The High Voltage and Currents in Hybrid Electric Vehicles (HEVs) create technical challenges for power conversion beyond those normally associated with vehicle electrical and electronic systems. High-voltage isolation, the potential for high electromagnetic interference, a large size, the need to remove a substantial amount of heat, and high cost are just a few of the problems. While 12 V automotive systems typically deal in power levels of a few 100 watts or less, voltages for hybrids can easily reach several 100 volts and drive power levels into the kilowatt or higher range. In hybrids, dc-dc converters boost the voltage from the high-voltage battery to power the high-voltage traction motor and also reduce the voltage for lower system voltages such as 14 V or intermediate levels. This report will cover ongoing improvements to existing hybrids as well as systems being developed for newer vehicles including plug-in hybrids (PHEVs) that also require ac-dc conversion. These ongoing improvements and new developments attest to the increasing interest in power conversion.
POWER CONVERSION IN TODAY'S HYBRIDS
High-voltage dc-dc converters for vehicles build on existing design approaches. In many cases, they are simple compared to converters used in computers, telecommunication and other high-performance electronic equipment. However, the automotive environment has driven unique design aspects and significant improvements have occurred in more recent designs. Due to cooling requirements, the dc-dc power converter is frequently mounted in an assembly with the inverter for the motor. This simplifies water cooling the entire unit and only uses a single set of water-in and water-out connections. As shown in Figure 1, the power electronics unit for the belt alternator system (BAS) initially used on the 2007 Saturn Vue Green Line performs these functions, as well as housing the transaxle electric oil pump controller and auxiliary inputs and outputs. Engine coolant circulated by the vehicle's cooling system provides the thermal regulation. For the 2008 model year, this system is also available on the Saturn Aura Green Line and Chevrolet Malibu hybrid sedans.
Honda chose to mount the dc-dc converter close to the battery and use a fan to cool both units. In the new Civic Hybrid introduced in 2006, the integrated power unit (IPU) mounts in the back seat and cools the battery, dc-dc converter and air conditioning inverter by drawing air through an intake air duct with a cooling fan.
Higher-power dc-dc converters have benefited from ongoing improvements since their initial usage in vehicles. TDK increased the operating frequency from 70 kHz to 115 kHz and used a different core from the 2002 to 2006 models. The new PC95 ferrite core material developed by TDK for the 2006 design allowed improvements in the transformer. In addition, package revisions for the power semiconductors enabled improved heat dissipation. Combined with other design changes, including circuit optimization, the changes provide a 51% reduction in the module's volume, 154% increase in power density and 48% reduction in mass, while the output power was increased by 25%. Figure 2 shows a comparison of the changes.
Honda has attributed significantly higher efficiency over the entire output current range in the Civic Hybrid to the use of higher switching frequencies as well as a design change from hard switching to soft switching. As shown in Figure 3, the improvements occurred, while the battery pack's voltage increased from 144 V to 158 V.
Soft switching reduced the power loss caused by the overlap of current and voltage when switching the power elements in the dc-dc converter. According to Tatsuro Horie, assistant chief engineer for Honda R&D at the Tochigi R&D Center in Japan, “By converting the switching frequency to a two-stage variable frequency, and performing switching at the high frequency of 115 kHz in the high load area, where the soft switching effect and transformer loss reduction effect are high, and performing switching at a low frequency of 90 kHz in the low load area, where the soft switching effect is small, the efficiency was enhanced over the entire area.” TDK's module used in the 2006 Honda Civic Hybrid is shown in Figure 4. A 1.5 kW TDK dc-dc converter is also used in the Ford Escape and Mercury Mariner Hybrids to convert the 330 V battery's output to 14 V to control vehicle electrical loads.
Toyota, the leading supplier of hybrid vehicles, has made several changes to the voltages as well as improvements to the converters used in its vehicles. In the RX400h, Lexus increased the battery voltage up to 288 V (compared to the 201.6 V used in the Prius THSII) and used a boost dc-dc converter to increase the voltage for the inverters to 650 Vdc instead of 500 Vdc used in the Prius. A dc-dc converter also reduced the 288 V battery voltage to 42 V to handle the electric power steering and another reduced the 288 V to a lower voltage to charge a 12 V auxiliary battery and supply electrical power to traditional vehicle accessories such as head-lamps, wipers and the horn.
The Lexus GS450h, the first front-engine, rear-wheel drive hybrid uses a new Denso dc-dc converter for the buck converter (Figure 5). A complex dual-transformer topology reduces the energy loss by half compared to a conventional dc-dc converter and also decreases the amount of heat generated during power conversion. As shown in Figure 6, the 1.8 kW converter uses synchronous rectification on the output side of the complex dual transformer.
Previously, voltage conversion was achieved by using a transformer and a choke coil as separate magnetic components. In the new design, Denso used a unique combination of two transformers to accomplish this. The structure reduces the stress in the semiconductors and other circuit components in the converter and subsequently reduces internal energy loss. This allows air cooling for the dc-dc converter instead of water cooling. In addition to decreasing the amount of heat generated during voltage conversion, the dc-dc converter contributes to improving fuel consumption and provides flexibility for the unit's mounting location.
One of the newest hybrid designs, General Motors' GMC Yukon and Chevrolet Tahoe dual-mode hybrids, uses a 300 V battery pack with a dc-dc converter located under the vehicle's hood to power conventional 12 V accessories, such as interior lighting, climate control and the radio. A 330 V to 42 V buck converter supports the electric power steering. Table 1 provides a summary of the different types of dc-dc conversion performed in hybrid vehicles today as well as the ac to dc battery charging required in plug-in hybrids.
