High-power-density Inverter Technology for Hybrid and ...

1 Hitachi Review Vol. 63 (2014), No. 2 96. High-power-density Inverter Technology for Hybrid and Electric Vehicle Applications Takashi Kimura OVERVIEW: Green vehicles have become familiar sights on the road in Ryuichi Saitou recent years, with growing sales of HEVs and the arrival of the first mass Kenji Kubo production BEVs. With environmental regulations on CO2 (fuel economy). and exhaust emissions also becoming increasingly stringent, sales of these Kinya Nakatsu green vehicles are set to increase further in the future. Consequently, Hideaki Ishikawa because electric drive systems are made up primarily of electric motors, Kaname Sasaki inverters, and batteries, inverters are being called on to fulfill increasingly diverse roles and there is a need to decrease their size and cost while increasing their output. Along with supplying inverters for HEVs and EVs that are an ideal match with customer requirements, Hitachi also intends to continue contributing to progress toward a low-carbon society by developing technologies that will help make inverters more widely used.

2 INTRODUCTION to alternating current (AC) at the frequency required WITH the aim of creating a sustainable society, for vehicle speed and other system control to control vehicle fuel economy standards are becoming the electric motor speed, drive torque, and power . progressively tighter year by year in order to reduce The performance requirements for these electric drive carbon dioxide (CO2) emissions. It is forecast that the systems for HEVs and EVs are small size (important proportion of the total market accounted for by electric for ensuring that the system can be installed in the vehicles (EVs) and Hybrid electric vehicles (HEVs) vehicle), high efficiency to extend EV range, high will grow significantly by 2020. HEVs have been output to provide suitable acceleration performance, commercialized in a variety of forms. Hitachi draws and reliability to operate in the harsh environment on its strengths in power electronics to supply inverters inside a vehicle.

3 That suit diverse customer requirements. By utilizing its technologies for package structure There has been demand in recent years to of power module to develop a direct water cooling increase the output and reduce the size of inverters system, Hitachi has succeeded in delivering both so that they can fit into the limited space available small size and high performance. It is also working on in vehicles. In response, Hitachi has developed new making its inverters smaller still by adopting a double- power modules with double-sided direct cooling sided cooling method with direct water cooling and that deliver considerably higher power density than fully immersed cooling fins (see Fig. 1). past models. Hitachi has also developed a standard Depending on the type of drive used, electric drive power electronics platform for mounting these power systems require an Inverter capable of driving one or modules, and has commercialized it in the form of a two electric motors.

4 For compatibility with the motor standard Inverter . and battery, they also need to operate at a variety of This article describes the power modules with system voltages, such as 60 V or less, or 100 450 V. double-sided direct cooling used in automotive Hitachi commenced full-scale production of HEV. inverters, the configuration of the standard Inverter , inverters with direct cooling in 2005, producing Hitachi's own direct current (DC) converter that is models capable of driving one or two motors and integrated into the Inverter , and the steps Hitachi is operating at 42 V. power modules with single-sided taking to increase Inverter output further. direct cooling were used up to the second generation, and double-sided direct cooling was adopted from the PAST WORK third generation onward. Hitachi has also developed Automotive inverters accelerate or decelerate the its standard Inverter to achieve greater sharing of parts vehicle by converting the DC power from the batteries between models (see Fig.)

5 2). - 41 - 97 High-power-density Inverter Technology for Hybrid and Electric Vehicle Applications Insulating Insulating compatibility (EMC) performance to minimize the IGBT IGBT Generation III. substrate substrate radiation of electromagnetic noise due to heavy Inverter output power density (relative). Double-sided current switching, ease of installation (small size and direct water cooling light weight), failsafe functions in the event of a fault, Cooling Thermal Cooling Cooling Heat-dissipating water grease fins water base with fins long life with respect to thermal fatigue, excellent Generation II water and dust proofing, and insulation performance at high altitude. The inverters also need to satisfy these Direct water cooling demanding requirements at a low cost. Indirect Automotive inverters incorporate a wide variety water cooling Generation I. of components, including insulated-gate bipolar 1997 2005 2008 2011 2014. transistor (IGBT) power devices, power modules, (Fiscal year) high-voltage DC line capacitors, main circuit busbars, a power module drive circuit board, a motor control HEV: Hybrid electric vehicle EV: electric vehicle IGBT: insulated-gate bipolar transistor circuit board, three-phase current sensors, and DC and heavy-current AC connectors.

