Technologies and Components for Designing Electric Vehicles
Hybrid electric vehicles (HEVs) such as the Toyota Prius and the Chevy Volt and electric vehicles (EVs) such as the Nissan Leaf, BMW i3 and Tesla Model S are growing in popularity amid concern for global warming. It’s easy to see why. According to the U.S. Department of Energy, during a typical 100-mile trip, if an HEV averages 42 miles per gallon, it will use about 2.4 gallons of gasoline. Gasoline contains approximately 24 pounds of CO2 equivalent per gallon, resulting in 57 pounds of CO2 emissions for the trip. If a vehicle powered purely by gasoline gets 25 miles per gallon, 100 miles requires 4 gallons of gasoline, resulting in 96 pounds of CO2. Calculating the numbers for EVs is a bit more complicated, since the fuel used to produce the electricity that charges the vehicle's battery needs to be taken into account. Nonetheless, with EVs there is one inevitable truth: EVs produce zero tailpipe emissions.
In a series hybrid, the electric motor is the only means of providing power to turn the vehicle’s wheels. As its name suggests, the car runs off the electric motor until the battery output falls below a certain level, at which point the gas engine kicks in to power a motor/generator that runs the car (in addition to its normal role of helping to charge the battery pack).
In a series/parallel HEV design, the engine can either drive the wheels directly or be effectively disconnected from the wheels so that only the electric motor powers the wheels (as in the series drivetrain). The Toyota Prius has made this concept popular. At lower speeds, this dual drivetrain operates more as a series vehicle, while at high speeds, where the series drivetrain is less efficient, the engine takes over and energy loss can be minimized.
From a system perspective, HEVs and EVs comprise a number of drivetrain and energy storage modules (Fig. 1). The battery pack typically uses Li-ion cells (in the range of 400 V) and is managed and monitored by a battery management system (BMS) and charged via an on-board AC/DC converter module. A DC/AC inverter uses the high voltage of the battery to drive the electric motor; it is also used for regenerative braking, putting energy back into the battery pack. A DC/DC converter is needed to connect the high-voltage battery to the conventional 12 V automotive network.
Fig. 1: Basic electrical architecture of the HEV/EV. (Source: Texas Instruments)
The main inverter
Power inverters and converters are used to invert HV battery pack direct current (DC) to alternating current (AC) for motors that propel the vehicle down the road; they also convert AC to DC to charge the HV battery pack. With an electric drivetrain, the inverter (Fig. 2) controls the electric motor in a manner somewhat equivalent to how the Engine Control Unit (ECU) of a gas or diesel internal combustion engine vehicle determines the vehicle’s driving behavior; it also captures kinetic energy released through regenerative braking and feeds this back to the battery. As a result, the range of the vehicle is directly related to the efficiency of the main inverter.
Fig. 2: Block diagram of the main inverter. (Source: Infineon Technologies)
The IGBTs are a high-voltage, high-current switch connected directly to the traction motor in the HEV or EV. The more efficient the IGBT, the less power is lost to wasted heat, resulting in better mileage (sometimes called miles per watt of energy). As an example of a power module with IGBTs available to engineers, Infineon’s Easy 1B and Easy 2B series provide a platform for different HEV and EV applications. One such unit is the F4-75R07W1H3 EasyPACK 1B with the company’s fast Trench/Fieldstop IGBT H3 and Rapid 1 diode. Electrical features include increased blocking voltage capability to 650 V, low switching losses and low inductive design. Mechanical features include 2.5 kV AC 1 min insulation, integrated NTC temperature sensor and the company’s PressFIT solder-less mounting of power modules. Easy automotive power modules are available with different configurations like H-bridges (F4 modules) for DC/DC converter applications.
Unlike IGBTs, MOSFETs have no “tail” current when turned off. SiC MOSFETs, in particular, combine several desirable characteristics, such as high breakdown voltage, low on-resistance and fast switching speed, with their inherent advantages of high-temperature capability, high-power density and high efficiency. Moreover, their light weight and small volume favorably affect the whole powertrain system in an HEV and, thus, the performance and cost. As a result, it is expected they will begin replacing IGBTs in HEV/EV applications. Rohm Semiconductor’s SCT2080KEC, for example, features a breakdown voltage of 1200 V, 80 mΩ on resistance, and turn-on/ urn-off time of less than 70-90 ns, enabling switching frequency in the hundreds of kHz range.
