, circulating current, and conduction losses of the LLC resonant tank. Zero voltage switching and zero current switching are achieved for all the active switches and diodes, respectively, within the entire operation range. The operation principles are analyzed and a 3.5 kW prototype with 400 V input voltage and 150–500 V output voltage is built and tested to evaluate the feasibility of the proposed method.

Interleaved LLC Resonant Converter With Hybrid Rectifier and Variable-Frequency Plus Phase-Shift Control for Wide Output Voltage Range Applications

Renewable Energy Sources (RES) showed enormous growth in the last few years. In comparison with the other RES, solar power has become the most feasible source because of its unique properties such as clean, noiseless, eco-friendly nature, etc. During the extraction of electric power, the DC–DC converters were given the prominent interest because of their extensive use in various applications. Photovoltaic (PV) systems generally suffer from less energy conversion efficiency along with improper stability and intermittent properties. Hence, there is a necessity of the Maximum power point tracking (MPPT) algorithm to ensure the maximum power available that can be harnessed from the solar PV. In this paper, the most important features of the DC/DC converters along with the MPPT techniques are reviewed and analyzed. A detailed comprehensive analysis is made on different converter topologies of both non-isolated and isolated DC/DC converters. Then, the modulation strategies, comparative performance evaluation are addressed systematically. At the end, recent advances and future trends are described briefly and considered for the next-generation converter’s design and applications. This review work will provide a useful structure and reference point on the DC/DC converters for researchers and designers working in the field of solar PV applications.

https://www.mdpi.com/2079-9292/9/1/31/pdf

A Comprehensive Review of DC–DC Converter Topologies and Modulation Strategies with Recent Advances in Solar Photovoltaic Systems

Achieving high efficiency and high power density is emerging as a goal in many power electronics applications. LLC resonant converter has been proved as an excellent candidate to achieve this goal. To achieve smaller size of passive components, the resonant inductor in the LLC converter is usually integrated into the transformer by utilizing its leakage inductance. However, the leakage inductance of the transformer is usually insufficient and thus the LLC converter has to be operated in a limited frequency range (this limits the input voltage range accordingly), otherwise the power efficiency will drop dramatically. Therefore, a larger resonant inductance in the LLC converter is expected to operate in a wider input voltage range. This paper proposes a new method to create a larger resonant inductance by using a magnetic shunt integrated into planar windings. The accurate leakage inductance modeling, calculation, and optimal design guideline for LLC planar transformer, including optimal magnetic shunt selection and winding layout, are presented. A 280–380 V input and output 48 V–100 W half-bridge LLC resonant converter with 1 MHz resonant frequency is built to verify the design methodology. A comparison is made between two converters with the same parameters, one using magnetic shunt integrated transformer and the others using traditional planar transformer and external inductor. Experimental result shows the proposed converter with magnetic shunt is capable to achieve comparable high efficiency and regulation capability with the other under a wide input voltage, which verifies the optimal design methodology. Above all, this magnetics integration methodology reduces the whole converter's volume and thus increases the power density.

/pdf/high-frequency-llc-resonant-converter-with-magnetic-shunt-8z3xvhi8qq.pdf

High-Frequency LLC Resonant Converter With Magnetic Shunt Integrated Planar Transformer

In this paper, a new common inductor current sharing method is proposed for a multiphase LLC resonant converter for high-power applications. Automatic current sharing is achieved by using a common resonant inductor connecting the resonant inductors in each LLC phase in parallel. The current sharing performance of the proposed method is evaluated under first harmonic approximation (FHA) assumption. The proposed method can automatically share the primary resonant current and the load current for all phases without any additional circuit and control strategy. A 600-W two-phase LLC converter prototype based on the proposed method is built to verify the feasibility. Excellent current sharing performance (less than 0.5% current sharing error at full load) has been achieved.

A Passive Current Sharing Method With Common Inductor Multiphase LLC Resonant Converter

In high-power applications, parallel operation is required to improve both efficiency and reliability. In an LLC converter, it is difficult to achieve parallel operation due to the gain sensitivity to component tolerance. This paper proposes a new structure of a common resonant capacitor for a multiphase LLC converter, which can achieve very good current-sharing performance with no additional components or control. To evaluate the current-sharing performance, a new first harmonic approximation (FHA) model is built in this paper to analyze the LLC converter with coupled resonant tanks. Both FHA analysis and powersim (PSIM) simulation results demonstrate the load-sharing improvement of the proposed common capacitor LLC resonant converter over the conventional two-phase LLC converter. The current-sharing performance from the FHA analysis and the PSIM simulation are compared, and the error of the FHA analysis is evaluated and explained. A 600 W two-phase LLC converter prototype based on the proposed method is also built to verify the feasibility. Excellent current-sharing performance (6% resonant current sharing error at a wide load range) has been achieved.

