Automotive Power Electronics Multidisciplinary Design
By, Berker Bilgin and Ali Emadi
Power electronics is at the heart of the powertrain electrification. Higher powertrain efficiency targets for our transportation system will be achieved by designing hybrid and electric powertrains with high power density and high efficiency power electronic circuits. In addition, lower cost solutions will enable more affordable electrified vehicles, leading to a more sustainable transportation system with lower greenhouse gas emissions.
In hybrid, plug-in hybrid, and electric vehicles, various power electronic circuits are being applied for energy conversion (see Fig. 1). The traction inverter converts the DC voltage from the battery into AC voltage to drive the electric motor. In some cases, a boost converter is used between the battery and the traction inverter. It maintains an adjustable DC voltage at the input of the inverter to improve overall performance of the drive system under different torque-speed requirements. Battery chargers are used in plug-in hybrid and battery electric vehicles. They rectify the AC input voltage from the grid, usually with power factor correction, and regulate the output DC voltage and current to charge the battery. Auxiliary power module (APM) is another converter, which is utilized to reduce the high traction battery voltage to the vehicle electrical network level (usually 12V in light duty vehicles) in order to charge the low-voltage battery and supply power to the conventional vehicular loads (e.g., radio, control units, head lamps, etc.). These are the main power electronic circuits in an electrified vehicle and they usually have different operational requirements.
Fig.1 Power electronic converters in a typical plug-in hybrid electric vehicle.
A traction inverter is designed for high power and it directly drives the electric motor. In battery electric vehicles, there is generally a single traction drive system, but hybrid powertrains are usually equipped with two traction drives in order to utilize the energy from the internal combustion engine and the battery pack. The operating conditions of traction inverters are defined by the speed and torque requirements of the motor, which are highly dependent on the driving cycle. Since the traction motor operates over a wide speed range and at high torque conditions, the fundamental frequency and current of the inverter varies significantly. Especially at low speed and high torque conditions (including stall torque conditions), the transient thermal performance of the converter becomes important. This is because the thermal time constant of the power semiconductors is much lower than that of the cooling system. Without considering the transient thermal dynamics, the junction temperature of the semiconductor can surpass the maximum value at high torque low speed operation even though the average temperature is still within the limits. This could result in the breakdown of the semiconductor device.
As was mentioned, APM is another power electronic converter, which is commonly used in electrified powertrains; but, it has different operating conditions than the inverter. The power rating of a typical APM (around 3 kW) is much smaller than the traction inverter (around 55kW for a midsize hybrids). The operating conditions of APM are dependent on the activated loads in the vehicular electrical network, but not closely related to the traction requirements. Since the output of the APM is connected to the low voltage network, it has to satisfy very stringent voltage ripple and transient performance requirements. The APM draws power from the traction battery to power non-traction loads. For this reason, it has strict efficiency requirements over a wide load range. Otherwise, the power drawn from the traction battery for the non-traction loads would be higher, lowering the range for an all-electric vehicle and increasing fuel consumption for a hybrid electric vehicle.
Requirements for Traction Power Electronics
For traction power converters, the main design challenge is to meet the high efficiency, high power density, and high specific power targets for wide range of operating conditions cost-effectively. Power density is expressed in power per unit volume. In a practical sense, high power density means providing the same power in a smaller volume. When the volume gets smaller, there is less space to place the components and surfaces for the extraction of the generated heat from the power losses get smaller.
Specific power is expressed in power per unit weight. High specific power means providing a given power within lighter weight. This is related not only with the reduction in volume, but also with the effective utilization of lightweight materials. In this case, the lightweight material technology being used should satisfy the thermal conductivity requirements to provide adequate heat transfer. In addition, the material should have high enough strength to withstand vibrations, especially when the converter is used in the engine compartment or on the transmission.
Designing traction converters for a wide range of operating conditions require a multidisciplinary approach. Various operating conditions affect the electrical, thermal, and mechanical performance of the converter and they should be evaluated during the design process to meet the high efficiency, high power density, and high specific power targets. Therefore, the circuit parameters, materials, and component layouts should be carefully defined to manage high efficiency operation, adequate heat dissipation, and structural integrity (see Fig. 2).
