Breakthrough in EV Charging Tech: New Resonant DC-DC Converter Achieves 96.1% Efficiency
In a significant leap forward for electric vehicle (EV) charging and renewable energy integration, a team of Chinese researchers has unveiled a next-generation resonant DC-DC converter capable of delivering exceptional efficiency across an unprecedented voltage range. The innovation, detailed in a peer-reviewed study published in Power System Protection and Control, introduces a dual-mode resonant topology that overcomes longstanding limitations in power conversion, paving the way for faster, more reliable, and compact charging systems.
As global electrification accelerates, the demand for high-performance power electronics is intensifying. From fast-charging stations to onboard vehicle chargers and solar inverters, the ability to efficiently convert power across wide input and output voltages is paramount. Traditional solutions often face a trade-off between efficiency, size, and operational flexibility. This new research, led by Gong Chunyang from Shanghai University of Electric Power, addresses these challenges head-on with a novel approach that harmonizes wide gain capability and peak efficiency in a single architecture.
The heart of the problem lies in the physics of power conversion. Conventional LLC resonant converters, while popular for their soft-switching capabilities and high power density, struggle when faced with large variations in input or output voltage—common scenarios in EVs where battery states of charge can vary dramatically, or in photovoltaic systems where sunlight intensity fluctuates. To maintain regulation under such conditions, these converters typically rely on pulse frequency modulation (PFM), which adjusts the switching frequency. However, this method forces the converter to operate far from its optimal resonant point, leading to increased switching losses, electromagnetic interference, and a complex design process for magnetic components. Moreover, the gain characteristics of standard LLC converters are heavily dependent on load conditions, making control strategies less predictable and system stability harder to ensure.
Recognizing these bottlenecks, the research team set out to develop a solution that could decouple performance from load variations while maintaining high efficiency. Their answer was not a minor tweak but a fundamental rethinking of the resonant network and control strategy. The result is two distinct yet related converter topologies: one based on an LLC_LC resonant network and the other on an LLCLC configuration. Both designs incorporate a multi-mode PWM (pulse width modulation) voltage-multiplying rectifier on the secondary side, a key element that enables precise and flexible voltage gain control independent of the load.
The first topology, the LLC_LC multi-mode PWM converter, operates with a fixed switching frequency locked at the primary resonant frequency. This strategic choice simplifies magnetics design and minimizes core losses. Instead of varying frequency, the system achieves different voltage gains by dynamically reconfiguring the secondary-side rectifier circuit through PWM signals. It seamlessly switches between two modes: a full-bridge to double-voltage bridge (FD) mode and a double-voltage to quadruple-voltage bridge (DQ) mode. Each mode provides a stable and monotonic gain curve, unaffected by changes in the connected load. This load-independence is a critical advantage, ensuring consistent performance whether the EV battery is nearly empty or close to full.
By fixing the operating frequency, the converter avoids the pitfalls of wide frequency sweeps. Magnetic components can be optimized for a single, narrow band, leading to smaller, lighter, and more cost-effective designs. Furthermore, the use of PWM for gain control allows for very fine adjustments, enabling smooth transitions between operating modes without disruptive jumps in output voltage. This translates directly into a better user experience, with charging systems that can adapt fluidly to changing conditions.
The second proposed converter, built on the LLCLC resonant network, employs a hybrid PFM+PWM modulation scheme. Here, the primary resonant network uses pulse frequency modulation to achieve low-gain operation, while the secondary-side multi-mode PWM rectifier handles medium and high-gain scenarios. This combination is particularly powerful. It leverages the LLCLC network’s inherent ability to provide sub-unity voltage gain (less than one) through frequency adjustment, a feature essential for stepping down high input voltages. Simultaneously, it uses the PWM rectifier to achieve super-unity gains (greater than one) in a manner completely decoupled from both load and resonant parameters.
This hybrid approach effectively extends the overall voltage conversion range far beyond what either technique could achieve alone. The study reports a maximum output voltage range of 1 to 6.2, a figure that represents a substantial improvement over many existing commercial solutions. For EV applications, this means a single converter can handle the entire charging cycle—from initial bulk charging at high current to the final topping-off phase at higher voltage—without the need for multiple stages or complex auxiliary circuits.
