The GTO (Gate turn-off Thyristor) is a power switching device that can be turned on by a short pulse of gate current and turned off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anode current to be turned off. Hence there is no need for an external commutation circuit to turn it off. Because turn-off is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more capability for highfrequency operation than thyristors. GTOs have the I2t withstand capability and hence can be protected by semiconductor fuses. For reliable operation of GTOs, the critical aspects are proper design of the gate turn-off circuit and the snubber circuit.
--- K.V.S.Sundeep, M.Phil.
Thursday, January 6, 2011
Tuesday, December 7, 2010
Triple-Mode Transistors Show Potential
Rice University research that capitalizes on the wide-ranging capabilities of graphene could lead to circuit applications that are far more compact and versatile than what is now feasible with silicon-based technologies.
Triple-mode, single-transistor amplifiers based on graphene -- the one-atom-thick form of carbon that recently won its discoverers a Nobel Prize -- could become key components in future electronic circuits. the discovery by Rice researchers was reported this week in the online journal ACS Nano.
Graphene is very strong, nearly transparent and conducts electricity very well. But another key property is ambipolarity, graphene's ability to switch between using positive and negative carriers on the fly depending on the input signal. Traditional silicon transistors usually use one or the other type of carrier, which is determined during fabrication.
A three-terminal single-transistor amplifier made of graphene can be changed during operation to any of three modes at any time using carriers that are positive, negative or both, providing opportunities that are not possible with traditional single-transistor architectures, said Kartik Mohanram, an assistant professor of electrical and computer engineering at Rice. He collaborated on the research with Alexander Balandin, a professor of electrical engineering at the University of California, Riverside, and their students Xuebei Yang (at Rice) and Guanxiong Liu (at Riverside).
Mohanram likened the new transistor's abilities to that of a water tap. "Turn it on and the water flows," he said. "Turn it off and the water stops. That's what a traditional transistor does. It's a unipolar device -- it only opens and closes in one direction."
Triple-mode, single-transistor amplifiers based on graphene -- the one-atom-thick form of carbon that recently won its discoverers a Nobel Prize -- could become key components in future electronic circuits. the discovery by Rice researchers was reported this week in the online journal ACS Nano.
Graphene is very strong, nearly transparent and conducts electricity very well. But another key property is ambipolarity, graphene's ability to switch between using positive and negative carriers on the fly depending on the input signal. Traditional silicon transistors usually use one or the other type of carrier, which is determined during fabrication.
A three-terminal single-transistor amplifier made of graphene can be changed during operation to any of three modes at any time using carriers that are positive, negative or both, providing opportunities that are not possible with traditional single-transistor architectures, said Kartik Mohanram, an assistant professor of electrical and computer engineering at Rice. He collaborated on the research with Alexander Balandin, a professor of electrical engineering at the University of California, Riverside, and their students Xuebei Yang (at Rice) and Guanxiong Liu (at Riverside).
Mohanram likened the new transistor's abilities to that of a water tap. "Turn it on and the water flows," he said. "Turn it off and the water stops. That's what a traditional transistor does. It's a unipolar device -- it only opens and closes in one direction."
Cloud Formation Mystery Cleared..(for a change)
Until now, one of the largest mysteries associated with understanding how clouds form was how the atmospheric structures at times appear to form faster than the basic laws of physics allow for. Physicists now believe they may have uncovered the answer.
The mechanism through which “usual” clouds form is fairly straightforward and well-understood. Under specific atmospheric conditions, with certain levels of pressure and appropriate temperatures, water vapors in the air begin condensing.
They form small droplets, which then proceed to combine with each other until they form raindrops that have enough mass to start dropping towards the ground. Some studies indicate that the vapors first start coming together around aerosol particles.
But the intricacies of droplet aggregation are what skew most theories. There are reports of clouds appearing out of the blue within minutes, and of rain starting to fall from areas of the sky that were a short while before clear and blue.
In scientific measurements seeking to validate these claims, scientists have indeed determined that droplets measuring some 15 micrometers grow to 50 micrometers in less than half an hour.
The latter size is sufficiently large to allow for a rain shower to drench the ground below. The mystery physicists wanted to clear revolved around how this fast aggregation occurs.
Standard models of droplet formation cannot explain this phenomenon in the absence of ice formation. But Imperial College London (ICL) experts Vassilios Dallas and Christos Vassilicos have an answer.
The key to unlocking the puzzle lies in the work of 19th century Irish mathematician George Stokes, who developed an inertia-related number that bears his name.
This dimensionless quantity refers to how water droplets interact with each other in a flow of gas. The most important factor influencing this number is the scale of the medium in which these collisions take place, Technology Review reports.
