To meet increasing electrical power demands, automakers are moving toincrease vehicle battery voltage from today's 14V to approximately 42V.It's been more than 40 years since car makers switched from thestandard 6V system – a change triggered by similar powerconsiderations.
Since then, vehicle electrical power consumption has increased bymore than 50 percent. Next-generation cars will have even moreelectronics and require a power source with an output of more than 3kW,the limit of today's 14V system. A 42V system will deliver around 8kWand allow better management of the higher power requirements.
A 42V system sets the stage for advanced technologies that will allowa switch from mechanical belt-driven systems to electrically-poweredones. Possibilities
include electric power steering, electromechanical brakes, electricalHVAC systems, electromagnetic valve trains, integratedstarter-generators and electronic ride control systems.
The so-called “beltless engine” of the future will be another reasonfor lower weight packaging (because accessories can be located outsidethe engine compartment), leading to higher efficiency that improves gasmileage and reduces emissions.
Before 42V systems can be adopted widely, many engineering problemsmust be addressed, including the engine/electrical system architectureand a migration strategy (dual 14/42V systems vs. straight 42Vsystems). Short-term challenges associated with dual voltage systemsinclude more wiring, extra weight and added complexity.
Regardless of migration path, suppliers need time to develop newcomponents and a part identification system that distinguishes between14V and 42V parts.
|Figure1. The test system includes voltage and current sources integrated withmeasuring instruments and a switching matrix.|
Electrical and electroniccomponents evaluation
While 42V is not far from 14V in physical terms, real-world issues area cause for concern. Current 14V designs won't automatically work at42V; even simple fuses will not migrate, let alone dimmers and activeload controllers. Some fuse panel and harness makers have found thatcommon 14V mini- and maxi-fuses do not behave properly at 42V.
They can fail to interrupt excessive currents properly, causingserious overload conditions. Also, interconnection technologies haveevolved for optimal cost and performance in a 14V environment. Thepresent design of connectors, circuit breakers and relay contacts maynot be optimal at 42V.
Therefore, manufacturers must re-evaluate component suitability forthe higher voltage. Tests can range from simple continuity tests tofull electrical characterization of a component's functionalperformance at 42V.
At 42V and higher power levels, many components, such as wires andrelays, experience electrical stress three times higher than before.With higher stress, components tend to break down more often.Therefore, component and module manufacturers have to perform morereliability testing, such as burn-in and accelerated stress tests, toensure adequate service life.
Safe distribution of 42V power throughout a heavily optioned automobileis also a challenge. In the first place, the 42V standard wasestablished because higher voltages create human safety issues. Forexample, 50V can stop a human heart, and anything higher than 60Vrequires more heavily insulated wires and connectors, which add weight.To prevent fires, electrical distribution designs must allow forjump-starting at the higher voltage and provide protection if batteryconnections are reversed.
Component, conductor arcing problems
Relay, switch and conductor arcing is another problem that must beaddressed; its potential for serious damage is greatly increased in 42Vsystems. Recent research shows that 42V arc energy is 50 to 100 timeshigher than in a 14V system.
Such arcing can generate temperatures of up to 982.22°C, ignitefuel vapors, start a fire in plastic insulation, and even melt metal.Simply redesigning relays, switches and fuses for higher voltage, andusing flame-retardant materials is not a total solution—these componentdesigns should suppress arcs.
The same is true for other connections, particularly those thatcould be opened during replacement of fuses, batteries and othercomponents. Mechanical design features must ensure that electricalterminals are correctly seated and locked. Therefore, increased use ofclips, clamps and shields may be required.
The impact of 42 volt systems
Implementation of 42V systems will affect the design, manufacturing,assembly and testing of most electrical and electronic components.Electromechanical components such as alternators, motors and startersmay require more time on field coil-winding machines to get the samenumber of ampere-turns (given that the current and wire gauge will beone-third of what it was for 14V devices).
Other components will be redesigned or replaced. In many cases,suppliers will be asked to make them lighter, more efficient and lessexpensive. This probably means that semiconductors will replaceelectromechanical designs in some switch and relay applications. Thiswill call for higher power devices, such as trench MOSFETs in highervoltage packages.
While basic designs of existing assembly and test equipment shouldbe adequate for 42V components, the higher voltage will require somemodifications. For instance, additional production testing may berequired to verify arc suppression and EMI/EMC compliance.
