Integrated gate commutated thyristor IGCT Seminar REPORT
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31-12-2009, 07:34 PM
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Integrated gate commutated thyristor tutorial.pdf (Size: 431 KB / Downloads: 456) ABSTRACT
The Integrated Gate Commutated Thyristor (IGCT) combines the advantages of the hard driven GTO thyristor, including its dramatically improved turn- off performance, with technological breakthroughs at the device, gate-drive and application levels. Homogenous switching area of the IGCT up to the dynamic avalanche limits. Snubber circuits are no longer needed. Improved loss characteristics allow high frequency applications extending into the kHz range. A new IGCT device family with integrated high- power diodes has been developed for applications in the 0.5-6 MVA range, extending to several 100 MVA with series and parallel connections. A first 100 MVA inverter based on the IGCT has been in commercial operation and confirms the very high level of reliability of this new technology. Other new application using the IGCT platform includes ABBâ„¢s new ACS1000 drive for medium voltage applications.
Thyristor technology is inherently superior to transistor for blocking voltage values above 2.5kV, plasma distributions equal to those of diodes offering the best trade-off between the on-state and blocking voltages. Until the introduction of newer power switches, the only serious contenders for high-power transportation systems and other applications were the GTO (thyristor), with its cumbersome snubbers, and the IGBT (transistor), with its inherently high losses. Until now, adding the gate turn-off feature has resulted in GTO being constrained by a variety of unsatisfactory compromises. The widely used standard GTO drive technology results in inhomogenous turn-on and turn-off that call for costly dv/dt and di/dt snubber circuits combined with bulky gate drive units.
Rooting from the GTO is one of the newest power switches, the Gate-Commutated Thyristor (GCT). It successfully combines the best of the thyristor and transistor characteristics, while fulfilling the additional requirements of manufacturability and high reliability. The GCT is a semiconductor based on the GTO structure, whose cathode emitter can be shut off instantaneously, thereby converting the device from a low conduction-drop thyristor to a low switching loss, high dv/dt bipolar transistor at turn- off. The IGCT (Integrated GCT) is the combination of the GCT device and a low
inductance gate unit. This technology extends transistor switching performance to well above the MW range, with 4.5kV devices capable of turning off 4kA, and 6kV devices capable of turning off 3kA without snubbers. The IGCT represents the optimum combination of low loss thyristor technology and snubberles gate effective turn off for demanding medium and high voltage power electronics applications.
The IGCT (Integrated GCT) is the combination of the GCT device and a low inductance gate unit. This can be represented as shown below.
The two transistor equivalent circuit of IGCT is shown below.
The turn-on waveform of the IGCT is shown below.
The thick line shows the variation of anode voltage during turn-on process. The lighter shows the variation of anode current during turn-on process of IGCT. When the gate current increases, the anode voltage decreases from its initial value i.e. the voltage applied across the IGCT. The anode current increases from zero to peak value. When the current reaches the latching current value, the device is turned on.
The turn-off waveform of the IGCT is shown below.
The thick line shows the variation of the anode voltage during turn-off. The lighter shows the variation of the anode current during turn-off process of IGCT.
GTO and thyristor are four layer (npnp) devices. As such, they have only two stable points their characteristics-Ëœonâ„¢ and Ëœoffâ„¢. Every state in between is unstable and results in current filamentation. The inherent instability is worsened by processing imperfections. This has led to the widely accepted myth that a GTO cannot be operated without a snubber. Essentially, the GTO has to be reduced to a stable pnp device i.e. a transistor, for the few critical microseconds during turn-off. To stop the cathode (n) from taking part in the process, the bias of the cathode n-p junction has to be reversed before voltage starts to build up at the main junction. This calls for commutation of the full load current from the cathode (n) to the gate (p) within one microsecond. Thanks to a new housing design, 4000A/us can be achieved with a low cost 20V gate unit. Current filamentation is totally suppressed and the turn-off waveforms and safe operating area are identical to those of a transistor.
IGCT technology brings together the power handling device (GCT) and the device control circuitry (freewheeling diode and gate drive) in an integrated package. By offering four levels of component packaging and integration, it permits simultaneous improvement in four interrelated areas; low switching and conduction losses at medium voltage, simplified circuitry for operating the power semiconductor, reduced power system cost, and enhanced reliability and availability. Also, by providing pre- engineered switch modules, IGCT enables medium-voltage equipment designers to develop their products faster.
Figure shows, Cross-section through a GTO thyristor (left), diode (center) and GCT (right). Showing the transparent emitter and buffer layer of the GCT allows a thinner n-base layer. This permits integration of the freewheeling diode in the same structure, plus snubbreless operation in combination with low inductive packaging and gate drive circuitry.
