A Power Shortage Is Driving Automotive Applications To 42 V

Power MOSFETs
Any power semiconductors that operate off the 42-V supply voltage must be rated to withstand its worst-case overvoltage. According to the proposed 42-V PowerNet specification, the maximum transient overvoltage allowed on the bus is 58 V. In theory, a 60-V transistor could do the job, but vendors are generally developing more conservatively rated MOSFETs for this application. Typically the drain-to-source breakdown voltage rating is 75 V.

The higher rating affords a greater margin of safety for designers who must consider reliability as a critical concern. After all, as Clemente observes, the automotive industry is "risk aversive." Proof of this is the long qualification time for a new piece of electronics hardware—possibly two years or more. On the other hand, reducing the breakdown voltage rating lowers on-resistance for a given die size.

Vishay Siliconix is one of the semiconductor vendors developing components for the ISA power module application. Klaus Pietrczak, senior manager of automotive marketing, believes that over time, the 75-V rating will come down to about 65 V. That change could either lead to parts with lower R_DS(ON) or to smaller, cheaper dies. (Unfortunately, though, as the die gets smaller, heatsinking becomes more difficult.) These gains will be coupled with the usual evolutionary improvements in cell density.

But for now, 75-V MOSFETs are the norm in the ISA application. For example, Vishay-Siliconix has developed a 75-V n-channel enhancement-mode MOSFET in 6- by 6-mm die form. Maximum R_DS(ON) for this die (part number SUC85N08-04) in a TO-247 package is 4 mΩ at 25°C and V_GS = 10 V. This part is fabricated in a trench-cell process which, according to the company, affords a much greater cell density than a conventional planar process. Moreover, the trench process is far from its limits in terms of achievable cell density. Much denser devices will be coming out of this technology, Pietrczak claims.

Other vendors, though, have taken the planar route in MOSFET development. International Rectifier chose a striped planar design to produce its 75-V MOSFET (part number IRFC2907). The size 6 die features an R_DS(ON) of 2.7-mΩ maximum at 25°C and V_GS = 10 V. In the TO-247 version, R_DS(ON) is 4.5 mW max. Gordon Gray, technical marketing manager for discrete components, says that IR decided to produce this transistor in a striped planar process despite the ability that the trench process has to provide lower R_DS(ON).

Gray notes that at present, striped planar is a more rugged process in regards to avalanche voltage. One of the benefits of the IRFC2907 is that it guarantees a repetitive avalanche rating up to T_J maximum. In the future, however, the company will likely transition 75-V parts to the trench process as its performance on avalanche rating is improved.

In producing its 75-V MOSFET for ISA, Infineon Technologies also went with a planar process on account of its ruggedness. There are no immediate plans to develop similar parts using a trench process. The company is currently developing a 75-V die produced in its OptiMOS planar process. The part's R_DS(ON) is 2.8 mΩ at 25°C and V_GS = 10 V. This product also will be offered in a D²PAK with 7.5-mΩ R_DS(ON). Both parts are due to be released to production by the end of this calendar year.

Although the ISA is a primary application of 75-V MOS technology, the 42-V electrical system will afford many other opportunities for the development of power components. Infineon, for example, indicates that it's working on a triple-output power-supply chip that will convert the 42-V supply to 5, 3.3, and 2.5 V. At first the IC will step down 42 V to 7 V, and then use LDOs to generate the three clean outputs. In addition, the company has a number of 75-V high- and low-side protected switches in production. These can be driven directly by logic level inputs from a microcontroller for controlling electronic actuators.

The 75-V MOSFET technology developed for the ISA and other 42-V applications will be critical in making 42-V system power a reality. As the electronic content of the vehicle rises, these semiconductors will play a greater role in determining both the performance and price of the car. To start, this will be for luxury vehicles only, but eventually will extend to cars with more modest price tags. Down the road, these components will be at the forefront of efforts to develop even more advanced automotive technologies based on hybrid or fully electric designs.

References:

1. "Automotive Electronics Power Up," John G. Kassakian, et al. IEEE Spectrum, May 2000.
2. "Road Vehicles—Environmental Conditions for Electrical and Electronic Equipment for a 42-V PowerNet—Part 2: Electrical Loads." AG "Normung," Forum Bordnetzarchitektur, Stand. 31.01.2000. Preliminary, still in preparation. See www.sican.de.

For further information, check the MIT Consortium's web site at www.auto.mit.edu/consortium.