6 Delivering the expected Fig. 1 Development Roadmap for HEV/EV Inverters. high performance has in the past required the use of To satisfy the diverse requirements for automotive use, Hitachi is achieving smaller size and higher performance by utilizing special-purpose components. The components used engineering and analytical technologies taken from numerous in the high-voltage and heavy-current power sections different fields, including the electric power , industrial, and of the Inverter required a high level of insulation and consumer sectors, to develop direct water cooling. the ability to withstand high voltages. As a result, Hitachi has set out to reduce costs by standardizing components, and is seeking to satisfy TECHNOLOGIES FOR INVERTERS WITH diverse customer needs by designing inverters that use HIGH power DENSITY these standardized components. Features of High-power-density Inverter The newly developed High-power-density Inverter The requirements for automotive inverters include has the following features.

7 Control performance that extends from low to high (1) Use of power modules with low thermal resistance, motor speeds, robustness to withstand a harsh and IGBTs and diodes with low losses and high environment (heat and vibration), electromagnetic performance (Year) 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016. First-generation strong (I) Second-generation strong (II). Development Third-generation strong (III). First-generation mild (M). M. Example Tahoe vehicles (General Motors Company). Volt eSLS. Elf (General Motors Company) (Daimler AG). (Isuzu Motors Ltd.). Malibu (General Motors Company). Fig. 2 Use in HEVs. Hitachi commenced full-scale production of inverters with direct cooling in 2005, and has supplied three generations of high-output inverters to date. - 42 - Hitachi Review Vol. 63 (2014), No. 2 98. (2) Design with a low-inductance main circuit and G E G E G E. optimized DC capacitance n+ n+ n+.

8 (3) Water channel design that combines efficient p p p cooling with low pressure loss n n n . (4) Use of an application-specific integrated circuit n+ p+ n+. (ASIC) with built-in protection functions for gate drive n+. p+. C. (5) Motor control circuit board designed for functional C. safety p+. (6) Compact package design with small size and light weight (7) Miniaturized auxiliary Inverter (option) C. Vce (sat) = V V V. Double-sided Direct-cooling power Module Generation I Generation II Generation III. power Devices The IGBT and freewheeling diode (FWD) power Fig. 3 Evolution of IGBT Device Structure and Vce (Saturation devices need to deliver the performance required by Voltage). As IGBT losses account for the bulk of Inverter losses, losses the vehicle when operating at its maximum limits. have been reduced with each successive generation. Non-punch- As power devices account for the bulk of losses in an through devices were used for the first generation, punch- Inverter , they have a significant impact on electrical through devices for the second, and field-stop devices for the efficiency.

9 Accordingly, the power devices need to third. have low losses. Hitachi has achieved low losses by adopting the latest device Technology available at the Thermal IGBT. time for each of its Inverter generations (see Fig. 3). grease As the maximum DC link voltage is about 400 V Base plate (assuming no voltage booster is used), IGBTs need to be rated for 650 to 700 V. Other likely design IGBT. Heat sink IGBT. requirements are an effective current of 300 to 400. Arms and a carrier frequency of 5 to 12 kHz. To achieve low losses under these operating conditions, Relative thermal and to reduce Inverter size, Hitachi has adopted field- resistance stop, trench IGBTs for its third-generation models. Single-sided Single-sided Double-sided indirect cooling direct cooling direct cooling power Modules Previous design Hitachi Hitachi The requirements for power modules are to Generations I and II Generation III. minimize Inverter floor space and to get the best performance out of the power devices.

10 Hitachi has Fig. 4 Advances in power Module Cooling Design. been using direct cooling since its first- and second- Whereas past designs dissipated heat via grease to a heat sink, Hitachi has since its first-generation models adopted generation models. For direct cooling, the fins on the direct cooling in which the heat-dissipating fins are immersed base of the module are immersed in cooling fluid. in cooling water, allowing the IGBT current density to be This improves heat dissipation because it dispenses increased. From the third generation onwards, Hitachi has with the heat sink thermal resistance and the thermal developed a new double-sided cooling method in which fins are grease used in the past between the base plate and heat located on both sides of the chip. sink (see Fig. 4). The pin fin design is critical to direct cooling. While having more fins reduces the thermal resistance, this optimization, the single-sided cooling systems it increases the pressure losses in the cooling water used in the first- and second-generation inverters channels.