Automotive Power Requirements Span From “A Little” To “A Lot”
Adding more electronics to vehicles will create much-desired differentiation, but it also ups the power requirements. As a result, every system’s power consumption—during any and all operating conditions—goes under the microscope. This is especially true in electrically propelled powertrains, where wasted power means less range.
However, even in internal combustion engine vehicles, greater electrical power consumption means reduced fuel economy. Also, excessive power consumption in the key-off mode can deplete the battery and prevent the vehicle from starting after prolonged parking.
A Little: The Low Leakage Requirement
Vehicles with the highest amount of electrical content inevitably have the highest amount of leakage current. Not surprisingly, reducing standby current and quiescent current is a big trend in Europe, according to Joseph Notaro, director of Marketing & Applications, Automotive Business Unit EMEA at STMicroelectronics.
“We hear the carmakers saying that every microamp counts,” says Notaro. “Most ECUs have a 100-µA max standby current allotment.”
As a result, products and/or techniques that reduce quiescent current and standby currents become very important to all carmakers. Some modules, such as those for gateways or with an RF interface, may exceed 100 µA to perform their unique functions. In general, though, that level must be met. Electronic control units (ECUs) such as the body controller, the door modules, and other applications that must operate below the 100 µA maximum require an intelligent voltage regulator or power management device.
One example of such a device is the L99PM62GXP developed by STMicroelectronics. With the VBAT standby mode, the lowest quiescent current drops below 10 µA, down to 6 or 7 µA. In the application, the microcontroller (MCU) isn’t supplied power until the regulator is awakened through contact monitoring or through the physical layer. “So, really, you have the lowest quiescent current possible,” says Notaro.
While it depends on the current allotment to the module, other devices (e.g., analog or power switching device) don’t necessarily require a similar low value. “Most of the loads that need to be driven are on a switched power supply,” says Notaro. Having the actuator and its driver on a switched line avoids leakage current consumption.
The power-management device and the microcontroller draw the most current during a system’s off-state. A cooperative design approach can reduce their combined power consumption. “The PowerPC-based Bolero family goes into cyclic wake-up mode, which interfaces directly with the power-management device, and you can optimize the quiescent current for the whole system by having the two devices working together,” says Notaro. ST’s SPC56xB family of PowerPC MCUs interface with the power-conscious voltage regulators.
Are You Talking To Me?
Communicating with the vehicle is a growing concern for key-off as well as operating modes. “Anything that you need to keep on so that the vehicle will wake up will present some challenges,” says Notaro. With essentially all ECUs connected to the CAN bus in today’s vehicles, the entire CAN network is typically active.
“Any ECU that is on the bus that does not communicate is in the idle mode—a standby mode, but not the really low quiescent current mode,” says Notaro. To reduce power consumption, a consortium of European carmakers and semiconductor suppliers is working to define “partial networking” in its Selective, Wake-up Interoperable Transceiver for CAN High speed, or SWITCH, workgroup.
In existing networks, a message sent on the CAN bus wakes up every node. When all but one determines that the message is not for them, they shut down again. Obviously, this is wasted current consumption.
In contrast, partial networking establishes soft network zones (Fig. 1). In this case, devices identify specific commands that refer to the partial network and provide specific data information to wake up the right devices. The process limits power consumption in both key-off and running modes.
“When you are driving and you don’t need your rearview camera, it could be shut down completely,” says Notaro. Other modules that could be completely turned off in normal driving include door locks, or a trunk lid or trailer tow module without a trailer.
“It’s the same concept they have in cell phones,” says Notaro. “They shut down functions and wake them up when you need them.” In fact, that’s where the idea came from.
In Between: Alternate Technologies
In some cases, departing from the traditional to an alternate approach can provide an effective solution. In a white paper titled “Zero Emission Vehicles,” Valeo engineers explain how light-emitting diodes (LEDs) can significantly reduce traditional lighting loads. For example, a night lighting system that uses conventional bulbs (taking into account a weighted rate of use) consumes an average of 206 W; yet if it all functions use LEDs, consumption falls to just 41 W (Fig. 2).
LED usage in taillights has ramped up dramatically. However, for the highest lighting load—the headlamp—LEDs usually wind up only in premium vehicles. LED manufacturers’ ongoing efforts to improve the efficacy of LEDs, as well as reduce their cost, could add lower power consumption lighting to more vehicles.
LED-driver manufacturers are doing their part to make LED technology more viable for automotive headlights, too. For example, ROHM Semiconductor recently introduced the BD8381EFV-M for driving multiple high-brightness (HB) LEDs in high- and low-beam headlamps as well as daytime running-light (DRL) applications (Fig. 3). The white LED driver can withstand high input voltage (50 V max). It integrates a current-mode, buck-boost, dc-dc controller to achieve stable operation over varying voltage input. The boost mode increases the number of LEDs that can be connected in series.
The driver allows dimming via either a built-in PWM or linear control. It also operates with or without a microcomputer. Operating frequency can be set internally between 100 to 600 kHz, or externally synchronized from the internal oscillator frequency up to 600 kHz. Additional built-in protection functions include undervoltage lock-out (UVLO), overvoltage protection (OVP), thermal shutdown (TSD), overcurrent protection (OCP), and short-circuit protection (SCP) with an LED error status detection function for an OPEN/ SHORT circuit. The circuitry is housed in a HTSSOP-B28 package.
