At the Applied Power Electronics Conference in Austin, Steven Schulz, technical specialist for the General Motors Powertrain Division, discussed how the growth in hybrid electric vehicles is creating demand for power electronics. As Schulz explained on Monday in his plenary session talk, “Power Electronics for Electric and Hybrid Vehicles,” the fundamental power electronic technologies are in place for the continuing development of electric and hybrid electric vehicles. However, these technologies need to evolve to achieve lower cost, higher efficiency, and greater reliability, says Schulz. According to Schulz, the power electronics industry can help the automotive industry to achieve these goals by developing automotive-specific components and packaging.
In discussing the significance of power electronics in hybrid vehicles, Schulz presented data to quantify, “the incremental material cost for hybridizing a typical vehicle.” While noting that the battery represents the biggest cost adder, Schulz quickly noted that power electronics is the second greatest cost adder as it “can account for 20% or more of the incremental vehicle costs.”
Schulz points to the traction inverter as being the most important of the power electronic components in the vehicle. This inverter must handle input voltages ranging from 42 Vdc to 700 Vdc and output currents ranging from 100 Arms to 450Arms. However, he also noted the need for a dc-dc converter to stepdown the high battery voltage to 12 V dc or 42 V dc.
Furthermore, he commented that plug-in hybrids and extended-range EVs will require a battery charger, probably rated for 1 kW to 3.3 kW. An auxiliary oil is another requirement “that’s necessary to maintain the transmission oil pressure when the engine’s not spinning,” said Schulz.
The speaker singled out the HEV inverter for particular scrutiny taking the traction inverter used in the Chevy Tahoe to illustrate what factors contribute most to inverter cost and overall system cost. This vehicle “features two 86-kW three-phase inverters within one package. The dc bus range is from 250 V to 390 V and the ac output current is 300 Arms. It also features a dedicated cooling loop for the power electronics with a 75ºC max inlet temperature,” said Schulz.
In a typical HEV inverter, said Schulz, over half of the unit’s overall cost comes from the power modules and the bus caps. Additionally, he described current, voltage and position sensors as “big cost hitters”.
Since power module cost relates back to its semiconductor content, Schulz presented data showing the market forecasts for IGBT silicon out to the year 2012. In assessing the data, Schulz described the demand for this silicon in industrial motor drives and home appliances as “essentially linear over the next couple of years.” Meanwhile, demand for IGBT silicon for the EV and HEV market is forecast to be “exponential” during this period.
“By 2012, the EV/HEV demand [for IGBT silicon] could be almost equal to the industrial drives, said Schulz. “And what I think that means, is that, with such high volumes, the automotive market deserves custom silicon and power packaging.”
Customizing silicon for hybrid inverter applications means tailoring several parameters. “Of course, we’re always asking for lower losses, but we’d also like to operate at higher temperatures as well,” said Schulz. “We’re currently using a dedicated cooling loop, which costs money, but in the future we’d like to look at being able to use…transmission oil or the engine cooling loop. So possibly 200ºC operational facility would be a nice feature.”
Schulz also noted that the 600-V and 1200-V ratings offered by existing industrial IGBTs may not be the best choice for some HEV inverters. As a result, silicon rated for 900 V or another intermediate voltage may be preferable.
Similarly, there are opportunities for optimization of power semiconductor packaging to achieve better thermal performance and reliability. In contrast to the “industrial style power packaging” that’s typical in HEV hardware, Schulz pointed to an example of a “transfer molded package with double-sided cooling” as application-specific packaging.
Bus capacitors represent another component area that can be inproved through the use of thinner films, which will yield higher volumetric capacitance. Schulz explained that the higher capacitance per volume drives down both cost and parasitics. As with the silicon, a higher operating temperature—in this case he suggested 135ºC—would be beneficial in allowing coolants with higher temperatures.
Speaking of the impact of sensors, Schulz said, it was not so much component cost at issue, but their lack of reliability, which in turn creates requirements for diagnostics and more lines of code. “We might consider position sensorless operation,” said Schulz, who noted that the automotive industry has avoided this approach due to its conservative ways, despite its prevalence in industrial drives. “But the cost benefits are significant and we need to seriously consider [sensorless operation] in the future.”
Schulz also touched on areas affecting HEV power electronics such as motors and control software. He also prefaced his discussion on HEV power electronics by discussing projections for growth in the HEV market and the regulatory requirements that are pushing automakers to build hybrids. Schulz noted how the Energy Independence and Security Act of 2007 will make federal corporate average fuel economy (CAFÉ) standards more stringent.“The new standard calls for an approximate 3.3% annual improvement, reaching a goal or final value of 35 MPG combined average, that’s a weighted average between passenger cars and trucks in the year 2020. And that’s a pretty aggressive schedule,” said Schulz. He also noted that attempts by the California Air Resources Board to set tighter emissions on vehicles are another concern for the automakers. Though disallowed recently by the EPA, these emission standards—if ultimately adopted—would impose fuel economy standards even more stringent than the CAFÉ standards.