Usable as a hybrid powertrain driver, International Rectifier’s automotive-qualified AUIRB24427S high-current, dual low-side driver IC features output current in excess of 6 A per channel across the full temperature range and is designed to drive large IGBT and MOSFET gates in modules or discrete packages. Due to the device's extremely low output impedance in turn-on and turn-off mode, power losses are said to be low, allowing operation in harsh and high-temperature environments such as in HEV power supply stages as a primary- or secondary-side driver.
MCUs in control
At the risk of oversimplification, the motor controller consists of a microcontroller, a power output stage and the motor together with a rotor position sensor, which requires a resolver-to-digital converter (RDC) to determine the angular position and rate as quickly and precisely as possible. This information can be forwarded to the microcontroller, so that it can be taken into account in the motor control algorithms, or the RDC can be integrated in the MCU. This results in a simplified system architecture, where rotor position, sinusoidal and co-sinusoidal values, and the angular rate are available to the MCU at all times.
One of the newest MCUs developed for motor control in HEVs and EVs is the Renesas RH850/C1x Series of 32-bit controllers. Based on the 40 nm process, the RH850/C1x Series features the RH850/C1H and RH850/C1M MCUs, which integrate large flash memory capacity achieved through 40 nm metal oxide nitride oxide silicon (MONOS) process technology. MONOS characteristics include fast readout, low power consumption and large storage capacity. The parts also offer the necessary motor control peripherals and single/dual motor control options needed to support fine-grained motor control and functional safety. In particular, there are two product types: the RH850/C1M, which incorporates a single RDC for single motor control, and the RH850/C1H, which incorporates two RDCs for dual motor control. The RH850/C1M and RH850/C1H devices offer memory capacities of 2 and 4 MB, respectively. In addition, 32 KB of data flash memory, with essentially the same functionality as EEPROM, is included for data storage. Samples of the RH850/C1H and RH850/C1M MCUs are scheduled to be available in early 2015. Mass production is scheduled to begin in May 2016.
Different voltage levels are required by the various electronic components in a car or truck. The most basic requirement for DC/DC conversion is to power the traditional 12 V loads. When a standard combustion engine vehicle is operating, an alternator connected to the engine provides the power for all electrical loads and also recharges the battery. The internal combustion engine in HEVs can be off for extended periods of time, so an alternator cannot be relied upon to provide power to auxiliary loads. A DC/DC converter charges the 12 V battery from the HV bus, thus eliminating the 14 V alternator.
The system can be realized with an MCU controlling both the high- and low-voltage side of the converter such as in TI’s Piccolo Real-Time 32-Bit Fixed-Point TMS320F2802x/3x devices. These include 40 to 60 MHz variants, up to 128 kB of flash memory, a high-speed 12-bit ADC, high-resolution enhanced Pulse Width Modulators along with a host of other modules such as high-precision on-chip oscillators, analog comparators, communication interfaces and general-purpose I/O. Designers can get started easily with a variety of Piccolo hardware evaluation tools and application kits from TI.
To power the electric motors, large battery packs are made up of hundreds of cells installed in the vehicle and producing about 400 V of power. The battery packs are managed and monitored by a battery management system (BMS) and charged via an on-board AC/DC converter module, with voltages ranging from 110 V single-phase to 380 V three-phase systems.
The battery management system is a key element in the overall HEV and EV architecture. It can not only extend the battery’s lifetime, but it can also extend the possible range of the vehicle. The State of Health (SoH), State of Charge (SoC) and Depth of Discharge (DoD) of the battery are constantly checked. As battery cells age, the capacity of individual cells changes and negatively impacts the total battery capacity. Fortunately, cell supervision circuitry enables cell balancing during charging and discharging. While the vehicle power system sees the battery pack as a single high-voltage source, the battery control system must consider each battery’s condition independently. If one battery in a stack has slightly less capacity than the other batteries, then its SoC will gradually deviate from the rest of the batteries over multiple charge and discharge cycles. The more cells a pack has in a series, the greater the possible difference in state of charge, impedance and capacity affecting the energy delivery of the pack.
