Abstract—This paper presents a switched capacitor dc-dc converter based electric drive system for battery electric vehicles. The main idea is to replace the traditional IGBT boost converter by modular battery cell tied MOSFET switched capacitor converters. The system topology is presented, including the drive train architecture. The modeling approach for each electrical component, including the battery set, dc-dc and dc-ac converters, ac machines, and their control is discussed. Upon successful simulations, various efficiency analyses are performed. Finally, potential hardware implementation, including economic and spacing constraints, is discussed. Index Terms—Switched capacitor dc-dc converters, electric vehicles, ac drives, lithium-ion batteries I. INTRODUCTION witched capacitor (SC) converters have gained in popularity in recent years, and are being applied at increasing power levels [l]. SC converters are significantly different from power converters that use bulky magnetic energy storage elements. Fundamentally, SC converters have equivalent resistance that determines their performance, and is generally much higher than the output impedance of a converter that uses inductors to store energy [2-3]. With capacitors as only energy storage elements in SC, design and selection of capacitor technology is particularly important for high power converters [4]. Battery packs in existing battery electric vehicles (BEV) require a boost converter for batteries to connect with the dc bus before powering the ac drive and machine. The battery packs consist of many single-cell lithium-ion batteries, usually 3.2-4.0 V each depending on their state of charge (SOC). They are connected in series to form a 300-400 V source, which is boosted to around 700 V for the dc bus. [5] Instead of one main bulky IGBT based converter between the battery packs and the dc bus, battery cell attached modular SC converters, based on MOSFET, can be proposed. All the SC converters are connected in series at the output and form a 600-800 V dc bus directly. In existing power train topologies, the dc bus is regulated to a fixed voltage by the boost converter. Since the conversion ratio for a SC converter is fixed by its circuit topology, the output of all the SC converters will vary depending on the battery SOC. However, a design can be 1 Yue Cao and Zichao Ye are with the Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801, USA (email: yuecao2@illinois.edu, zye4@illinois.edu). selected to ensure a minimum dc bus voltage, and for higher voltages, the ac drive can be controlled to operate normally. Possible advantages for modular SC converters in the BEV application include reduced volume consumption, improved thermal flows, flexible structures, improved battery cells balance, and increased reliability/fault bypass, etc. However, many other factors are unknown, such as the proposed system’s feasibility, efficiency, cost, thermal effect, reliability, and also its impact on the motor drive due to floating voltages. This project is therefore to design and simulate such a system and compare it with an existing system. A more comprehensive analysis may also include variations in system topology or system operation points. Note that in this paper, the focus is at system level simulation and not at component level design. II. BEV SYSTEM TOPOLOGY WITH SC CONVERTERS Figure 1 shows the proposed system: the battery connects to the dc bus through a dc-dc converter; then a dc-ac inverter drives an ac induction machine. It is most important to observe the power and efficiency in each subsystem. Figure 1. Battery electric vehicle power system structure The traditional power system utilizes a 300-400 V battery pack and a main dc-dc converter. In the proposed system, a single cell battery can form a module with a SC converter, and the modules connect in series to form a dc bus. This is illustrated in Figure 2 and Figure 3, respectively. Note that there must be multiple columns of modules in parallel so that the total output current meets the ac drive demand while at the same time each battery cell does not exceed its recommended discharge current rating (usually 1C, e.g., in a 2.2 A-h battery it is 2.2 A). Figure 2. A single battery cell and a SC converter formed power module Simulation and Analysis of Switched Capacitor dc-dc Converters for Use in Battery Electric Vehicles Yue Cao, Zichao Ye 1 , Student Member, IEEE S 1 978-1-4799-7949-3/15/$31.00 ©2015 IEEE