Efficient power electronics for hybrids and EVs

  • 04-Jun-2014 04:33 EDT
tmc_sic_pcu.jpg

A production version of a silicon power control unit (PCU) is compared to the target package for a future PCU based on high-performance silicon carbide (SiC) semiconductors.

Although the electrification of road transportation has improved overall energy efficiency, the ledger for hybrid and electric vehicles has always had a significant entry in the debit column that marks the large energy losses from the silicon-based power electronics.

Take a hybrid car’s power control unit (PCU), which manages the flow of battery power to the motor for speed control and sends recovered braking energy to the battery for storage. A goodly portion of the strong currents and high voltages that pass through the silicon power transistors and diodes dissipates as heat, accounting for about a quarter of the vehicle’s total electrical power loss. That’s around a fifth of the loss that is associated with the power semiconductors alone.

PCUs and other power devices suffer two kinds of operational losses; some occur during conduction, but most waste occurs when the current switches on and off.

Ever since Toyota engineers introduced the first Prius hybrid in the late 1990s, they have sought more efficient power electronics that they hoped would be based on new, low-loss semiconductors, but these have taken decades to materialize.

Silicon to silicon carbide

Recently, however, the automaker said that it is starting to convert the power-hungry silicon-based IC chips in its PCUs to more efficient devices made from silicon’s cousin, silicon carbide (SiC). These next-generation electronics run at higher temperatures, voltages, and switching frequencies, which can mean fewer energy losses, better performance, and greater efficiency.

Power electronics devices using the SiC semiconductors could raise the fuel efficiency of hybrid vehicles by as much as 10%, Toyota claims.

Integrated circuits that are fabricated on SiC substrates lose one-tenth less electrical power than their conventional silicon counterparts and can run at 10 times the drive frequency. The technology would also enable reductions of the size of future PCUs by as much as four-fifths. (For more information, see this video: http://www.youtube.com/watch?v=9FGSOK5l6s0.)

SiC power electronics could find use in the battery chargers on plug-in hybrids and electric vehicles, drivetrain systems, as well as inverters and dc-to-dc voltage conversion systems.

Since the early 1980s, Toyota Central R&D Labs has collaborated on SiC semiconductors with Denso Corp., the Toyota Group’s biggest parts supplier. In recent years, Denso has demonstrated production of 4-in (10.2-cm), then 6-in-diameter (15.2-cm-diameter) wafers of high-quality crystalline silicon carbide, one of the hardest materials in nature. The costly wafers are difficult to process because of the ceramic’s high hardness and temperature resistance, said company documents.

SiC power semiconductors lose little power when switching on and off, which supports efficient current flow even at higher frequencies. This capability will allow engineers to miniaturize the coil and capacitor, which accounts for approximately 40% of the size of a current PCU.

Denso specialists are now perfecting the crystallization process and working to establish stable, high-yield processing to mass-produce even larger, high-quality SiC wafers.

Efficient power chips

The new semiconductor power chips unveiled recently at the 2014 Automotive Engineering Exposition in Yokohama were a 5-mm (0.2-in) square SiC transistor and 6-mm (0.24-in) square SiC diode. From silicon power chip designs, the ICs borrowed a trench chip structure with a buried vertical gate electrode.

Toyota reported that it has already achieved a 5% improvement in fuel efficiency in test vehicles (JC08 test cycle) using the new technology, and it expects to begin test-driving prototypes in vehicles on public roads in Japan within a year. In December of last year, the company completed a clean room at its Hirose Plant, an R&D facility in Japan that is dedicated to development of SiC semiconductors.

Company representatives said it is aiming to implement SiC in hybrids by lowering costs to the same level as the current silicon semiconductors through mass production. The aim is to commercialize the technology by 2020.

Wide bandgap semiconductors

Silicon semiconductors’ use in power electronic devices is coming under pressure from new wide bandgap (WBG) semiconductors such as SiC and speedy but costly gallium arsenide.

The term “bandgap” refers to the amount of energy required to make electrons jump off their atoms and begin conducting electricity through a material. Conductors such as copper often have no bandgap, which makes them good conductors. Silicon-based semiconductors feature narrow bandgaps, and most insulating materials have very wide bandgaps. A semiconductor with a wider bandgap enable electronics devices to run hotter at higher voltages and switching frequencies.

Up to now, SiC commercialization has been hampered by the need to remove crystal defects from the extremely hard, temperature-resistant ceramic. Apart from crystal quality, problems with the interface between the SiC and silicon dioxide have slowed the development of SiC-based power MOSFETs (metal oxide semiconductor field-effect transistors), though nitriding treatments seem to have helped. The new SiC chips are said to be robust and reliable.

Growing supplier network

The biggest obstacle to adoption is high costs. SiC MOSFETs cost from 10 to 15 times more than silicon MOSFETs, according to an analysis conducted in 2013 by IMS Research (part of IHS), a market research firm based in the U.K. Beyond the processing problems, the report also cited the industry’s reluctance to change because the conversion to SiC is not a drop-in swap for silicon. Nevertheless, the IMS study predicted that sales of SiC PCUs in hybrids and EVs would start picking up by 2016 and would constitute a $200-million market by 2022.

Other SiC device makers and developers include Cree, Danfoss, Fairchild, Fuji Electric, GeneSiC Semiconductor, Global Power Device Co., Infineon, Microsemi, Mitsubishi Electric, Rohm Semiconductor, Renesas, Semikron, ST, United Silicon Carbide Inc., and Vincotech.

The problem of inefficient power electronics extends way beyond the automotive industry; the U.S. Department of Energy projects that power electronics will consume a staggering 80% of all electrical energy by 2030.

In January, a DOE-supported research center devoted to WBG technology, the Next Generation Power Electronics National Manufacturing Innovation Institute, was established at North Carolina State University. The institute, funded by a consortium of businesses and universities, has a mandate to develop “the next generation of energy-efficient, high-power electronic chips and devices by making wide bandgap semiconductor technologies cost-competitive with current silicon-based power electronics in the next five years.”

Meanwhile, the Japanese government also considers advanced SiC power semiconductors to be a high priority; it currently funds a joint industry and university R&D program that includes Toyota, Honda, and Nissan.

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