“Micro hybrids with stop-go functionality are going to become very big in Europe,” predicts Benjamin Jackson, product manager, Automotive Products Business Unit at International Rectifier (IR). “Some of our customers are putting huge forecasts out for up to 50% of all the new cars in Europe for the next few years to be micro hybrids,” he says.

An integrated starter alternator in a micro hybrid provides stop-go functionality. This allows shutting the engine off at idle to reduce fuel consumption, yet restarting within an undetectable timeframe for drivers. A short restart is just one system consideration. “When it re-starts, the entertainment system mustn’t cut out,” says Jackson. Other loads, such as air conditioning, need to stay on as well so a micro hybrid has to change from belt driven to electrically driven loads requiring a DC-DC converter. As a result, DC-DC conversion and other switching power usage such as Class D audio amplifiers are now becoming quite prevalent on vehicles as carmakers put more value on reducing weight, size and energy consumption.

Converting between different battery voltages is an issue in all hybrids. Stepping 300 V, 600 V or 900 V down to 14 V for cabin or so called ”hotel loads” also requires DC-DC converters. Product ratings for MOSFETs and insulated gate bipolar transistors (IGBTs) used in higher voltage systems can range from 300 V to 1200 V. The voltage specification for MOSFETs controlling 14-V loads also varies considerably depending on the application according to Jackson, but 40 V is an important and common rating.

Higher Efficiency Alternators

Another micro hybrid function is regenerative braking that involves operating the alternator at high output power levels to charge the battery during deceleration. In addition to micro hybrids, alternators in non-hybrid vehicles need more power output as well. With increasing electronic content, carmakers are limited by how much power they can provide from the battery or the alternator. At the North American International Auto Show (NAIAS) 2010, DENSO announced efforts to more effectively manage energy consumed in vehicles that included ongoing development to improve the efficiency of alternators.

To improve the alternator’s efficiency, synchronous rectification techniques used in higher-voltage hybrid starter-generators can also be applied to 14-V alternators. “We are now seeing people seriously looking at synchronous rectification,” says Jackson. “While a traditional alternator is maybe 50% to 60% efficient, with synchronous (rectification) you can be getting up in, I guess, the 80% range.” For a given amount of input power, the higher efficiency can easily mean 50 A or more of additional output power in a high-output alternator.

Packaging Power Semiconductor Technology

To help automotive suppliers more efficiently power the loads in micro hybrids, synchronous rectifier alternator designs and other systems, IR has improved the design and increased the size of its DirectFET package (Fig. 3). This surface mount technology (SMT) design has been used extensively in high-performance computer power supplies, but now meets automotive AEC-Q101 reliability requirements and has a Moisture Sensitivity Level rating of 1 (MSL1).

Basic design characteristics of the package that have not changed include lowest die-free package resistance, dual-sided cooling, low package inductance and simple package construction with no leadframe, no wire bonds and no molding. “We made changes to the passivation layer to improve its robustness and integrity,” says Jackson. Also, some of the materials used in the die attach were changed so the package is 100% lead free. In addition, the original maximum junction temperature of 150°C has been increased to 175°C.

With the MSL 1 rating, the automotive qualified DirectFET can address applications such as transmission-mounted controls. For that type of application, power density is obviously important, as well as ruggedness and reliability. “Power density is a term that five years ago was only used for servers and enterprise power,” says Jackson.  “Now we visit automotive customers and they regularly use this term, power density and efficiency.”

Figure 3. The same internal construction is used in all three of the automotive-qualified DirectFET packages. Select figure to enlarge.

The first two automotive products are the AUIRF7739L2 and AUIRF7665S2. The former is a 40-V, large can design with 700 µΩ typical (1000 µΩ max) RDS(on) and a maximum drain current rating of 270 A. “This is the lowest RDS(on) MOSFET in the world today at 40 V,” proclaims Jackson. Gate charge is about 220 nC. The large can is rated at 375 A maximum. Motor control, for example in electric power steering, DC-DC conversion, and battery switches in micro hybrids, are ideal applications for this device.

The AUIRF7665S2 is a 100-V, small can device with 51 mΩ typical (62 mΩ max) and 8.3 nC typical gate charge. This product targets the switching requirements in Class D audio, achieving 100 W/channel into 8 Ω without requiring a heatsink, as well as DC-DC conversion and fuel injection.