THE SECRET OF IGCT TECHNOLOGY
In IGCT technology, a combination of design innovations permits the thousands of individual power switching structures in a modified GTO thyristor to switch fast and simultaneously. What is more, the low on and off-state losses inherent in thyristor designs are retained.
The first of two key innovations is the buffer layer design, which has allowed the on-state and switching losses to be reduced by a factor of 2 to 2.5 and makes the optimum doping profile of a GTO and a diode virtually identical. Previously integrating a diode with a GTO has resulted in serve degradation of the diodeâ„¢s performance. Although the idea of the buffer layer is almost as old as GTO itself, it has never been used before for the following reason: to reduce the switching losses, the charge stored in the device in the conducting phase has to be GTO and a diode virtually identical. In a conventionally designed GTO this function is performed shorts, which provide a path for the electrons to flow out. The combination of anode shorts and a buffer layer, however, leads to high trigger and holding currents. To solve this problem the anode shorts have been omitted. Instead the anode is made transparent, i.e. permeable, to the electrons, with the result that the trigger currents are reduced by the order of the magnitude compared with a conventional GTO without buffer.
The second innovation address the gate control GTO an thyristors are four layer (npnp) devices. As such, they have only two stable points in their characteristics Ëœonâ„¢ and Ëœoffâ„¢. Every state in between is unstable and results in current filamentation. The inherent instability is worsened by processing imperfection. This has lead to the widely accepted myth that a GTO cannot be operated without a snubber.
Essentially, the GTO has to be reduced to a stable pnp device i.e. a transistor, for the few critical microseconds during turn-off. To stop the cathode (n) from taking part in the process, the bias of the cathode n-p junction has to be reversed before voltage starts to build up at the main junction. This calls for commutation of the full load current from the cathode (n) to the gate (p) within one microsecond. Thanks to a new housing design, 4000A/us can be achieved with a low cost 20V gate unit. Current filamentation is totally suppressed and the turn-off waveforms and safe operating area are identical to those of a transistor.
FUSION OF POWER DEVICE AND CIRCUIT DESIGN EXPERIENCE
The IGCT technology is the result of intense collaboration between device designers at ABB semiconductors and power circuit designers at ABB industrial systems. In fact, it was the co-development of power silicon, the packaging and additional circuitry needed to make the power switch suitable for industrial applications that made the IGCTâ„¢s unique combination of characteristics possible in the first place.
IGCT technology brings together the power handling device (GCT) and the device control circuitry (free wheeling diode and gate driver) in an integrated package. By offering four levels of component packaging and integration, it permits simultaneous improvement in four interrelated areas; low switching and conduction losses at medium voltage, simplified circuitry for operating the power semiconductor, reduced power system cost, and enhanced reliability and availability. Also, by providing pre- engineered switch modules, IGCT enables medium-voltage equipment designers to develop their products faster.
CHARACTERISTICS OF THE IGCT
The characteristics targeted in the IGCT development programme were achieved in steps that closely resembled the approach adopted for the IGBT.The development goals were:
1. Improved GTO switching characteristics for operations without dv/dt snubbering at high current density.
2. Reduced on state and turn off losses through minimization of the silicon thickness.
3. Reduced gate drive requirement, especially during conduction.
4. Development of anti-parallel diodes capable of snubberless turn-off at high di/dt.
5. High frequency operation for continuous and dynamic condition.
6. Integration of main switches (Gto thyristor and diodes) In one semiconductor packages.
For high power application some additional characteristics Were necessary;
7. Enhanced reliability per MVA by reducing the complexity and number of components.
8. Extension of the power range to several 100MVA by means of cost effective, reliable series and parallel connections.
IMPROVED GTO SWITCHING CHARACTERISTICS
A dramatic improvement in the GTO turn off SOA is achieved at high power (3kA/4.5kV) with the so called hard drive-in which an integrated gate drive commutates the cathode current to zero before the anode voltage starts to rise. The very low gate circuit inductance that is required is obtained by means of a coaxial gate feed-through combined with a multilayer PCB gate drive connection; as a result values = 5-6 kA Ã‚Âµs can be achieved with a gate voltage of 20V.When the cathode current is zero, the remaining anode current is fully commutated to the gate unit, which remains in low impedance mode. The gate-drive energy consumption is minimized by avoiding gate over drive. The hard gate drive converts the thyristor from its pnpn latching state to a pnp mode within 1Ã‚Âµs.Turn off takes place entirely mode, thus eliminating any possibility of latching. Turn-off is homogenous as a result, enlarging the SOA to a full dynamic avalanche[peak turn off energy equal to 250kW cm] The device can be operated at the full area physical limit of silicon. The possible turn off currents per unit silicon area are on a par with the former snubbered turn off current of a best in class GTO device.2.shows the typical turn off wave form without dv/dt snubber.
