Design of HV to LV DC/DC converter using resonant topology

Abstract

The increasing demand for high-voltage electric vehicle architectures requires efficient power converters. Additionally, the increasing adoption of 800 V battery architectures alongside conventional 400 V systems has increased the requirements placed on the high-voltage to low-voltage (HV-LV) DC-DC converter. Hence, a wider variation in battery voltage requires converters with a wide input-voltage range while maintaining high efficiency, compact size, and low voltage stress on the components. This thesis presents the design and evaluation of a 9 kW isolated HV-LV DC-DC converter based on a Stacked Half-Bridge (SHB) LLC resonant topology, designed to operate over an input-voltage range of 330–950 V while providing a regulated low-voltage output. The proposed converter uses the LLC resonant tank together with the SHB structure to achieve the required wide voltage-gain range. The large variation in input voltage would otherwise require a conventional LLC converter to operate over a wide gain range, potentially increasing the required operating frequency range or requiring additional conversion stages. The SHB structure provides an additional voltage gain, allowing the required conversion ratio to be achieved over the 330–950 V input range. At the same time, the LLC resonant tank enables soft-switching operation, while the SHB structure reduces the voltage stress across the primary-side switches. The converter design includes the development of the LLC resonant tank, highfrequency transformer, resonant inductor, modulation strategy, and control structure. First Harmonic Approximation (FHA) was employed for resonant tank design, and detailed loss models were implemented in MATLAB/Simulink to evaluate converter performance under practical operating conditions. Several capacitor-voltage balancing techniques were investigated, and an interleaved PWM modulation strategy was selected due to its improved balancing performance. Simulation results demonstrate that the proposed converter achieves the required voltage conversion over the specified operating range while maintaining capacitorvoltage balance and high efficiency. A maximum efficiency of 97.2% was obtained under nominal operating conditions. From the results, it can be concluded that the SHB LLC converter is a promising solution for wide-input-voltage automotive DC-DC conversion applications. It offers high efficiency, reduced component stress and increased voltage gain range of the converter.