To design the new 42V components properly, car makers and theirsuppliers must understand critical engineering and performance issues.As a result, there will be increased R&D activity involving theelectrical characterization of devices and their designs.
Typically, this entails electrical measurements under various loadconditions, insulation resistance and hi-pot testing, and very lowresistance measurement of relay contacts and connector terminals.
Simplifying, speeding upmeasurements
Many 42V tests will require only common instruments, such as loadbanks, high current power supplies and DMMs. More specialized tests ofconductors and insulators require instruments designed specifically forthe measurement extremes involved in low-resistance, high-resistanceand low-current testing.
Complex devices, such as DC/DC converters, inverters, airbag ignitersystems and other electronic controllers require more extensive testingand multifaceted test systems.
Many of these devices contain large numbers of conductor pathways,have many sensor inputs (e.g. temperature, vibration and humidity) andrequire multiple measurements. Thus, signal switching systems are avaluable test tool. Matrix switches support fully automated testing,reduce the number of instruments required, simplify test procedures andreduce test time.
In such a test system, the measuring instruments, signal switchingand other critical components should be selected for ease ofintegration and optimum overall performance. Better still, the use of afully integrated data-logging and switch system eliminates the need tointegrate many of the test system components.
Application specific measurements
Many automotive electrical tests are essentially resistancemeasurements to verify continuity or low leakage currents during hi-pottesting. Nevertheless, production testing may dictate multiplemeasurements in a specific sequence to check for proper assembly andwiring, which creates complexity in simple resistance measurements.
For instance, the electrical check on a vehicle's primary airbaginflator verifies proper characteristics in the pyrotechnic initiator,a fusible wire with a typical resistance of about 2-3 ohms. A secondtest checks the safety shorting clip to verify that initiator pins areshorted together, a safety feature that prevents accidental activationduring airbag handling and installation.
The shorting clip is removed after installation is complete. A thirdtest is a high voltage isolation resistance measurement to ensure thatno electrical leakage path (i.e. low resistance) is between theinitiator and the grounded metal housing of the system's electronicmodule, which otherwise could cause a “no-fire” condition. Somemanufacturers perform additional electrical tests on their airbagmodules and wiring harnesses using the same test stand.
Often, developing such a system from individual instruments andswitching components, and then implementing it on the production floorcan be quite costly. When available, an application-specific testsystem can save the user time and money by providing tightly integratedcomponents in a single ready-to-run unit.
Figure 1 above illustratesan airbag inflator hi-pot electrical test circuit using such a system.The test system includes voltage and current sources integrated withmeasuring instruments and a switching matrix.
The need for Ethernet-based testsolutions
The switch to 42V systems will be on a “fast track,” so test datasharing across the enterprise will be important. Today, that oftenmeans feeding data to multiple departments across an Ethernet bus (Figure 2, below ). HavingEthernet-ready instruments with tightly integrated measurement andswitching functions greatly simplifies this task.
|Figure2. Burn-in chambers may be located in either R&D or productiondepartments and may include vibration.|
An additional benefit of Ethernet-based measurement solutions isthat test engineers don't have to trade measurement accuracy forconvenience and cost-effective data collection. While PC plug-in cardsprovide low cost, measurement quality is usually much lower than thatavailable with benchtop instruments.
When the benchtop instrument also has an Ethernet-ready interface,test engineers get the best of both worlds. This becomes increasinglyimportant in a production environment with many test stations ormultipoint sensors. In such cases, it's often more cost-effective touse an Ethernet-based instrument than to install multiple PC-basedplug-in card systems.
Once the transition to a 42V power architecture is completed, wiregauges will be reduced, cable bundles will shrink, smaller connectorscan be used and wiring weight will drop. Cable and labor costs will bereduced due to simpler installation.
Full benefits of the new architecture will include increasedelectrical power for cellphones, GPS units and audio systems, reducedsize and mass of motors and other accessories, more flexible andlighter weight packaging, more efficient operation (improved fueleconomy and lower emissions), the potential for redundant powersources, faster temperature change in the HVAC system and longerservice life for many components and assemblies.
Qi Wang has served as leadapplications engineer and currently is involved in product marketing atKeithley Instruments. Dr. Wanghas more than 15 years experience in semiconductors, optical physics,and DC and RF measurements. He received a Ph.D. in physics from TexasA&M University and a B.S. in physics from Beijing NormalUniversity.