Tiny Power Box - Thermal Investigations for Very High Power Density Onboard Chargers

State-of-the-art onboard charger (OBC) designs often require bidirectional power transfer between the grid and high voltage battery, galvanic isolation, and even additional low voltage output(s). This leads to increasing part count and complexity of the power electronic system, yet the efficiency should be as high as possible and for automotive applications the design should be compact and lightweight. Consequently, power-and loss density increases in almost all parts of the OBC system. Semiconductor components and magnetic components are typically the main loss contributors and therefore the most critical considerations of any elaborate cooling system design. This paper will guide through the thermal aspects of OBC design and optimization by means of one design example achieving beyond state-of-the-art power density for OBC applications (>3.1kW/dm3 including housing). For the presented 7kW single phase OBC design, a liquid cooling concept is investigated. 3D-finite element analysis (FEA) and detailed high fidelity component models are used to identify hot spots in the complex designs which guide the cooling system designer to adopt the cooling system accordingly. The cooling system's optimization targets and system modelling aspects are discussed in detail and compared with the real system's thermal performance.