When droplets have a small Stokes number, they don't have sufficient inertia, and therefore rarely collide with each other. Conversely, when the number is large enough, the droplets carry a lot of inertia, and simply can't avoid collisions, and subsequent aggregation.
The ICL experts say that turbulence is what causes the Stokes number to go up, and add that these phenomena can occur at a variety of scales, including the micrometer one.
When turbulences at this scale occur, they cause big variations in the Stokes number. As a consequence, the water droplets begin colliding more and more, and the cloud formation process is accelerated.
If the new model turns out to be accurate, then it could have significant implications for current climate models. Scientists will soon be able to calculate in more detail the total amount of sunlight clouds bounce back into space.
The mechanism through which “usual” clouds form is fairly straightforward and well-understood. Under specific atmospheric conditions, with certain levels of pressure and appropriate temperatures, water vapors in the air begin condensing.
They form small droplets, which then proceed to combine with each other until they form raindrops that have enough mass to start dropping towards the ground. Some studies indicate that the vapors first start coming together around aerosol particles.
But the intricacies of droplet aggregation are what skew most theories. There are reports of clouds appearing out of the blue within minutes, and of rain starting to fall from areas of the sky that were a short while before clear and blue.
In scientific measurements seeking to validate these claims, scientists have indeed determined that droplets measuring some 15 micrometers grow to 50 micrometers in less than half an hour.
The latter size is sufficiently large to allow for a rain shower to drench the ground below. The mystery physicists wanted to clear revolved around how this fast aggregation occurs.
Standard models of droplet formation cannot explain this phenomenon in the absence of ice formation. But Imperial College London (ICL) experts Vassilios Dallas and Christos Vassilicos have an answer.
The key to unlocking the puzzle lies in the work of 19th century Irish mathematician George Stokes, who developed an inertia-related number that bears his name.
This dimensionless quantity refers to how water droplets interact with each other in a flow of gas. The most important factor influencing this number is the scale of the medium in which these collisions take place, Technology Review reports.
When droplets have a small Stokes number, they don't have sufficient inertia, and therefore rarely collide with each other. Conversely, when the number is large enough, the droplets carry a lot of inertia, and simply can't avoid collisions, and subsequent aggregation.
The ICL experts say that turbulence is what causes the Stokes number to go up, and add that these phenomena can occur at a variety of scales, including the micrometer one.
When turbulences at this scale occur, they cause big variations in the Stokes number. As a consequence, the water droplets begin colliding more and more, and the cloud formation process is accelerated.
If the new model turns out to be accurate, then it could have significant implications for current climate models. Scientists will soon be able to calculate in more detail the total amount of sunlight clouds bounce back into space.
Friday, November 19, 2010
The future of Power Electronics
The future of Power Electronics is expected move in the following directions:
- The future will demand Integrated Systems for electronic power processing. A more multi-disciplinary approach is needed for new achievements in integration, packaging, reliability and cost reduction.
- Intelligent control and energy management will come easily. Thermal and passive component integration is equally important.
- Large penetration of power electronics into power systems, mainly in distributed generation.
- Large-scale use of power electronics in automotive applications. This largely depends on political decisions as well as technology advances.
- Advances in high current, higher voltage devices will have a major impact on traction applications.
- Emerging applications in commercial / residential areas: HVAC, Induction cooking, lighting, computer power etc.
- The cost of using power electronics will depend on factors like: • Recycling • Standards regarding EMI • Intelligence / protection • Modular converters • Reliability considerations • Thermal engineering • Electromagnetic Integration
- Silicon carbide is an important future development in power semi-conductors.
- Distributed generation and power quality are important future considerations.
- For very high-power applications, modularization will be expanding, provided the system cost, efficiency, flexibility and EMC are acceptable.
- For medium-power applications, the total system integration still leaves a lot of room for improvements in cost, power modules, control and sensing, passives, reliability and performance.
- Fast energy storage is required in numerous applications. At present, super capacitors seem to be the most promising solution for energies upto 5 kWh. In the next ten years, all other energy storage options will continue to be considered.
- Applications in power transmission include HVDC converter stations, Flexible AC Transmission Systems (FACTS).
- Applications in power distribution include DC-DC converters, Dynamic filters, frequency conversion and Custom Power Devices.
The on-going development of interconnection standards and regulations will present both market opportunities and technology challenges for the power electronics industry. Future research and development efforts will need to focus on improving efficiency and reliability, communication and interface, thermal management, reduce parts and points of failure, packaging and bringing down cost.
The range of applications continue to expand in areas such as power supplies to motion control, factory automation, transportation, energy storage, multi-megawatt industrial drives, and electric power transmission/ distribution.
Power electronics....?
Power electronics is the technology associated with the efficient conversion, control and conditioning of electric power by static means from its available input form into the desired electrical output form.
