sábado, 24 de julio de 2010

Wide Band Gap Devices in Power Systems

Silicon carbide (SiC) is a wide band gap semiconductor material which is ideal for the production of power switching devices. It has excellent power handling and high-temperature operation capabilities. The defense industry has long been interested in the use of SiC technology for its high power applications, such as electric ships, high power weapon systems, hybrid electric vehicles, and More Electric Aircrafts (MEAs). Power electronics converter systems with SiC-based power semiconductor switching devices are lighter, more compact, and more efficient, making them ideal for high-voltage power electronic applications.

These highly desirable device improvements would also substantially trim the amount of undesirable power losses in electric motor drive power conversion applications. For example, SiC high-temperature electronic sensors and controls on an automobile engine will lead to better combustion monitoring and control, which would result in cleaner burning, more fuel efficient cars. SiC MOSFETs, Schottky diodes or PIN diodes, IGBT, Cascode modules, and JFETs are among the choices of the technologies, depending on maturity levels.

The U.S. Navy's DD(X) next generation destroyer requires power control, distribution, and Integrated Power System (IPS) power conversion at multi-MW levels. The power conversion modules (PCMs) for use in the Integrated Fight Through Power (IFTP) of the IPS would benefit substantially from SiC technology. PCM-1 contains DC-to-DC converters with a required rating of at least 750kW. The transfer switch cabinet has a 5000 amp capacity rated DC switch with 1000VDC. The Ship Service Conversion Modules (SSCMs) in PCM-1 convert the DC input (1000VDC) to a lower DC voltage output (800VDC). Si IGBT modules with a 100KW power rating are currently in use. PCM-2 contains DC-AC inverters (Ship Service Invert Modules/SSIMs) with a required rating of at least of 500kW. PCM-4 consists of the hardware and software necessary to convert AC, three-phase, three-wire (the source neutral is grounded via high impedance at the source), 60Hz power from the propulsion bus to DC power. A PCM-4 consists of a transformer and rectifier to convert 4160VAC (3-phase, 60Hz) to 1000VDC.

The development of More Electric Aircraft (MEA) begins by addressing the need for generation and control of significantly increased on-board aircraft electric power. The present distributed controls and actuators for MEA flight control systems and surfaces are typically electrically-controlled but hydraulically-powered. Future systems should be controlled optically and powered electrically. Devices for generation and protection (circuit breakers) will range from 600 to 1000V and will require reliable devices at current levels up to 1000A. One of the key design requirements for new power generation will be the ability to provide high quality power generation at variable generator frequencies.

Replacement of constant frequency generators having variable frequency capability will eliminate the need for integrated drive subsystems (currently used to cancel out engine speed variations while providing constant generator speeds). Operability in the 300-400°C range with continuous operation will be required. Enabling technologies are: high quality power generation, optical control (optical bi-polar transistors), and smart actuator controls operating at higher voltages, to eliminate the cost and added weight of power conditioners. Since control will generally be provided by short, high-current pulses, the control devices will require both high voltages and high currents (though not as high as power generation and control). Typical ranges are from 600-1000 volts and from 5 to more than 600 amps. SiC diodes and transistors provide the primary enabling technology. The desired device types are SiC MOSFETs, with baseline comparison devices being Si IGBTs, which have been demonstrated above 1 kA current levels. Switch mode power conversion transistors are considered better than thyristors for these applications.

Solid-state systems, including flexible alternating current transmission systems (FACTS), use a series of silicon power transistors to control the flow of current. They have been used on a limited basis on the power grid; however, the use of SiC rather than silicon could greatly simplify FACTS, while significantly improving performance. For example, SiC could double the voltage per device, reducing the number of transistors needed in a series in each "electrical valve." This could increase operating frequency from about 500Hz to about 20kHz. Utilities could realize significant savings in auxiliary controls and reduce the size, cooling requirements, and maintenance of systems.

In the near future, one of the largest potential commercial applications for SiC Schottky rectifiers is in the continuous conduction mode (CCM) power factor correction (PFC) circuit. In traditional, off-line AC/DC power supplies used in computer and telecom applications, the AC input sees a large inductive (transformer) load, which causes the power factor to be substantially lower than 1. SiC Schottky diodes are suitable for applications that require blocking voltage under 3KV, and SiC PiN diodes are suitable for higher blocking voltage applications. The performance improvements include higher switching speed and lower switching loss. For example, in a test case power converter, replacing the best available 600V Si diodes with a 1500V SiC diode, resulted in an increase of power supply efficiency from 82 to 88 percent for switching at 186kHz, along with a reduction in EMI emissions.

SiC Schottky diodes have been considered as replacements for silicon PiN diodes in many high-frequency motor drive applications. A recent study by a market analysis group predicted that the global market for SiC Schottky diodes and transistors will increase from $13 million in 2004 to more than $53 million in 2009 – a compound annual growth rate of 32 percent. Just about any household or industrial electric motor in the world requires a power electronic drive which can be made smaller and more efficient with the use of SiC devices; however, SiC must become less expensive and more readily available before it can compete in the commercial industry of motors and motor drivers.

Other applications for SiC electronics are piezoelectric accelerometers. The traditional approach (used in 90 percent of high-temperature applications) is to move the electronics off of the sensor and down to an external charge amplifier, in a cooler location. This introduces significant noise to the signal. An alternate approach which could be used on some aircraft engine designs, is to use high-temperature electronics inside the sensor housing up to 210°C. SiC should increase this temperature limit to 370°C. The application of SiC high-temperature electronics would allow performance improvements to sensors that could not obtain such improvements otherwise.

SiC sensors have been tested on automotive applications with positive results. Both military and commercial aircrafts would gain significant enhancements to their performance. Crucial to realizing these new components is the ability to sufficiently amplify low-level sensor signals (vibration, temperature, pressure) to enable transmission over moderate distances. The availability of an operational amplifier (op-amp) functioning reliably at temperatures as high as 370°C would serve this purpose. It would also provide a fundamental building block for sensor manufacturers to design circuits that amplify low-level signals and signal the condition of the electrical output from pressure and temperature sensors operating at high temperatures. With the availability of high-temperature op-amps, A/D converters, and resistive and capacitive components, circuits could be designed that would read the bridge resistance output at the high-temperature strain gauge pressure sensor. These circuits would also perform temperature and pressure linearity compensation and signal amplification. Such a circuit would be a major step towards a high-temperature "smart" sensor.

Overall, the implementation of SiC devices in high power systems would provide significant improvements to system performance, reduce system size and power loss, and potentially lower the overall system cost. The challenges of implementing SiC devices are lack of high quality materials and lack of suitable high temperature packages. Over the past decade, the wide band gap semiconductor industry has been working aggressively to improve material quality, the device fabrication process, and device reliability, making impressive advances. The Department of Defense has also made significant investments to advance wide band gap technology. Currently, SiC Schottky diodes are available commercially from a few companies. SiC MOSFETs and PIN diodes have been demonstrated and are expected to further advance in the next 12 to 18 months.

Freddy Vallenilla, EES SECC 2

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