REDUCED ON STATE AND TURN OFF LOSSES
A reduction in the device thickness of up to 30% for the same forward breakdown voltage can be achieved by introducing a buffer layer on the anode side. Buffer layer power semiconductors easily outperform conventional devices, the main benefit of such thin devices being an improved technology curve with lower on state and switching losses. The buffer layer, with a four device the buffer layer is combined with a transparent anode i.e. a pn junction with current dependent emitter efficiency. By means of an appropriate design, the electrons are extracted during turn off as efficiently as with anode shorts. Table 1 compares the improved characteristics of the new IGCT devices with this of a conventional hard driven and a standard GTO thyristor.
Due to its operation in thyristor mode, the IGCT has inherently lower conduction losses than a comparable IGBT device. The buffer layer which can be used with the IGCT as well as with the IGBT, the common denominator that causes there two devices to have comparable switching losses (ignoring the higher turn on losses of the IGBT, which implements gate controlled di/dt for the diode recovery protection).The improved loss characteristic and snubberless operations allow cost-effective IGCT application with switching frequencies in the range of 500Hz to 2kHz.
REDUCED GATE-DRIVE REQUIREMENTS
The characteristics usually having an impact on the design of a GTO thyristor gate drive are;
1. The gate turn off current (750a at 3kA turn off) has to be pumped into a 300-nh gate circuit inductance. This requires a high gate charge (Q) per pulse (Q=0.5ts*Ig ; where t is the storage time and Ig the peak turn off current).Also high losses are produced at the output MOSFETs.
2. The high gate back porch current(typically 4to8 a for a 3-kA device) that is required, especially at low temperatures.
Considerably lower demands are made on the new GCT gate drives with respect to these characteristics;
3. The storage time t is reduced by a factor of approximately 20.Despite the increase in the required gate charge is reduced by a factor of about 4.Due to this and the large number of parallel devices needed to carry the high turn off current pulse, the MOFSET losses are significantly reduced.
4. The use of transparent anode technology reduces the back porch current by factor of 20.
5. An additional requirement of the IGCT is low inductive connection to the gate drive, which therefore has to be located as close possible to the GCT. To achieve maximum robustness and compactness the gate drive has to surround the GCT, form an integral unit with the GCT and cooler, and carry only those parts of the circuit which are necessary for the gate drive. As a consequence, the number of gate drive component, the heat dissipation, electrical stress and internal thermal stress are all reduced, which significantly lowers the cost and failure rate of the gate drive. The IGCT, with its integrated gate drive, is easily snapped into the correction position in the stack and connected to its power supply and fiber optic control. The carefully designed pressure contact system ensures that release of the spring causes a defined pressure to be exerted on the GCT, establishing both the required electrical and thermal contact. Maximum ease of assembly and the highest reliability are achieved in this way.
SNUBBERLESS TURN-OFF AT HIGH di/dt WITH ANTI-PARALLEL DIODES
Due to the snubberless operations of the IGCT, operations of its anti-parallel diode also have to be snubberless. Upgraded high-powered press pack diodes manufactured using improved irradiation processes in combinations with the classical (non structured) processes fulfill these requirements. The di/dt which is possible scales directly with the diode area.
HIGH-FREQUENCY OPERATION FOR CONTINOUS AND DYNAMIC CONDITION
One of the impressive capabilities of the IGCT is its ability to handle high frequency turn-on / turn-off pulse bursts, whereas traditional GTO thyristors require a fairly long time between two consecutive turn-off operations. During turn-off current redistribution across the GTO and current crowding lead to a non-uniform temperature distribution (which additionally provokes non-uniform turn-on).This situation can rapidly create hot spots and cause thermal runaway. Thus, the minimum time between consecutive GTO switching operations is basically determined by the time needed to return to uniform junction temperature. The GCT, however, overcomes this limitation because of its extremely uniform switching behavior.
The heat that is generated during turn-off is evenly distributed across the entire device which means that the GCT has no Ëœthermal memoryâ„¢ other than its virtual junction temperature. Therefore, the only parameter limiting the GCT switching frequency is its Ëœthermal budgetâ„¢.
Since the thermal capacitances are much lower for short heating pulse durations than for steady state heating short pulse bursts can be executed without excessive temperature excursions.
The anti parallel diode can be operated without a dv/dt snubber,but di/dt control is still necessary to keep the reverse recovery of the diode within its safe operating area. Due to its thyristor nature, the GCT cannot provide di/dt control. Instead, di/dt is control is achieved with an inductor, clamped by a diode and a resistor as in standard GTO circuits. This additional di/dt clamp limits the current in the very unlikely event of a shoot-through.