An onboard battery management and protection system controls battery state during charging and discharging to enable the longest possible battery life. Battery monitoring devices integrate all necessary components for voltage and current measurement, signal isolation and safety monitoring. Since most EV and HEV battery packs are now Li-ion formulations, battery protection and monitoring are a necessity. At the higher operating voltages experienced in electric vehicles, overvoltage can be catastrophic.
Components such as Texas Instruments’ BQ76PL536A (Fig. 3) include circuitry for bringing the Li-ion cells back into balance. The BQ76PL536A is a stackable three to six series cell lithium-ion battery pack protector and analog front-end that incorporates a high-accuracy ADC, independent cell voltage and temperature protection, cell balancing and a precision 5 V regulator. The BQ76PL536A can be stacked vertically to monitor up to 192 cells without additional isolation components between ICs. Each BQ76PL536A device protects the battery pack from overcharge, overdischarge and overtemperature for system safety.
Fig. 3: Functional block diagram of the BQ76PL536A. (Source: Texas Instruments)
Because batteries have a finite energy capacity, HEVs and EVs must be recharged on a periodic basis, typically by connecting to the power grid. With an onboard charger unit, the battery can be charged from a standard power outlet. For most users, 120 VAC at 15 to 20 A will be the most readily available power supply that all onboard chargers should be capable of handling. But since charging time is an important factor for car drivers, some users can take advantage of 240 VAC that will allow for faster charging times, but will require a more robust power source.
The onboard charger converts electrical power from AC to DC and controls the power flow to the high voltage battery. The charging system consists of an AC/DC rectifier to generate a DC voltage from the AC line, followed by a DC/DC converter to generate the DC voltage required by the battery pack. The long-term trend will be to move towards bi-directionality, where the charger also feeds power from the car to the smart grid. In this case the incoming power will need to undergo power factor correction (PFC) to boost the power factor to meet regional regulatory standards.
Optimized for onboard chargers and battery management in electric cars and plug-in HEVs (PHEV), Vishay Intertechnology’s AY2 capacitors are AEC-Q200-qualified, AC line-rated ceramic disc safety capacitors designed to provide high reliability for Class X1 (440 VAC) and Y2 (300 VAC) automotive applications in accordance with IEC 60384-14.3, 3rd edition. Featuring U2J, Y5S and Y5U ceramic dielectrics, the AY2 series offers a capacitance range from 10 to 4,700 pF — with tolerances down to ± 10 percent — over a temperature range of - 55 C to +125 C. The Vishay AY2 capacitors are said to be able to withstand more than 2,000 temperature cycles without a single failure, twice the AEC standard.
Resources for engineers
Not only can Avnet provide and support electronics products and systems for EV/HEV applications, it also serves automotive OEM customers by providing reference designs, system solutions, and a variety of design chain and supply chain services and capabilities. For example, the Xilinx® Zynq®-7000 Programmable SoC/Analog Devices Intelligent Drives Kit combines the Xilinx Zynq-7000 SoC ARM dual-core Cortex-A9 + 28 nm of programmable logic with the latest generation of Analog Devices’ precision data converters and digital isolation to enable high performance motor control. Also included in the kit is a Zynq reference design of field-oriented control featuring Analog Devices’ Linux framework to provide an infrastructure for quickly adding custom control algorithms.
In addition to the powertrain applications for HEVs and EVs discussed in this article, Avnet uses the know-how of our suppliers — world leaders in automotive electronics — in areas such as body control, driver information and infotainment systems, automotive lighting, ignition systems, car security and safety applications. Engineers will find Avnet well-situated to provide innovative, high-performance semiconductor solutions with best-in-class technologies for traditional as well as hybrid and electric vehicles.