BATTERY CHARGING FOR PLUG-IN HYBRIDS
One of the highly anticipated changes for hybrid vehicles is the capability to replenish the battery's lost energy from a fixed electrical power source. Plug-in hybrids, such as the Chevy Volt concept vehicle, may have only a 40-mile range on a charge without dipping into the gas tank, but this fulfills the needs of 78% of commuters. To recharge the battery without using the gasoline engine, plug-in hybrids require a 110/220 Vac to dc converter.
Since General Motors' announcement earlier this year, several manufacturers have revealed plug-in hybrid programs. The Ford HySeries drive in its Airstream concept vehicle uses a 336 V lithium-ion battery and fuel cell to recharge the battery during normal driving. An on-board charger converts 110/220 Vac to recharge the batteries from available grid power when the vehicle is parked.
Using its established Ni-MH battery technology, Toyota has produced Prius PHEV prototypes that can operate on electric power for a range of about seven miles and can re-charge in three to four hours from a 110 V outlet. The Prius PHEV may be offered as a 2009 model.
Unfortunately, production PHEVs are not currently available. However, a plug-in hybrid lithium power adapter kit available from Hymotion, a recent acquisition of A123Systems, allows the conversion of Prius or Ford Escape Hybrid vehicle to a plug-in hybrid within a couple of hours with minimal modification to the vehicle. Installed in the spare tire well of the vehicles, the battery range extender module (BREM) converts a hybrid into a PHEV capable of achieving 100 or more miles per gallon.
Figure 7 shows the BREM and its internal components including the power electronics, the ac-dc converter. For the Toyota Prius, the BREM is a 5 kW/hr unit that uses a charge voltage of either 120 Vac or 240 Vac with a resulting charge time of 5.5 or 4.0 hours. A BREM for the Ford Escape is a 12 kW/hr unit that has a charge time of 12 or 6 hours from a charge voltage of 120 Vac or 240 Vac.
Since only 120 (110) or 240 (220) Vac is currently being proposed for plug-in hybrids, one of the controversies from the earlier EV charging is resolved, whether to use isolated or direct charging. In his presentation at the SAE Hybrid Vehicle Technologies 2007 Symposium, Mark Duvall, manager of technology development for electric transportation at Electric Power Research Institute (EPRI) strongly encouraged the use of strictly 120 Vac in the United States due to cost and availability. The charging time for obtaining 20% state of charge ranged from a low of four hours for a compact sedan to a high of 8.2 hours for a full-size SUV using a 1.2 kW to 1.4 kW power supply at 120 Vac with 15 A.
CONNECTING VEHICLES TO THE POWER GRID
Payback has always been an issue for electric vehicles and now hybrid electric vehicles. For many drivers, the increase in fuel economy and the environmentally friendly aspect are insufficient incentives to purchase a hybrid. Designing plug-in hybrids to interact with the electrical grid could change the rules.
Vehicle-to-grid or V2G technology allows plug-in hybrid vehicle owners to sell electricity stored in their vehicles back to the power company. When a number of vehicles connected to the grid can source power from nearly fully charged batteries, the power companies gain by using the cumulative storage capacity to address peak power requirements, stabilize the grid, and even store the output from wind and solar generation.
Support for this effort comes from many companies. In 2007, the U.S. House Committee on Science and Technology developed H.R. 3776, the Energy Storage Tech-nology Advancement Act of 2007, which authorizes the Department of Energy's involvement in research, development and demonstration of energy storage systems for stationary and vehicular applications. Recently, the Federal Energy Regulatory Commission (FERC) sponsored a public demonstration of real-time vehicle-to-grid (V2G) technology. In the test, an electric vehicle connected at FERC headquarters received power commands from PJM, the regional grid operator. A communication package developed by Pepco Holdings Inc. (PHI) and the University of Delaware linked the commands to the vehicle. The vehicle, an AC Propulsion eBox EV (a Scion xB conversion) responded to the commands by charging or discharging its battery in short bursts that helped to balance supply and demand of power on the grid.
For V2G to become a reality, a high-power plug is involved. Instead of the 1 to 1.5 kW levels being proposed for plug-in hybrids, a 20 kW battery charger could be required. With the higher cost of this type of unit, from $2,000 to as much as $6,000 per year per vehicle could be paid to the vehicle's owner. This cash back aspect could offset the higher cost of the vehicle in less than five years.
Perhaps one of the more surprising supporters of V2G technology is Google. Through its Google.org philanthropic wing, Google supports climate change efforts including V2G technology. Google has several 120 Vac and a few 240 Vac charging stations for a small fleet of Prius and Ford Escape Hybrid vehicles that have been converted to PHEVs using Hymotion's system. The fleet is available for all Google employees to use.
“The cars as they are now are not actively vehicle to grid and we are not doing any demand side regulation,” said Alec Proudfoot, a mechanical engineering manager at Google who volunteers for Google.org. However, demand side regulation is the goal. With millions of vehicles connected to the grid, even those with small batteries and low charge rates can make a difference. Proudfoot noted, “I think where they are first going to make a difference is on the demand side of the equation, where there is a signal that goes between an iso or the utilities and the vehicle that says it is OK to charge now or don't charge now as a way of helping to regulate the load.”
ABOUT THE AUTHOR
Randy Frank is president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, AZ. He is an SAE and IEEE Fellow and has been involved in automotive electronics for more than 25 years. He can be reached at [email protected].