Power Electronics deals with the study of
• Power semiconductor devices
- their physics, characteristics, drive requirements and their protection for optimum utilisation of their capacities,
• Power converter topologies involving them,
• Control strategies of the converters,
• Digital, analogue and microelectronics involved,
• Capacitive and magnetic energy storage elements,
• Rotating and static electrical devices,
• Quality of waveforms generated,
• Electro Magnetic and Radio Frequency Interference,
• Thermal Management
Power Electronics deals with the study of
• Power semiconductor devices
- their physics, characteristics, drive requirements and their protection for optimum utilisation of their capacities,
• Power converter topologies involving them,
• Control strategies of the converters,
• Digital, analogue and microelectronics involved,
• Capacitive and magnetic energy storage elements,
• Rotating and static electrical devices,
• Quality of waveforms generated,
• Electro Magnetic and Radio Frequency Interference,
• Thermal Management
Wednesday, November 10, 2010
The Insulated Gate Bipolar Transistor (IGBT)
IGBT is a voltage controlled four-layer device with the advantages of the MOSFET driver and the Bipolar Main terminal. IGBTs can be classified as punch-through (PT) and non-punch-through (NPT) structures. In the punch-through IGBT, a better trade-off between the forward voltage drop and turn-off time can be achieved. Punch-through IGBTs are available up to about 1200 V. NPT IGBTs of up to about 4 KV have been reported in literature and they are more robust than PT IGBTs particularly under short circuit conditions. However they have a higher forward voltage drop than the PT IGBTs. Its switching times can be controlled by suitably shaping the drive signal. This gives the IGBT a number of advantages: it does not require protective circuits, it can be connected in parallel without difficulty, and series connection is possible without dv/dt snubbers. The IGBT is presently one of the most popular device in view of its wide ratings, switching speed of about 100 KHz a easy voltage drive and a square Safe Operating Area devoid of a Second Breakdown region.
Power Semiconductor devices-history(brief)
Power electronics and converters utilizing them made a head start when the first device the Silicon Controlled Rectifier was proposed by Bell Labs and commercially produced by General Electric in the earlier fifties. The Mercury Arc Rectifiers were well in use by that time and the robust and compact SCR first started replacing it in the rectifiers and cycloconverters.
The necessity arose of extending the application of the SCR beyond the line-commutated mode of action, which called for external measures to circumvent its turn-off incapability via its control terminals. Various turn-off schemes were proposed and their classification was suggested but it became increasingly obvious that a device with turn-off capability was desirable, which would permit it a wider application. The turn-off networks and aids were impractical at higher powers.
The Bipolar transistor, which had by the sixties been developed to handle a few tens of amperes and block a few hundred volts, arrived as the first competitor to the SCR. It is superior to the SCR in its turn-off capability, which could be exercised via its control terminals. This permitted the replacement of the SCR in all forced-commutated inverters and choppers. However, the gain (power) of the SCR is a few decades superior to that of the Bipolar transistor and the high base currents required to switch the Bipolar spawned the Darlington. Three or more stage Darlingtons are available as a single chip complete with accessories for its convenient drive. Higher operating frequencies were obtainable with a discrete Bipolars compared to the 'fast' inverter-grade SCRs permitting reduction of filter components. But the Darlington's operating frequency had to be reduced to permit a sequential turn-off of the drivers and the main transistor.
Further, the incapability of the Bipolar to block reverse voltages restricted its use.The Power MOSFET burst into the scene commercially near the end seventies. This device also represents the first successful marriage between modern integrated circuit and discrete power semiconductor manufacturing technologies. Its voltage drive capability – giving it again a higher gain, the ease of its paralleling and most importantly the much higher operating frequencies reaching upto a few MHz saw it replacing the Bipolar also at the sub-10 KW range mainly for SMPS type of applications.
Extension of VLSI manufacturing facilities for the MOSFET reduced its price vis-à-vis the Bipolar also. However, being a majority carrier device its on-state voltage is dictated by the RDS(ON) of the device, which in turn is proportional to about V2.3DSS rating of the MOSFET. Consequently, high-voltage MOSFETS are not commercially viable.
Improvements were being tried out on the SCR regarding its turn-off capability mostly by reducing the turn-on gain. Different versions of the Gate-turn-off device, the Gate turn-off Thyristor (GTO), were proposed by various manufacturers - each advocating their own symbol for the device. The requirement for an extremely high turn-off control current via the gate and the comparatively higher cost of the device restricted its application only to inverters rated above a few hundred KVA.The lookout for a more efficient, cheap, fast and robust turn-off-able device proceeded in different directions with MOS drives for both the basic thysistor and the Bipolar.
