markets wide bandgap electronics
Today’s HEVs are fitted with silicon power electronics, and use two separate cooling systems. That’s because the inverter mounted on or near the internal combustion engine block must be maintained at a lower temperature than the engine. When SiC devices start to take the place of silicon IGBTs that have inferior thermal capabilities, it may allow the inverter to run at the same temperature as the engine casing and consequently use the same cooling system.
Unfortunately, none of the current SiC switch technologies are sufficiently mature to match the reliability of silicon devices or even of SiC Schottky diodes. This will delay adoption of SiC transistors in HEVs. SiC JFETs should meet the reliability requirements first, because they have the same basic structure and materials as diodes. But these transistors may not dominate the market, because there could be significant competition from SiC BJTs, which seem to offer good reliability in terms of life test, high- temperature operation and temperature cycling, plus robustness to shocks and vibrations.
In the long term, GaN transistors and diodes are of great interest for HEVs, because they could eventually undercut the price of equivalent products built from SiC. Discrete GaN devices will probably only find deployment in the battery charger, but it is possible that they might also be used in drivetrain power modules. As with SiC devices, adoption of GaN will not take place until devices have proven reliability to automotive specifications.
Industrial motor drives The greatest revenue opportunity for SiC power devices, particularly for power modules, is in the industrial motor drive market. Here, SiC can be used in equipment for driving and controlling industrial motors within factory automation systems. This equipment can be single-phase or three-phase; drive AC, DC or servo motors; and operate at variable speed or variable frequency. The main benefit of this equipment is improved control and efficiency when driving motors.
The increase in power density resulting from the switch from silicon devices to those built with SiC trims the inverter’s size, its weight and its cooling requirements. However, this market is extremely cost-sensitive, with drives sold on price versus performance and size. To win sales, the prices of SiC devices must plummet to a level comparable with silicon equivalents.
Power conversion efficiency values will become increasingly important. The market for low-voltage AC and DC drives should benefit from European Union legislation that has established minimum motor efficiency requirements as a result of the Eco-design of Energy-related Products Directive. Beginning in 2015, a mandate that has led from this directive has dictated the
use of either an IE3 efficiency motor (which operates at ‘premium efficiency’), or an IE2 efficiency (which operates at ‘high efficiency) motor coupled with a drive. Most modern motors are IE2 efficient, and we believe that many customers will take the cheaper option of adding a drive to an existing IE2 motor, rather than replacing it with a new IE3 motor. This should lead to ramping sales in the drives market.
Designers of systems with long operational life, such as industrial motor drives, only want devices with excellent reliability. These are some unanswered questions concerning the reliability of SiC devices, and diodes and transistors will only be qualified after extensive load-cycling, temperature-cycling and life-test data has been produced and evaluated. However, we predict that this scrutiny will not reveal any major weaknesses, and that there will come a time when discrete SiC and GaN power devices are used in single- phase drives, and SiC power modules in three-phase, industrial motor drives.
There are two significant, far from obvious benefits that result from the use of SiC or GaN devices in industrial motor drives: Higher switching frequencies are possible, leading to quieter motors; and there is an opportunity to modify the inverter circuit topology to incorporate a transformer. Go down the latter route and it is possible to generate sine-wave voltage and current outputs, which can drastically diminish RF electromagnetic compatibility noise.
Boosting PV efficiencies Owners of all forms of photovoltaic system want to generate as much energy as possible. Increasing the efficiency of the inverter helps, which is possible by replacing silicon devices with those made from SiC or GaN. Even an improvement of just 0.5 percent is viewed as highly worthwhile, because it allows the end-user to make more money selling the electricity to power companies.
Consequently, it is of no surprise that the PV inverter market is the second largest one for SiC Schottky diodes and it will be the first one to start using SiC transistors in volume. An inverter built with SiC diodes and transistors can operate at a switching frequency that is two-to-three times that of a silicon-based equivalent, and potentially be half the size.
Despite their relatively high price, 600 V SiC Schottky diodes should win deployment in micro-inverters and single-phase string inverters. However, we believe that those devices will subsequently be displaced by lower- priced GaN Schottky diodes, which will make inroads once their reliability is proven.
Moving on to transistors, our view is that those made from GaN could eventually replace high-voltage MOSFETs in 600 V systems for string inverters and
As with SiC devices, adoption of GaN will not take place until devices have proven reliability to automotive specifications
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www.compoundsemiconductor.net July 2012
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