As an example, the value of the input capacitance of the traction inverter defines the ripple content on a DC-link, which is usually limited by standards. Higher values of DC-link capacitance means lower voltage ripple, but also larger size, both for the capacitor and the converter. If the capacitor is connected to the switching devices through a bus bar structure, the larger size of the capacitor affects the size of the bus bar, which might lead to a higher stray inductance and, hence, higher electromagnetic interference (EMI). Higher EMI might limit the operation of the inverter and reduce the power density due to the larger size of the EMI filters.
Fig.2 Multidisciplinary design in traction power converters.
In traction applications, the thermal management system is designed to extract the heat generated by the losses in the converter. The thermal performance is dependent on the heat dissipation path and the thermal resistances. A lower thermal resistance provides better heat dissipation for a material. If the heat generated by the losses cannot be extracted outside of the converter, the temperature inside the converter increases, potentially reducing both reliability and the life of the converter.
Losses in the semiconductor devices are dependent on the junction temperature. Similarly, in the case of the boost converter, battery charger, and auxiliary power module, resistivity and, hence, copper losses increase with temperature. Therefore, a careful evaluation and calculation of the losses is critical during the design of the converter, considering their interdependent relationship with thermal system dynamics and packaging constraint.
Powertrain electrification is receiving growing attention from the automotive industry to meet the fuel efficiency targets, reduce our dependency on fossil fuels, lower greenhouse gas emissions and, hence, to create a more sustainable transportation system. Power electronic circuits are at the heart of the powertrain electrification since they enable the utilization of different energy sources (e.g., battery charger and auxiliary power module) and provide controllability of the power sources (e.g., traction inverter).
Lower-cost, higher efficiency, and higher-power density power electronic circuit solutions are being developed to provide more affordable electrified powertrains and facilitate widespread use of these vehicles. Considering the interdependent relationship between the electrical, thermal, and mechanical characteristics of power electronic circuits, a multidisciplinary approach is crucial during the design process.
This approach is being applied in the Canada Excellence Research Chair (CERC) in Hybrid Powertrain Program at McMaster Institute for Automotive Research and Technology (MacAUTO), where advanced power electronic circuit designs are being developed for various traction applications. For any converters, the design process starts with selecting the right circuit topology for the application, where the efficiency, cost, reliability, size, component requirements, and controllability are of main concerns. Then, the design parameters of the circuit components are carefully defined, based on the power losses, thermal analysis and, more importantly, packaging constraints. In the CERC program, the same approach is applied in many ongoing projects for the development of traction power converters from a few kilowatts to a few hundred kilowatts.
Berker Bilgin received his PhD degree in Electrical Engineering from Illinois Institute of Technology. He is current working as the Senior Principle Research Engineer in Canada Excellence Research Chair Program in Hybrid Powertrains in MacAuto at McMaster University, Hamilton, ON. His research area focuses on design of electric machines and power electronic converters and motor drives for electrified powertrain applications. Berker Bilgin was in the organizing committee for 2013 IEEE Transportation Electrification Conference and Expo. He is an active reviewer for IEEE Transactions on Power Electronics, Industrial Electronics, Vehicular technology and Industrial Informatics, and also for IEEE Conferences such as APEC and ITEC.
In addition to his duties at the McMaster Institute, Dr. Ali Emadi is the Canada Excellence Research Chair in Hybrid Powertrain and also a guest editor-in-chief for the IEEE Transportation Electrification Initiative, specifically aimed at accelerating the development and implementation of new technologies for the electrification of transportation.
Before joining McMaster University, Dr. Emadi was the Harris Perlstein Endowed Chair Professor of Engineering and director of the Electric Power and Power Electronics Centre and Grainger Laboratories at the Illinois Institute of Technology. Dr. Emadi is internationally recognized for his in-depth research on hybrid electric vehicle powertrains and electric drives having authored over 250 publications and conference papers, as well as several books.