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.
Fighting ESD And EMI
With the number of automotive electronic functions rising steadily, the need for transient suppression and EMI control becomes increasingly important, particularly when many of the functions reside on a network bus such as CAN. Rising data rates on the bus tend to exacerbate the problem, creating a more urgent need for EMI suppression. A variety of methods can be applied to attenuate ESD transients and EMI. These include RC networks, LC networks, back-to-back Zener diodes with an EMI capacitor, and even on-chip transient protection.4
Another approach employs multilayer varistors (MLVs), such as those developed by AVX Corp. The MLV is a ceramic device fabricated from a conductive zinc-oxide (ZnO) material that's doped with bismuth, cobalt, manganese, or some other metal oxide. The device can be modeled as two back-to-back Zeners in parallel with an EMC capacitor.
The techniques employed in fabricating MLVs are similar to those used in multilayer chip capacitors (MLCCs) with some added proprietary methods that are unique to the ZnO materials. The same process capability that allows MLCCs to be produced with very thin dielectrics (less than or equal to 1 mil) also permits MLVs to be made with working voltages of as low as 3.3 V.
While working voltage is a function of ZnO dielectric thickness, the MLV's ability to absorb transient energy is a function of the number of layers of di-electrics and electrodes. The device's capacitance, which results from the combination of electrodes with the ZnO ceramic dielectric, ranges from about 3 pF up to several thousand picofarads. As with chip capacitors, packaging for MLVs is in standard EIA surface-mount cases.
In comparison to silicon transient-voltage suppressors (TVSs), MLVs are claimed to have somewhat faster response times due to the inductance introduced by the silicon TVS's lead frame and wire bonds. Moreover, MLVs will tolerate a greater number of transient strikes and handle more inrush current than the diode TVS. Plus, in the off-state, the MLV's capacitor can provide as much as 40 dB of EMI attenuation across a frequency spectrum of several hundred megahertz.
Housed in packages as small as the 0612 standard, AVX's TransFeed is a four-terminal version of the technology that improves upon the MLV's turnon time, broadband EMI attenuation, and size, versus MLV and filter combinations (Fig. 3a). In the TransFeed version of the MLV, some of the device's parallel inductance has been converted to series inductance. This effectively lowers the injected transient peak current and the clamping voltage, while also reducing turnon time down below 250 ps.
For surface-mount components such as the MLV, there are extensive opportunities in developing automotive designs because as Ron Demcko of AVX says, "You need a varistor on just about everything on the CAN bus," as well as on every airbag. Newer cars tend to include more of these safety devices.
Another interesting aspect of the MLV technology is that it creates an attractive path to passive integration. For example, AVX has just announced a four-element varistor housed in 0508 and 0612 packages that effectively combines four Zener pairs, four EMC capacitors, four inductors, and four resistors. It's fabricated by depositing thick-film nickel-barrium terminations on the MLV's ZnO body.
In addition to MLVs, other space-reducing alternatives are being called on to perform EMI suppression. One of them is the capacitive technology pioneered by X2Y Attenuators LLC.5 Its multilayer capacitor structure, which was introduced last year, achieves very low values of ESR and ESL, so that it's possible to implement single-component broadband EMI filtering for dc motors.
Furthermore, given that as many as 100 motors are currently in a typical luxury vehicle, the need for simple low-cost filters is already significant and should only increase in the future. Aside from just motors, many other applications for EMI filtering exist throughout the vehicle. Hall Effect, ABS, and other sensor applications, which typically have power, ground, and signal lines coming off the sensor, also require broadband filtering.
As an example of the technology's promise, consider test results obtained when a single-component X2Y filter was used to suppress the EMI from a 12-V dc motor (Fig. 3b). The 1410-size chip attenuated the motor's radiated emissions over the 150-kHz to 1000-MHz frequency range by 25 to 50 dB, pushing EMI down almost into the noise.6 In doing so, the single-component filter outperformed more traditional filters that were built with up to seven components.
Such technology also is attractive in automotive applications because its failure mechanisms meet automotive requirements for reliability. If bombarded with excessive RF, the device may go open circuit. "More than likely, this is the only failure," claims Tony Anthony of X2Y. According to X2Y, the motor filter chip can handle 1.5 to 2 A of RF with no limit on dc current because the device operates in bypass. In addition, the device's performance doesn't degrade over the typical automotive temperature range.
Initially, this technology was only produced in a ceramic dielectric with breakdown voltages of up to about 100 V. But development of X2Y designs in other dielectrics like film, which boosts breakdown voltage ratings of up to 600 V (and with still lower ESR), could expand the technology into the area of bulk capacitance.
That raises the possibility that X2Y-type capacitors could replace electrolytic and ceramic styles. Such a development could be applicable to automotive designs, as would the use of MOV dielectric materials to build single-component solutions for transient suppression and broadband filtering (as with the MLVs from AVX).
Beyond the filtering and transient-suppression applications discussed here, numerous other opportunities exist for passive components in future automotive designs. Passives might not be the key enabling technologies in most cases, but they will undoubtedly affect the performance, reliability, and cost of many new electronic features.
- "Ceramic Solutions For Automotive," DuPont Microcircuit Materials, www.dupont.com/mcm/cerautomotive.html.
- For a look ahead at the rising electronic content planned for future automobiles, see diagrams of General Motors' Precept, a PNGV hybrid-electric concept vehicle. The Precept contains complex networking structures to support a host of electromechanical devices. Further information is available in "Low Power Flexible Controls Architecture for General Motors Partnership for a New Generation (PNGV) Precept Vehicle," by Joe LoGrasso and Kevin Kidston, General Motors Corp. and Walton Fehr, Motorola Inc., Convergence 2000 proceedings, paper number 2000-01-C060, p. 395-409.
- "Power Electronics And Electric Machinery Innovations-U.S. Government's Role in PNGV," by Donald J. Adams, Oak Ridge National Laboratory, Convergence 2000 proceedings, paper number 2000-01-C063, p. 427-435.
- "Multilayer Varistors In Automobile Mux Bus Applications," by Ron Demcko, AVX Advanced Products Group, SAE International Congress and Exposition, Feb. 23-26, 1998, paper number 981106.
- "Capacitive Technology Performs Filtering And Decoupling With Fewer Parts," Electronic Design, Feb. 7, 2000, p. 25-26.
- "Broadband Testing Of Low Cost Filter Solutions For DC Motors," by James P. Muccioli, Anthony A. Anthony, and others; Interference Technology Engineer's Master (ITEM) Update 2000.