Time and again, it has been said that the automotive world presents electrical engineers with some of the most daunting design challenges. To begin with, automotive electronic as-semblies must be built to last. Members of the industry speak of designing components to last for 10 years and 150,000 miles, and in some very harsh environments. When placed under the hood, equipment might be faced with ambient temperatures reaching widely varying extremes—from as low as −55°C to as high as 150°C (Fig. 1). Conditions are even worse for devices mounted to the wheel. These components may encounter temperatures of up to 250°C.
Aside from the rough thermal environment, electronics hardware must withstand mechanical shock, vibration, humidity, and exposure to contaminating dirt, salt, chemicals, gases, and radiation.1 Meanwhile, sensitive semiconductors require protection against electrical threats from ESD, EMI, and fault conditions.
Despite these imposing requirements, automotive applications are extremely cost sensitive. As a result, every component and subassembly has to lend itself to high-volume, low-cost production. Nevertheless, there's an ongoing growth in the amount of electronic equipment being designed into new vehicles.
As the list of electronic functions grows, the amount of semiconductor content increases and with it, so does the need for passive components. Among their various functions, these components provide energy storage, filtering, transient and EMI protection, and sense and control functions in a variety of vehicle applications. Although space restrictions usually aren't as severe as in the handheld consumer applications (where passives proliferate), there's still a demand to minimize the pc-board real estate or volume occupied by resistors, capacitors, inductors, filters, and protection components.2
Capacitors For Power Electronics
Hybrid powertrains typically require power inverter stages to drive the electric motor used for propulsion. In existing designs, much of the bulk of the inverter comes from its large electrolytic capacitors that supposedly account for approximately 40% of the inverter's volume (Fig. 2).3 The capacitors now available are said to also be too costly for the inverter application and have limitations with respect to operating life, reliability, and temperature range.
As part of its efforts in the Partnership for a New Generation of Vehicles (PNGV) program, the U.S. government is sponsoring research to develop better capacitors based on new materials. Three Department of Energy (DOE) labs are presently conducting capacitor research projects. According to David Hamilton of DOE, one of the main objectives in these projects is to meet reliability goals. "In cars, we want the capacitors to last 150,000 miles or the life of the car, within an environment of −40°C to 95°C with low-frequency vibration modes in excess of 3G and shock as high as 10G," Hamilton says.
Hamilton explains that automotive power modules require several capacitor types, depending on where they're used in the application. These include a low-frequency or energy capacitor of about 600 µF; lower-value, middle-frequency capacitors distributed on the bus; and high-frequency components that must be placed closest to the switching devices.
At Sandia National Laboratories, scientists are working to develop replacements for aluminum electrolytics that now serve as the dc bus capacitors. Using high-temperature polymer dielectric film technology, the lab is working to create capacitors with better dielectric properties than aluminum electrolytics. The polymer film capacitors should have a similar or smaller size, yet have longer operating life as demanded by the application.
Meanwhile, at Argonne National Laboratories (ANL) and Pennsylvania State University, researchers are jointly working on a low-cost multilayer ceramic technology that could replace those Coke-can-size electrolytics. Balu Balachandran, manager of the ceramics department at ANL, indicates that the company's research focuses on developing a ceramic material based on barium strondium titanate.
Using the new ceramic material, researchers have achieved an energy density of 20 joules/cm3 in a thin-film device. This work, though, hasn't yet reached the stage of building actual capacitor prototypes, which is perhaps still a couple of years away, Balachandran says. Ultimately, this thin-film technology is expected to produce capacitors that are half the size (or less) of existing aluminum electrolytics.
Another project, being carried out at Lawrence Livermore National Laboratory (LLNL) in conjunction with the Office of Naval Research, is the development of a multilayer ceramic capacitor to take the place of electrolytic snubber capacitors. These capacitors should exhibit high breakdown strength, low loss, and low temperature dependence. Troy Barbee of LLNL says that so far, they were able to create a 130-nF, 600-V capacitor that measures just 0.5 mm high by 3 cm long by 1.5 cm wide.
Almost all of this unpackaged component's size is substrate, which should be reduced further. Barbee says that in the end, they expect to shrink their snubber capacitor down to 0.25 mm by 1.5 cm by 1.5 cm. Additionally, 600 V is merely the part's operating voltage. It is actually rated to survive up to 900 V.
As for temperature performance, the capacitor endured testing (in an inert atmosphere) over a temperature range of −196°C to 250°C. Despite the extreme variations in the range, the part's performance didn't degrade over temperature. The lab is seeking a partner to develop this technology commercially. Barbee expects that the first commercial components might be out in beta sites in about two years.