Modern power lockable thyristors
The creation of semiconductor devices for power electronics began in 1953 when it became possible to produce high-purity silicon and the formation of large-size silicon disks. In 1955, a semiconductor controlled device was first created, which has a four-layer structure and is called a thyristor.
It was turned on by applying a pulse to the control electrode with a positive voltage between the anode and the cathode. Turning off the thyristor is provided by reducing the direct current flowing through it to zero, for which many schemes of inductive-capacitive switching circuits have been developed. They not only increase the cost of the transformer, but also worsen its mass-dimensional parameters, reduce reliability.
Therefore, simultaneously with the creation of the thyristor, studies began to ensure its shutdown via the control electrode. The main problem was to ensure the rapid resorption of charge carriers in the base areas.
The first such thyristors appeared in 1960 in the USA. They are called Gate Turn Off (GTO). In our country, they are better known as lockable or off thyristors.
In the mid-90s, a lockable thyristor with a ring output of the control electrode was developed. It was named Gate Commutated Thyristor (GCT) and became a further development of GTO technology.
Lockable thyristor is a fully controlled semiconductor device, based on the classical four-layer structure. Turn it on and off by applying positive and negative current pulses to the control electrode. In Fig. 1 shows the symbol (a) and the block diagram (b) of the thyristor being turned off. Like a conventional thyristor, it has a cathode K, an anode A, a control electrode G. Differences in the structures of devices consist in a different arrangement of horizontal and vertical layers with n- and p-conductivities.
Fig. 1. Lockable thyristor:
The device of the cathode layer n has undergone the greatest change. It is divided into several hundreds of elementary cells, uniformly distributed over the area and connected in parallel. This design is caused by the desire to ensure a uniform decrease in current over the entire area of the semiconductor structure when the device is turned off.
The base layer p, despite the fact that it is made as a single whole, has a large number of contacts of the control electrode (approximately equal to the number of cathode cells), which are also uniformly distributed over the area and connected in parallel. The base layer n is made similar to the corresponding layer of a conventional thyristor.
The anode layer p has shunts (zones n) connecting the n-base with the anode contact through small distributed resistances. Anode shunts are used in thyristors that do not have the reverse blocking ability. They are designed to reduce the device off time by improving the conditions for extracting charges from the base area n.
The main execution of the GTO thyristors is a tablet with a four-layer silicon plate, sandwiched through thermocompensating molybdenum disks between two copper bases, which have high thermal and electrical conductivity. A control electrode with a lead in a ceramic housing is in contact with the silicon wafer. The device is clamped by contact surfaces between the two halves of the coolers, isolated from each other and having a design determined by the type of cooling system.
In the cycle of operation of the thyristor GTO, four phases are distinguished: on, conducting, off, and blocking.
On the schematic section of the thyristor structure (Fig. 1, b) the bottom output of the anodic structure. The anode is in contact with the layer p. Then from the bottom upwards follow: base layer n, base layer p (having a control electrode lead), layer n in direct contact with the cathode lead. Four layers form three pn junctions: j1 between layers p and n; j2 between layers n and p; j3 between layers p and n.
Phase 1 - power on. The transition of the thyristor structure from the blocking state to the conductive (inclusion) is possible only when a direct voltage is applied between the anode and the cathode. Transitions j1 and j3 are displaced in the forward direction and do not interfere with the passage of charge carriers. All voltage is applied to the middle j2 junction, which is shifted in the opposite direction. Near the j2 transition, a zone depleted of charge carriers is formed, called the space charge region. To turn on the thyristor GTO, a positive polarity U G is applied to the control electrode and the cathode along the control circuit (pin "+" to layer p). As a result, a circuit of current I G flows through the circuit.
Lockable thyristors impose stringent requirements on the steepness of the dIG / dt front and the amplitude of the control current IGM. Through the j3 junction, besides the leakage current, the inrush current I G begins to flow. The electrons creating this current will be injected from layer n into layer p. Further, some of them will be transferred by the electric field of the base j2 junction to the layer n.
At the same time, the counter injection of holes from the layer p into the layer n and further into the layer p will increase, i.e. there will be an increase in the current created by the minor charge carriers.
The total current passing through the base j2 junction exceeds the switching current, the thyristor is opened, after which charge carriers will freely pass through all four of its areas.
Phase 2 - conductive state. In the direct current flow mode, there is no need for a control current I G if the current in the anode circuit exceeds the value of the holding current. However, in practice, in order for all the structures of the thyristor to be switched off to be constantly in a conducting state, it is still necessary to maintain the current provided for this temperature mode. Thus, all the time of switching on and conducting the state, the control system generates a current pulse of positive polarity.
In the conducting state, all regions of the semiconductor structure ensure uniform movement of charge carriers (electrons from the cathode to the anode, and holes in the opposite direction). Through the j1, j2 transitions the anode current flows, through the j3 transition the total current of the anode and the control electrode.