The Insulated Gate Bipolar Transistor (IGBT) – basically a MOSFET driven Bipolar from its terminal characteristics has been a successful proposition with devices being made available at about 4 KV and 4 KA. Its switching frequency of about 25 KHz and ease of connection and drive saw it totally removing the Bipolar from practically all applications. Industrially, only the MOSFET has been able to continue in the sub – 10 KVA range primarily because of its high switching frequency.
The IGBT has also pushed up the GTO to applications above 2-5 MVA.Subsequent developments in converter topologies – especially the three-level inverter permitted use of the IGBT in converters of 5 MVA range. However at ratings above that the GTO based converters had some space. Only SCR based converters are possible at the highest range where line-commutated or load-commutated converters were the only solution. The surge current, the peak repetition voltage and I2t ratings are applicable only to the thyristors making them more robust, specially thermally, than the transistors of all varieties.
Presently there are few hybrid devices and Intelligent Power Modules (IPM) are marketed by some manufacturers. The IPMs have already gathered wide acceptance. The 4500 V, 1200 A IEGT (injection-enhanced gate transistor) or the 6000 V, 3500 A IGCT (Integrated Gate Commutated Thyristors) which are promising at the higher power ranges. However these new devices must prove themselves before they are accepted by the industry at large.
The necessity arose of extending the application of the SCR beyond the line-commutated mode of action, which called for external measures to circumvent its turn-off incapability via its control terminals. Various turn-off schemes were proposed and their classification was suggested but it became increasingly obvious that a device with turn-off capability was desirable, which would permit it a wider application. The turn-off networks and aids were impractical at higher powers.
The Bipolar transistor, which had by the sixties been developed to handle a few tens of amperes and block a few hundred volts, arrived as the first competitor to the SCR. It is superior to the SCR in its turn-off capability, which could be exercised via its control terminals. This permitted the replacement of the SCR in all forced-commutated inverters and choppers. However, the gain (power) of the SCR is a few decades superior to that of the Bipolar transistor and the high base currents required to switch the Bipolar spawned the Darlington. Three or more stage Darlingtons are available as a single chip complete with accessories for its convenient drive. Higher operating frequencies were obtainable with a discrete Bipolars compared to the 'fast' inverter-grade SCRs permitting reduction of filter components. But the Darlington's operating frequency had to be reduced to permit a sequential turn-off of the drivers and the main transistor.
Further, the incapability of the Bipolar to block reverse voltages restricted its use.The Power MOSFET burst into the scene commercially near the end seventies. This device also represents the first successful marriage between modern integrated circuit and discrete power semiconductor manufacturing technologies. Its voltage drive capability – giving it again a higher gain, the ease of its paralleling and most importantly the much higher operating frequencies reaching upto a few MHz saw it replacing the Bipolar also at the sub-10 KW range mainly for SMPS type of applications.
Extension of VLSI manufacturing facilities for the MOSFET reduced its price vis-à-vis the Bipolar also. However, being a majority carrier device its on-state voltage is dictated by the RDS(ON) of the device, which in turn is proportional to about V2.3DSS rating of the MOSFET. Consequently, high-voltage MOSFETS are not commercially viable.
Improvements were being tried out on the SCR regarding its turn-off capability mostly by reducing the turn-on gain. Different versions of the Gate-turn-off device, the Gate turn-off Thyristor (GTO), were proposed by various manufacturers - each advocating their own symbol for the device. The requirement for an extremely high turn-off control current via the gate and the comparatively higher cost of the device restricted its application only to inverters rated above a few hundred KVA.The lookout for a more efficient, cheap, fast and robust turn-off-able device proceeded in different directions with MOS drives for both the basic thysistor and the Bipolar.
The Insulated Gate Bipolar Transistor (IGBT) – basically a MOSFET driven Bipolar from its terminal characteristics has been a successful proposition with devices being made available at about 4 KV and 4 KA. Its switching frequency of about 25 KHz and ease of connection and drive saw it totally removing the Bipolar from practically all applications. Industrially, only the MOSFET has been able to continue in the sub – 10 KVA range primarily because of its high switching frequency.
The IGBT has also pushed up the GTO to applications above 2-5 MVA.Subsequent developments in converter topologies – especially the three-level inverter permitted use of the IGBT in converters of 5 MVA range. However at ratings above that the GTO based converters had some space. Only SCR based converters are possible at the highest range where line-commutated or load-commutated converters were the only solution. The surge current, the peak repetition voltage and I2t ratings are applicable only to the thyristors making them more robust, specially thermally, than the transistors of all varieties.
Presently there are few hybrid devices and Intelligent Power Modules (IPM) are marketed by some manufacturers. The IPMs have already gathered wide acceptance. The 4500 V, 1200 A IEGT (injection-enhanced gate transistor) or the 6000 V, 3500 A IGCT (Integrated Gate Commutated Thyristors) which are promising at the higher power ranges. However these new devices must prove themselves before they are accepted by the industry at large.
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