A cornerstone of the technology’s high efficiency is its intelligent use of harmonic content. Unlike traditional converters that treat harmonics as unwanted noise, these new designs actively harness the third harmonic of the resonant current for power transfer. By carefully designing the resonant network—specifically by setting the notch frequency (where impedance is very high) at three times the fundamental resonant frequency—the system creates a pathway for the third harmonic to contribute meaningfully to the total power delivery. This not only boosts overall efficiency but also reduces the burden on the fundamental frequency component, lowering conduction losses and thermal stress on components.
The practical benefits of this harmonic utilization are profound. In the LLC_LC variant, the consistent high level of third-harmonic power transfer across the entire gain range contributes significantly to its excellent efficiency profile. In the LLCLC version, the careful selection of quality factor (Q) ensures minimal reactive circulating currents in all three operating modes—low-gain PFM_FR, medium-gain PWM_FD, and high-gain PWM_DT. Minimizing these wasteful currents is crucial for maintaining high efficiency, especially at partial loads, which are common in real-world operation.
The researchers didn’t stop at theoretical modeling. To validate their claims, they constructed physical prototypes and conducted rigorous testing. The experimental platforms confirmed the simulation results, demonstrating smooth, glitch-free transitions between operating modes and stable output voltage regulation under various load conditions. Most impressively, the tests revealed peak conversion efficiencies of 93.3% for the LLC_LC converter in its highest gain mode and a remarkable 96.1% for the LLCLC converter in its PWM_DT mode. Achieving such high efficiency, particularly above 96%, is a notable achievement in the field of power electronics, where every fraction of a percent saved translates into significant reductions in heat generation, cooling requirements, and energy waste over the system’s lifetime.
Beyond raw efficiency, the new converters offer compelling advantages in terms of reliability and manufacturability. All active switches are able to achieve zero-voltage switching (ZVS) turn-on, a soft-switching technique that drastically reduces switching losses and electromagnetic noise. This leads to cooler-running, quieter, and more durable power systems. The control strategies, while sophisticated in their effect, are relatively simple to implement, requiring only the management of one switch or a complementary pair per operating mode. This simplicity enhances robustness and lowers the barrier to commercial adoption.
The implications of this research extend far beyond the laboratory. For the automotive industry, these converters could revolutionize onboard chargers (OBCs), enabling lighter, more compact units that support ultra-fast charging protocols. They are equally applicable to DC fast chargers, where their wide input voltage range can accommodate different grid connections and their high efficiency reduces operating costs and environmental impact. In the realm of renewable energy, the technology can improve the performance of solar microinverters and string inverters, maximizing energy harvest from photovoltaic arrays even under non-ideal lighting conditions.
Moreover, the modular nature of the design opens doors for scalability. The principles demonstrated could be adapted for higher-power industrial applications or integrated into emerging technologies like solid-state transformers for smart grids. The ability to achieve wide gain with high efficiency in a single-stage converter could simplify power architectures across numerous sectors, from data centers to aerospace.
While the results are impressive, the authors acknowledge that further work is needed. Future research directions include exploring even more advanced resonant networks, such as L3C configurations, and developing novel modulation techniques like frequency-adaptive phase-shift control to further minimize reactive currents. Nonetheless, the current work represents a substantial step forward, offering a clear path toward the next generation of power conversion technology.
In conclusion, the development of these wide-gain, high-efficiency resonant DC-DC converters marks a pivotal moment in power electronics. By ingeniously combining multi-mode PWM rectification with advanced resonant networks, Gong Chunyang and his colleagues have created a solution that meets the demanding needs of modern energy systems. As the world moves towards deeper decarbonization, innovations like this will be essential in building the efficient, resilient, and sustainable infrastructure required for a clean energy future.
Gong Chunyang, Xia Xiao, Bao Jun, Zheng Jian, Chen Hui, Chen Xiaolin, Wang Zhixin, Huang Dongmei; Shanghai University of Electric Power, Shanghai Jiao Tong University, Schneider Electric (China) Co., Ltd. Shanghai Branch, Shanghai Xilong Technology Co., Ltd., Shanghai Chint Power Co., Ltd.; Power System Protection and Control; DOI: 10.19783/j.cnki.pspc.230666