Phase 3 - shutdown. To turn off the GTO thyristor with a constant voltage polarity U T (see Fig. 3), a negative voltage UGR is applied to the control electrode and the cathode along the control circuit. It causes a shutdown current, the flow of which leads to the resorption of the main charge carriers (holes) in the base layer p. In other words, there is a recombination of holes that have entered the p layer from the base layer n, and electrons that have entered the same layer through the control electrode.
As the base transition j2 is freed from them, the thyristor begins to lock. This process is characterized by a sharp decrease in the forward current I T of the thyristor in a short period of time to a small value I TQT (see Fig. 2). Immediately after locking the base transition j2, the transition j3 begins to close, however, due to the energy stored in the inductance of the control circuits, it is still in the ajar state for some time.
Fig. 2. Graphs of changes in the anode current (iT) and control electrode (iG)
After all the energy stored in the inductance of the control circuit is consumed, the j3 transition from the cathode side is completely locked. From this point on, the current through the thyristor is equal to the leakage current, which flows from the anode to the cathode through the control electrode circuit.
The process of recombination and, therefore, turning off the lockable thyristor depends largely on the steepness of the front dIGQ / dt and the amplitude I GQ of the reverse control current. In order to provide the necessary steepness and amplitude of this current, a voltage UG must be applied to the control electrode, which should not exceed the value allowed for j3 junction.
Phase 4 - blocking state. In the blocking state mode, a negative polarity voltage U GR from the control unit remains applied to the control electrode and the cathode. A total current I GR flows through the control circuit, consisting of a thyristor leakage current and a reverse control current passing through j3 junction. Transition j3 is shifted in the opposite direction. Thus, in the thyristor GTO, which is in the direct blocking state, two transitions (j2 and j3) are shifted in the opposite direction and two space charge regions are formed.
All the time off and blocking the control system generates a pulse of negative polarity.
The use of thyristors GTO, requires the use of special protective circuits. They increase the mass-dimensional parameters, the cost of the converter, sometimes require additional cooling devices, however, are necessary for the normal functioning of the devices.
The purpose of any protective circuit is to limit the rate of rise of one of the two parameters of electrical energy when switching a semiconductor device. In this case, the capacitors of the protective circuit CB (Fig. 3) are connected in parallel with the protected device T. They limit the rate of rise of the forward voltage dUT / dt when the thyristor is turned off.
LE chokes are installed in series with the device T. They limit the rate of rise of direct current dIT / dt when the thyristor is turned on. The values of dUT / dt and dIТ / dt for each device are normalized, they are indicated in reference books and passport data on devices.
Fig. 3. Protective circuit diagram
In addition to capacitors and chokes, additional elements are used in protective circuits that ensure the discharge and charge of reactive elements. These include: a diode DB, which shunts the resistor RВ when the thyristor T is turned off and the capacitor CB is charged, the resistor RВ, which limits the discharge current of the capacitor CB when the thyristor T is turned on
The control system (SU) contains the following functional blocks: a circuit comprising a circuit for generating a trigger pulse and a signal source to maintain the thyristor in the open state; contour of formation of the blocking signal; circuit to maintain the thyristor in the closed state.
Not all types of control systems require all of the listed blocks, but the contours of the formation of unlocking and locking pulses must contain each control system. In this case, it is necessary to ensure the galvanic isolation of the control circuit and the power circuit of the thyristor being switched off.
To control the operation of the switched-off thyristor, two main SUs are used, differing in the methods of applying a signal to the control electrode. In the case shown in fig. 4, the signals generated by the logic block St are galvanically isolated (potential separation), after which they are fed through the keys SE and SA to the control electrode of the thyristor T, in the second case, the signals first act on the keys SE (on) and SA (turn off ), which are at the same potential as the control system, then through the galvanic isolation devices, UE and UA are fed to the control electrode.
Depending on the location of the keys SE and SA, there are low potential (NPSU) and high potential (VPSU, Fig. 4) control schemes.
Fig. 4. Option control circuit
The control system of the NPSU is structurally simpler than the VPSU, but its capabilities are limited in terms of generating control signals of long duration operating in the mode of flow through the forward current thyristor, as well as in ensuring the steepness of the control pulses. For the formation of signals of long duration here it is necessary to use more expensive push-pull circuits.
In VPSU high steepness and increased duration of the control signal is achieved easier. In addition, here the control signal is used completely, while at the NPSU its value is limited by a potential-separation device (for example, a pulse transformer).
The information signal — the command to turn on or off — is usually supplied to the circuit via an optoelectronic converter.
In the mid-90s, the firms ABB and Mitsubishi developed a new type of thyristors, Gate Commutated Thyristor (GCT). Actually, the GCT is a further improvement of the GTO, or its modernization. However, the fundamentally new design of the control electrode, as well as the markedly different processes occurring when the device is turned off, makes it worthwhile to consider it.