Other automotive applications continue to drive LED technology to reduce power consumption, too. For instance, National Semiconductor recently introduced an LED driver with dynamic headroom control for automotive LCD backlight applications. The LM3492 controller, a member of the PowerWise energy-efficiency family, drives two independently dimmable LED strings. The IC’s dynamic headroom control feature dynamically adjusts LED supply voltage through the boost-converter feedback to the lowest level required for optimal system efficiency.
A Lot: Hybrids, EVs Raise The Power Bar
At the extreme end of the power spectrum, electric vehicles continue to push power and, in some instances, the voltage requirements to new levels. Topologies have yet to be defined, let alone standardized, for plug-in hybrid and hybrid electric vehicles.
“You see many different topology requirements going from a couple of kilowatts to hundreds of kilowatts,” says STMicroelectronics’ Notaro. This encompasses electrical bikes and scooters, small city cars between 4 and 20 kW (the 500 kg market), city midsized cars from 10 to 80 kW, and sports cars above 80 kW. Notaro sees device voltage requirements going from low voltages below 100V, up to a couple of 100, to 1200V.
“The Japanese are driving the higher voltages,” he says. “The highest I’ve been seeing in Europe is in the 420 to 480-V range.
“There is a significant drive right now to get below 100 V, not only for stop-start systems, but also for small electric city vehicles –those below 500 kg,” says Notaro. City vehicles represent a new market pursued by many carmakers.
Batteries are much smaller and lighter when using 5- to 15-kW electric motors. In this voltage range, power MOSFETs perform the switching. To efficiently handle these applications, manufacturers have come up with a variety of products, e.g., ST’s MDmesh V power MOSFETs. The 200- and 250-V devices employ traditional semiconductor packaging: D2PAK, PowerFLAT 5x6, and TO-220.
At higher voltage requirements—IGBT territory—designers prefer bare die or power modules. “You are looking at 200 A per leg for a 600-A module,” says Notaro. ST is developing the STA-1 power module for electrical traction requirements in vehicles.
These applications could have a configurable three-phase full-bridge inverter, rated at 600 V and 200 A, or a half-bridge inverter with the three legs in parallel for 600-V and 600-A capability. The module goes with aluminum ribbon bonding instead of wire bonding to increase the module’s efficiency. ST also uses ribbon bonding in conventional TO-247, D2PAK, TO-220, and other packages.
Changing The Rules
Supplier R&D is in high gear to satisfy carmakers’ high power switching requirements. This includes suppliers seeking to supply non-traditional technology to automotive applications. For example, Vicor uses silicon power devices and advanced design topologies to deliver dc-dc converters with 95% efficiency at 1 kW/in3 power density.
Vicor is perhaps best known for creating the brick packaging terminology (full, half, and quarter brick) and advanced technology, which has led to standard packaging in computer and telecom power. Using an approach called adaptive cell topologies, the company developed a flexible, scalable power-system methodology for high-power automotive applications. According to Keith Nardone, director of Business Development for Vicor Automotive, the technology is currently scalable to 2 kW, and expected to reach 4 kW. Ultimately, it reduces the space and weight of existing modules (Fig. 4).
“With the higher efficiency, the thermal management becomes easy,” says Nardone. “We are at the state of proving the technology to end users. The next step would be getting into the packaging and customization of it.”
Though silicon MOSFETs are the technology of choice in the lower voltage range, and IGBTs likewise in the higher voltage range, the time for silicon carbide (SiC) could be drawing near. “According to famous car makers, implementation of SiC electronics in HEV inverters will improve fuel consumption by +10%,” says Dr. Philippe Roussel, senior manager BU Compound Semiconductors & Power Electronics at Yole Développement. The improvement results from a lighter system, better conversion efficiency, removal of a dedicated water cooling system, and more.
“With a 104 g CO2/km ratio, this will lead to avoiding 200 kg CO2/year of emissions per car,” says Roussel. For example, expected cumulative sales (from 2010 to 2015) of EV/HEV—if equipped with SiC technology—could eliminate 3.5 million tons of CO2 emissions over that period.
In addition, SiC will allow the same water cooling system to be used for both the thermal engine and the power inverter for the electric drive. The advantages become viable if costs are significantly reduced at the system level. Roussel thinks this is achievable in the 2014-2015 timeframe, but it requires the availability of SiC products in 2012 due to the typical two-year qualification time.
“According to the recent device announcements from CREE, SemiSouth, ROHM or TranSiC, we feel comfortable saying that we are on the good tracks,” says Roussel.
In his presentation at the SAE 2011 Hybrid Vehicle Symposium & EV Symposium (Anaheim, Calif.), Jochen Hanebeck, president of Infineon’s Automotive Division, concluded that the SiC JFET is the best future device for high inverter efficiency thanks to its low conduction and switching losses (Fig. 5). Infineon’s 2011 SiC roadmap for its automotive sector reveals wafer size increasing from 4 to 6 inches, and a diode introduction at the end of year.
Some companies are working on SiC and other advanced technologies. For example, STMicroelectronics is pursuing SiC for higher voltages and gallium nitride (GaN) for 100- to 800-V traction motor applications.
Challenges for managing power consumption across all automotive operating modes remain daunting. Still, suppliers keep coming up with products that offer viable solutions to carmakers.