Design, Simulation, and Testing of a GaN-based Input-Series Output-Parallel LLC Resonant Converter for Motorsport Applications

The accelerating shift toward electrified powertrains, driven by increasingly stringent European Union (EU) decarbonization targets and the broader goals set by the Paris Agreement, has intensified research into every component of the electrical drivetrain, from the main traction inverter to the auxiliary power unit (APU). This thesis focuses on the final conversion stage of the powertrain chain: an isolated DC–DC converter responsible for stepping down an 800 V battery bus to the 36 V domain used to power onboard electronics in motorsport vehicles. The thesis begins with an extensive review of gallium nitride (GaN) transistors, demonstrating their suitability for high-frequency, high-efficiency conversion. Compared to conventional silicon (Si) and even silicon carbide (SiC) devices, GaN high electron mobility transistors (HEMTs) exhibit lower on-resistance, reduced gate charge, and faster switching transitions. These properties enable the use of smaller passive components and higher power density by pushing switching frequencies into the hundreds of kilohertz. Consequently, GaN HEMTs are the natural choice for both the HV and LV switching bridges in the proposed converter. The architecture explored in this thesis is an LLC resonant converter in an input-series output-parallel (ISOP) configuration, which is particularly well-suited to the 800 V input range. By splitting the input voltage equally across modules, each LLC module needs to handle only a fraction, thereby enhancing system efficiency and power density. The design procedure is based on an analytical framework grounded in first harmonic approximation (FHA) and time-domain analysis at resonance, enabling the determination of the resonant tank parameters necessary to ensure soft-switching operation. The design of the converter and transformer parameters was addressed through a multiobjective genetic algorithm (GA) optimization that simultaneously minimized losses in the power devices and in the transformer while reducing the transformer’s volume. This results in a Paretooptimal front of solutions, enabling an optimum trade-off between efficiency and power density. Following the design phase, LTspice simulations using manufacturer-provided behavioral models validated the analytical predictions across six representative operating points. These simulations confirmed soft-switching behavior throughout the operating range and showed strong agreement between calculated and simulated root mean square (RMS) and peak current values, particularly at full-load. A hardware prototype of the LLC building-block module was designed in Altium Designer, assembled, and tested across twelve operating points comprising three input voltages and four output power levels. The converter achieved an efficiency of approximately 96% over a wide portion of the load range, consistently exceeding 94% at nominal and maximum input voltages. Analytical and experimental current values agreed within 10% at high load, with larger deviations at light load attributed to below-resonance operation and nonideal synchronous rectification (SR) behavior. These results demonstrated that a GaN-based ISOP LLC converter operating at 400 kHz is a compelling solution for high-density, high-efficiency auxiliary power conversion in motorsport and, more broadly, for any 800 V electrified platform requiring a compact step-down stage.