Design of an LLC resonant converter experimental platform

To address global warming, the transportation sector, a significant contributor to carbon emissions, is transitioning toward electric vehicles (EVs), which are rapidly gaining market share. Despite their growth, EVs still face many technical challenges, particularly those related to battery, charging, and motor drive technologies. Among these, charging technology is the primary concern for users. Studying experimental technologies related to EV charging can enrich the knowledge of students within the power electronics discipline.

Currently, onboard chargers mostly utilize a two-stage structure comprising AC–DC and DC–DC conversion stages. The DC–DC conversion stage often employs topological structures such as phase-shifted full-bridge, dual active bridge, and LLC resonant converters. Among these, the LLC resonant converter is particularly notable due to its wide soft-switching range, low electromagnetic interference, high efficiency, and wide output gain. Its output voltage is regulated by adjusting the switching frequency, allowing for analysis in time and frequency domains. The inductive and capacitive components in this converter exhibit frequency-selective characteristics, and the converter operates by transferring energy through the fundamental harmonic of the current. Thus, the fundamental harmonic analysis method can be used to derive the gain expression of the LLC resonant converter and plot its gain curve, facilitating parameter design.

Based on theoretical analysis, an experimental prototype platform with an output power of 3.3 kW was designed. During the design phase, parameters of the resonant tank, transformer, and resonant inductor were calculated, and the prototype was constructed accordingly. We also designed the main, driver, and sampling circuits to ensure stable and efficient converter operation. Steady-state experimental results indicated that the primary-side switching transistors achieved zero voltage switching under varying output voltages and currents. Operating in the below-resonance region allowed the secondary-side synchronous rectifier switches to achieve zero-current switching, ensuring that all switches achieved soft-switching operation. The converter showed high overall efficiency, with the peak efficiency exceeding 96%, and a wide soft-switching range. Dynamic characteristic experiments revealed that the converter quickly stabilized the output voltage following sudden changes in settings or loads, showcasing the reliability of the PI closed-loop control.

The experimental results confirmed that the converter met the design requirements, with steady-state and dynamic performances fulfilling expectations. This experimental platform, which pertains to EV charging applications, serves as a tool for conducting various experiments related to LLC resonant converters, enhancing students' intuitive understanding of resonant converters in power electronics. DC–DC resonant converters are crucial components of power electronics technology. The experimental platform improves students' hardware debugging and software programming skills, enhancing their comprehensive skills in the power electronics discipline.