A great simplified three-phase inverter circuit can be obtained with only one di/dt limiter for all three phases. The preferred three-phase IGCT inverter therefore only needs 11 electrical components:
1. 6 IGCT
2. 1 inductor
3. 1 clamp diode
4. 1 clamp capacitor (for high-inductive DC link only)
5. 1 clamp resistor
6. 1 gate-drive power supply
The capacitor connected at the input terminals tends to make the input dc voltage constant. This capacitor also suppresses the harmonic fed back to the source. The output voltage waveforms are similar to that of a three-phase bridge inverter.
Output waveforms of 3 phase inverters
EXPLANTION OF POWER RANGE BY SEVERAL 100 MVA
Typical maximum values for GTO technology are at present 6kV for the dynamic blocking voltage and 6 kV for the turn-off current. These figures correspond to a maximum NPC inverter power(NPC= neutral-point clamped or 3-level inverter) without series-connected devices of about 15 MVA. The future power electronics system market will, however, require converter powers which clearly exceeded this value. Inverter rating of several 100MVA will be needed in the near future. The key to future high-power the applications lie in the series connection of controlled turn-off devices.
Series-connected thyristor are a proven high voltage DC transmission technology, and such configuration has been used successfully for decades in the area. Due to its inherently short storage time of 1 Ã‚Âµs,the IGCT allows the simple, reliable series connection which is fundamental to the design of extra-high power inverters.
In series connections, the established method used to ensure maximum equipment availability is to insert more IGCTs in series than are necessary. This makes sense since it improves the installation in a number of always:
1. In the event of an IGCT or anti-parallel diode failure operation will continue without interruption. This is because the IGCT, being a press back device, is designed to behave as a short circuit when it fails. The failure is detected by an electronic circuit and signaled over fibre-optic cables. The failed device can be replaced at leisure, during routine maintenance.
2. The addition of redundant IGCTs reduces the voltage load of each individual device, including any eventual snubber circuit. It is know that the life time of an individual device depends strongly upon the voltage stress. When it is reduced, eg by one third, the average lifetime of the device is increased by a factor of about 20.
3. The in corporation of redundant devices reduces the risk of shoot-through in a converter phase, thereby allowing fuse less high-power converters phase, thereby allowing fuse less high-power converters to be built. The fuse less design improves converter reliability and efficiency. Thus shoot-through in a converter phase is very unlikely. Nevertheless, the converter is constructed to withstand the stresses during such fault conditions.
APPLICATIONS OF IGCT
The core of IGCT performance advantage is its ability to turn off in 2 microseconds and conduct like a thyristor. IGCT technology therefore permits simple inverter designs with half the losses of alternative technologies. For the first time, a power silicon technology has been matched to medium voltage, high power applications. This enables equipment designers working with IGCTs to build less costly, more reliable and more compact power control systems, including:
Â¢ Railway power supply frequency changers
Â¢ Static var compensators for power factor control
Â¢ Pump and fan drives for chemical, oil and power sectors
Â¢ Locomotive drives
Â¢ Static breakers
Future innovations that may be expected in the application of IGCTs include:
Â¢ Future reduction or elimination of the du/dt snubber for the series-connected turn off devices.
Â¢ Higher switching frequencies of up to 1kHz or more
Â¢ An anode-derived drive supply, made possible by the reduced gate drive power requirement of transparent anode IGCTs .
Â¢ The inherent reliability of IGCT operation coupled with device with device redundancy is the cornerstone on which future extra high-power converters will be built, in particular for the new emerging FACTS and custom power markets.
The IGCT combines all the important innovation needed for the future power electronics application and will become the key component for future medium to high voltage application in the range from 0.5MVA to several 100MVA.It inherently enables simple and robust series connection of turn-off devices for high power applications. The additional advantages of the IGCT over other turn-off devices (eg; low cost, low complexity and high efficiency) mean that it has no real competition in this power range. The IGCT is available today from several manufactures of high power semiconductors.
1. ABB Review on Power Semiconductor May 2000
2. Power Electronics by Dr. P.S. Bimbra
3. Power Electronics by Mohd. H Rashid
4. Power Electronics by Ned Mohan
5. Power Electronics by P.C. Sen
I express my sincere gratitude to Dr.Nambissan, Prof. & Head, Department of Electrical and Electronics Engineering, MES College of Engineering, Kuttippuram, for his cooperation and encouragement.
I would also like to thank my seminar and presentation guide Mrs. Sobha M (Asst. Professor, Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge, Department of EEE) for their invaluable advice and wholehearted cooperation without which this seminar and presentation would not have seen the light of day.
Gracious gratitude to all the faculty of the department of EEE & friends for their valuable advice and encouragement.
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