# Numerical Analysis of the Cooling Performance in a 7.2 kW Integrated Bidirectional OBC/LDC Module
## 1 Introduction
The method considering SOH has been proposed as a method for checking battery life and water solubility to improve performance degradation problems and stability problems due to overheating, and research for checking the effects of temperature, energy efficiency, and battery aging has been carried out [22; 23]. In previous studies, efforts have been made to improve circuit efficiency and improve cooling performance. However, most of the previous studies are designed considering only specific conditions or conditions for main modes. The power conversion device used in this study operates in various modes depending on the situation, and the circuit characteristics change accordingly, so it is necessary to consider the design stage to cope with this. In particular, the 7.2 kW integrated bidirectional onboard battery charger/low-voltage DC-DC converter (OBC/LDC) module is very intensively designed in order to increase power density; thus, it is indispensable to understand the characteristics of each operation mode to effectively dissipate heat generated from internal circuits. In this study, a numerical analysis method is proposed that considers the power loss and heat flow characteristics in the design of a 7.2 kW integrated bidirectional OBC/LDC module. In addition, the cooling system design takes into account changes in the heating characteristics depending upon the operating modes. A 7.2 kW integrated bidirectional OBC/LDC module for an EV consists of four main parts: A totem-pole power factor correction (PFC), series resonant converter, high-voltage buck converter, and low-voltage buck converter. Each part is composed of an inductor to induce a voltage in proportion to the current change rate, a diode that rectifies the current by allowing it to flow in only one direction, a metal-oxide-semiconductor field-effect transistor (MOSFET), which is a type of transistor that performs amplification and switching by adjusting the current or voltage flow, and a transformer that increases or lowers the AC voltage via the mutual inductance principle. The main sources of heat in this type of module are the diode, rectifier, and inductor, and the heat generated from these devices has the potential to degrade the performance and safety of the system if not properly handled. Thus, the arrangement, cooling method, and operating conditions are critical factors in the stable performance of the power converter. In addition, it is essential that the cooling performance be examined in light of the heat generated by each element and its supporting structure. The conventional approach to the cooling problem involves numerical analyses with the objective of minimizing the internal temperature of each device, while considering the heat value produced by each major heat source to be constant. However, as the size and position of the heat sources in the proposed 7.2 kW integrated bidirectional OBC/LDC module varies depending on the operating mode, the conventional approach cannot be used in that design. To overcome this limitation, a numerical analysis method is introduced to support a cooling system design that supports changes in the operating mode of the module. The proposed method involves an unsteady state analysis to determine the changes in the circuit and heat flow in each operating mode. Then, the obtained circuit heating and internal cooling characteristics are analyzed based on the numerical results. The circuit and heat flow analysis method used in this study are depicted in Figure 1. The circuit was configured based on the results of the numerical analysis, and the power loss and efficiency were then calculated in each operating mode (Modes 1 to 4) using MATLAB/Simscape. The calculated power loss of each device was then converted to a heating value and used in the subsequent analysis. A heat flow analysis model constructed in the ANSYS Fluent 19.2 software package was used in the evaluation of the cooling performance.