A Model to Estimate the Effect of DC Bus Voltage on HEV Powertrain Efficiency Mark G. Thompson, Craig J. Hoff, and James E. Gover Kettering University Flint, Michigan Abstract—The efficiency of the electric drive system in a Hybrid Electric Vehicle plays a significant role in overall hybrid propulsion system efficiency and influences design choices for other mechanical components in the system. Increasing the DC bus voltage is thought to improve overall powertrain efficiency. In this paper, models are developed to predict the effect of DC bus voltage on the power loss in a PWM inverter motor drive and a brushless DC electric machine. The models indicate that a higher bus voltage will improve powertrain efficiency in some, but not all, operating conditions. Keywords-Hybrid Electric Vehicle; efficiency; inverter; IGBT; conduction loss; switching loss; drivetrain ; iron loss; windage loss I. INTRODUCTION The efficiency of the electric drive system (inverter and traction motor) in a Hybrid Electric Vehicle (HEV) plays a significant role in the overall hybrid propulsion system efficiency and influences design choices for other mechanical components in the system [1]. The loss mechanisms in the semiconductor power devices, (Insulated Gate Bipolar Transistors (IGBT’s) and diodes), of the power inverter and the loss mechanisms of the traction motor can be defined from a foundation of fundamental material properties, device physics and electromagnetic interactions. The balance of losses introduced into an HEV electric drive system can be calculated based on these fundamental concepts applied to the specific type of inverter circuit employed and the design of the specific traction motor utilized. Furthermore, to be useful to the system designer, the method of calculating losses must be simple and accurate enough to yield useful design information and employ readily available parameters that can be extracted from manufacturer supplied datasheets. A complete inverter / motor efficiency map is an important tool in the HEV design process in that it facilitates system level simulation of HEV propulsion systems. The electric drive system losses not only affect the system efficiency directly, but also influence the design and selection of other components throughout the HEV system [2,3]. A notable example is the design of the cooling system for the power electronics. Understanding the influence of the electrical loss mechanisms is necessary for the development of efficient HEV propulsion systems. This paper investigates the influence of DC bus voltage level on the power losses in the inverter and traction motor and develops an efficiency map model for design of HEV propulsion systems. II. ELECTRIC DRIVE SYSTEM LOSS MODELS A. Loss Model for the Inverter In this section, we discuss analytical models to predict the losses in the IGBT’s and diodes of a 3-phase voltage source inverter driving an HEV traction motor [4,5,6,7,8]. For this purpose, the motor is modeled as a 3-phase wye connected inductive load. For the purposes of developing the inverter loss model and the motor loss model, the traction motor load is assumed to be a brushless DC motor (BLDC). The specific inverter circuit modeled is a pulse width modulated (PWM) inverter implemented with IGBT power switches and anti- parallel diodes. The diodes can be either the intrinsic diodes of the IGBT’s or external free-wheeling diodes. A sine wave PWM control scheme is assumed for development of the analytical loss model. A simplified schematic of a PWM controlled inverter drive system is shown in Fig. 1. The power loss in each semiconductor device will be modeled as the average power loss over one motor electrical cycle. In this way, the calculated average power loss in each IGBT and each diode will be equal. It will be assumed that the PWM switching frequency, f s , is much higher than the motor electrical frequency, f m . A simple piece-wise linear conduction model for both IGBT’s and diodes can be used to calculate the conduction losses in these devices. A good approximation of the current – voltage characteristics in conduction can be achieved with a series connection of a DC voltage source and a resistance, both parameters being temperature dependent. These parameters can be easily extracted from device datasheets. Conduction Model for IGBT: v CE = V CE0 + r CE i C (1) Conduction Model for Diode: v F = V F0 + r D i F (2) Representative manufacturer datasheet graphs are shown in Fig. 2 that can be used to extract the conduction model parameters (V CE0 , r CE , V F0 , r D ) for (1) and (2). The off-state blocking losses of both the IGBT’s and diodes will be This research is funded by Paice LLC.