Common Capacitor Multiphase LLC Converter With Passive Current Sharing Ability

The on-board low-voltage dc–dc converter (LDC) in electric vehicles (EVs) is used to connect the high-voltage battery with the low-voltage auxiliary system. With the advancement of auxiliary equipment in EVs, the output current of the LDC can be hundreds of amperes, which will cause high-conduction loss and severe thermal concern. In this article, a high-efficiency high-power-density on-board LDC is presented. To reduce current stress and improve efficiency, three-phase interleaved LLC dc–dc converters are paralleled to provide 270 A load current. Synchronous rectifier is used to reduce secondary conduction loss. zero-voltage-switching (ZVS) turn- on of primary switches and ZCS turn- off of secondary switches are achieved, thus switching loss can be reduced significantly. Moreover, phase-shedding technology is used to improve light load efficiency. Switch-controlled capacitor (SCC) technology is used to achieve accurate load current sharing among the three phases, which protects the devices against high-current stress, reduces the conduction loss, and improves the reliability of the system. As SCC switches achieve ZVS turn- on and turn- off by its nature, the loss of the SCC circuit is of less concern with regard to the rated output power. In addition, GaN HEMTs are used in the primary side to improve the power density and eventually help achieving light weight. A 3.8-kW (14 V/270 A) LDC prototype is developed and tested. Experimental results show good current balancing among the three phases. A peak efficiency of 96.7% at 140 A load and a full load efficiency of 95.8% are achieved with 3 kW/L power density and 1.5 kg weight.

A High-Efficiency High-Power-Density On-Board Low-Voltage DC–DC Converter for Electric Vehicles Application

A passive-impedance-matching (PIM) technology is proposed to achieve automatic current sharing for multiphase resonant converters through matching the input impedance of each phase. The series inductors (or series capacitors) of each phase are connected in parallel to achieve a couple of virtual resistors including positive and negative resistors and variably series inductors (or capacitors). A virtual positive (or negative) resistor increases (or decreases) the input impedance of the respective phase, and the variably series inductors can also compensate the component tolerance such that the impedance of each phase is matched. The current-sharing performance of the common-inductor two-phase LLC resonant converter (as one example) is evaluated under the first-harmonic-approximation assumption. The virtual positive and negative resistors and variably virtual inductors are calculated. The proposed method can share the primary resonant current and the load current for all phases without any additional circuit and control strategy. The PIM technology is extended to other resonant converter topologies, including common-inductor or common-capacitor series-resonant converter, LCC, CLL resonant converter, etc. A 600-W 12-V common-inductor two-phase LLC resonant converter prototype is built to verify the feasibility and demonstrate advantages of PIM technology.

A Passive-Impedance-Matching Technology to Achieve Automatic Current Sharing for a Multiphase Resonant Converter

This paper proposed a new half-bridge (HB) LLC resonant converter family with pulse-width modulated (PWM) auxiliary switch (sLLC converter) for hold-up mode operation. The proposed sLLC converters could operate with synchronous rectifier, which is suitable for low-output voltage applications. In the proposed converters, an auxiliary switch is added at the primary side to provide charging path for the series resonant inductor. For nominal 400-V input, sLLC achieves same performance as conventional LLC converter, and all the good features such as soft switching are naturally retained. For slight input voltage fluctuation, frequency modulation is used to regulate the output voltage. When the input voltage reduces further, HB switches will operate at constant minimum frequency, and the auxiliary switch will operate in the PWM mode to energize the resonant inductor with the bus voltage directly during hold-up period. Thus, the converter achieves higher voltage gain, and the output voltage can be maintained at desired level. To verify the effectiveness of the proposed sLLC converter, operational principle and equivalent circuits will be carefully explained and analyzed in this paper. A 300-W prototype is built for 250–400-V input 12-V output application.

Yang Chen

Papers

A Passive Current Sharing Method With Common Inductor Multiphase LLC Resonant Converter

Common Capacitor Multiphase LLC Converter With Passive Current Sharing Ability

A High-Efficiency High-Power-Density On-Board Low-Voltage DC–DC Converter for Electric Vehicles Application

A Passive-Impedance-Matching Technology to Achieve Automatic Current Sharing for a Multiphase Resonant Converter

An LLC Converter Family With Auxiliary Switch for Hold-Up Mode Operation