INDUSTRY POWER ELECTRONICS
Cutting costs
In 2013, Cree took a tremendous stride in increasing the affordability of its MOSFET line up by launching a range of second-generation devices that roughly halved the cost-per-amp. By reducing resistances, such as trimming the specific on- resistance from 8 mΩ cm-2
to about 5 mΩ cm-2 , engineers were
able to maintain current ratings while shrinking die size. In turn, this led to an increase in yield, further trimming production costs.
Circuit designers are embracing the new products. “Gen II is now the majority of MOSFET sales,” says Palmour. “We’ve had a very good adoption rate there.”
One reason for this is that these second-generation devices can lead to costs savings. “Even though the component cost is higher than silicon, it saves money at the system level,” says Palmour.
A Cree fabrication engineer removes a 100 mm SiC wafer from a spin rinse dryer, which is a fabrication tool used to clean the SiC wafer as part of the fabrication process used to make SiC MOSFETs.
This is the case in solar inverters, a market where Cree is seeing a lot of activity. The company’s 1200 V MOSFET is now being deployed in Delta Energy Systems’ 11 kW PV inverter.
Expanding Cree’s portfolio
One move that Cree has made to increase the competiveness of its MOSFETs is to broaden its portfolio, by offering different products with different current ratings. This approach, which is one that makers of silicon IGBTs and MOSFETs have taken for many years, means that customers don’t have to buy a bigger, more expensive chip when a smaller one with a lower current rating will suffice.
“There is no point in making [customers] pay more – that only hurts both of us,” argues Palmour. He points out that if prices are too high, customers will not buy these parts, and that is detrimental to the adoption of the company’s SiC MOSFETs.
Expansion of the Cree portfolio has not included the launch of a 600 V SiC MOSFET to complement its 600 V SiC Schottky barrier diode. The reason is the competition from silicon: 600 V diodes are bipolar, so inherently slow, whereas 600 V transistors can be unipolar, super-junction MOSFETs. “They are quite fast,” admits Palmour. “That does not mean that SiC could outperform it, because our capacitances would be far lower. But we choose to take on bipolar devices at higher voltages.”
Going up in voltage makes a lot of sense. The recovery losses for SiC are one-fifth of those for silicon at 1200 V, but just one-tenth at 1.7 kV, and one-thirtieth at 3.3 kV. To allow customers to benefit from this superiority at higher voltages, Cree launched a 1.7 kV device in 2012.
“Another area where we have had a lot of success is industrial high-frequency power supplies,” says Palmour. These units, which are being deployed in semiconductor processing equipment, enable an increase in the power from a rack- mounted power supply. “You can double the amount of power they get out of the same sized box.”
Expansion of the family of Cree’s second-generation products continues. “We recently announced a 50 A packaged discrete: That’s a lot of juice for a discrete package, and we’ve had a lot of interest in that,” reveals Palmour.
Within the research and development group, efforts are focused on generation III products, which continue the die shrink approach applied during the move from generation I to generation II.
At a recent conference, Palmour presented results for generation III products that can operate at 900 V, 1200 V, 1700 V, 3.3 kV, 6.5 kV, 10 kV and 15 kV. “So we’ve done it across the board, and the question is what the first product will be,” says Palmour. “We’ll have to wait and see, but I would not expect it to be that long – I would expect that somewhere in 2015 we’d see a gen III product announced.”
Although the number of suppliers of SiC MOSFETs is on the rise, Palmour still sees silicon as the main competitor for chip sales. “It’s not all done in silicon. There is still room to improve, and it has a thirty year head-start on us.”
Left: European power electronics giant ST Microelectronics is launching a 1200V SiC MOSFET this autumn.
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www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor
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