Semiconductor technology closes in on 'ideal switch'

Milwaukee, Wis.-A new semiconductor technology introduced by ABB Industrial Systems Inc. has significantly advanced electrical power switching, ABB claims. According to the company, the high-power semiconductor device, known as an integrated gate commutated thyristor (IGCT), approaches the "ideal switch" long sought throughout the industry.

The requirements of the "ideal switch," which power converter designers covet, include the ability to rapidly switch high currents at high voltages with low losses at high frequency as well as high reliability and compact construction. IGCTs make it easy and cost-effective to switch up to 10 megawatts of power at medium voltage levels, claims ABB.

The IGCT switch combines a new powerhandling device, a gate commutated thyristor (GCT), and the device control circuitry in an integrated package. "IGCT, a unique high-speed switch for high-horsepower applications, complements the IGBTs [insulated gate bipolar transistors] used widely in low-voltage, lower-horsepower applications," said Anders Nilarp, president and CEO of ABB Semiconductors AG. Both IGCT and IGBT devices will be made at newly expanded manufacturing facilities in Lenzburg, Switzerland, he said.

Hal Stillman, vice president of technology and innovation at ABB Ltd., Zurich, Switzerland, called it "an unexpected development and a surprise." Stillman said it was a surprise in the sense that IGCT resulted from a synergy-separate developments within the company's R&D efforts coming together to create what he called an "exponential leap" in semiconductor technology.

ABB claims the new IGCT megawatt switches offer five times better reliability than existing IGBT switches, require 50% fewer components for fabrication, reduce on-state losses by 50%, increase switching speed threefold, and lower power-stage equipment costs at least 30%.

As shown in the table, IGCTs switch like an IGBT and conduct like a gate turn-off thyristor (GTO). According to the company, ABB's new GCT switch is designed so that the thousands of individual monolithic elements on the wafer can switch quickly and simultaneously. The new silicon switch also retains the low onand off-state power losses inherent in thyristor designs.

To turn off large currents very quickly (essential for fast switching and high frequency), the IGCT incorporates two technical innovations: a transparent anode design that allows charges to flow out of the device quickly and a reduced silicon wafer thickness that limits the amount of stored charge and permits diode and switch to be fabricated on the same wafer.

These design features create the conditions in which low-loss switching can occur at the device level. Because signals that control the gates of the power structures must enter and exit the GCT quickly, that capability is made possible through a special low-inductance package and lowinductance gate driver circuitry that is colocated with the GCT.

The combination of this switch design and gate driver permit the thousands of individual power-switching structures in the GCT to switch simultaneously and without a snubber, according to ABB's Stillman. "When all elements switch at once, device failures (due to localized/uneven current flows) are eliminated, and the result is an unprecedented, robust, highly reliable power-switching device with low losses," he claims.

Typical control system applications of IGCTs, at voltage levels of 2.3, 4.16, and 6.9 kV, will include industrial motor drives, traction and ship propulsion, transmission lines, and power flow controllers for utilities. John A. Hill

Copyright Instrument Society of America Sep 1997
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