GCT was developed as a device, devoid of the shortcomings characteristic of the GTO, so you first need to dwell on the problems that arise when working GTO.
The main disadvantage of the GTO is the large energy loss in the protective circuits of the device when it is switched. Increasing the frequency increases the loss, so in practice the GTO thyristors are switched with a frequency of no more than 250-300 Hz. The main losses occur in the resistor RВ (see Fig. 3) when the thyristor T is turned off and, consequently, the capacitor CB is discharged.
The capacitor CB is designed to limit the rate of rise of the forward voltage du / dt when the device is turned off. By making the thyristor insensitive to the du / dt effect, it was possible to abandon the snubber circuit (the switching path formation circuit), which was realized in the design of the GCT.
Management feature and design
The main feature of the thyristors GCT, as compared with the GTO devices, is the fast shutdown, which is achieved both by changing the control principle and improving the design of the device. Fast shutdown is realized by turning the thyristor structure into a transistor when the device is locked, which makes the device insensitive to the du / dt effect.
The GCT in the turn-on, conducting, and blocking states is controlled in the same way as the GTO. When you turn off the control GCT has two features:
- the control current Ig is equal to or exceeds the anode current Ia (for thyristors GTO Ig is less by 3-5 times);
- the control electrode has a low inductance, which makes it possible to achieve a control current rate of dig / dt equal to 3000 A / μs or more (for GTO thyristors, the value of dig / dt is 30-40 A / μs).
Fig. 5. The distribution of currents in the structure of the thyristor GCT when turning off
In fig. 5 shows the distribution of currents in the structure of the thyristor GCT when switching off the device. As mentioned, the turn-on process is similar to the inclusion of thyristors GTO. The shutdown process is different. After supplying a negative control pulse (-Ig) equal in magnitude to the anodic current (Ia), all the direct current passing through the device deviates into the control system and reaches the cathode, bypassing the j3 junction (between areas p and n). The j3 junction is shifted in the opposite direction, and the cathode transistor npn closes. Further turning off the GCT is similar to turning off any bipolar transistor, which does not require an external limitation of the forward voltage rise rate du / dt and, therefore, allows the absence of a snubber circuit.
The change in the design of the GCT is due to the fact that the dynamic processes that occur in the device when it is turned off, proceed one to two orders of magnitude faster than in the GTO. So, if the minimum off time and blocking state for a GTO is 100 µs, for GCT this value does not exceed 10 µs. The rise rate of the control current when the GCT is turned off is 3000 A / µs, the GTO does not exceed 40 A / µs.
To ensure high dynamics of switching processes, the design of the output of the control electrode and the connection of the device with the pulse shaper of the control system were changed. The output is made annular, encircling the device around the circumference. The ring passes through the ceramic case of the thyristor and contacts: inside with the cells of the control electrode; outside - with the plate connecting the control electrode with the pulse shaper.
Now GTO thyristors are made by several large firms in Japan and Europe: Toshiba, Hitachi, Mitsubishi, ABB, Eupec. Instrument parameters for voltage UDRM: 2500 V, 4500 V, 6000 V; on current ITGQM (maximum repeatable lockable current): 1000 A, 2000 A, 2500 A, 3000 A, 4000 A, 6000 A.
Thyristors GCT produced by the company "Mitsubishi" and "ABB". The devices are designed for UDRM voltage up to 4500 V and ITGQM current up to 4000 A.
Currently, the GCT and GTO thyristors are mastered at the Russian enterprise Electrovypryamitel (Saransk). Thyristors of the TZ-243, TZ-253, TZ-273, ZTA-173, ZTA-193, ZTF-193 series are produced (similar to GCT ) and others with a silicon wafer diameter of up to 125 mm and a voltage range of UDRM 1200-6000 V and current ITGQM 630-4000 A.
In parallel with lockable thyristors and for use in combination with them, Electrovypryamitel OJSC has developed and mastered fast-production diodes for damping (snubber) circuits and reverse current diodes, as well as a powerful pulse transistor for output stages of a control driver (control system).
Thanks to the concept of hard control (fine regulation of alloying profiles, mesatechnology, proton and electron irradiation to create a special distribution of controlled recombination centers, the technology of so-called transparent or thin emitters, the use of a buffer layer in the n - base region, etc.) when you turn off. The next major achievement in the technology of tightly controlled GTO (HD GTO) in terms of instrument, control and application was the idea of controlled devices based on a new "lockable thyristor with an integrated control unit (driver)" (English Integrated Gate-Commutated Thyristor (IGCT)) . Thanks to the hard control technology, uniform switching increases the safe operation of the IGCT to the limits limited by avalanche breakdown, i.e. up to the physical capabilities of silicon. No protective circuits are required to exceed du / dt. The combination with improved power loss allowed us to find new applications in the kilohertz range. The power required for control is reduced by 5 times compared to standard GTO, mainly due to the transparent design of the anode. The new IGCT instrument family, with monolithic integrated high-power diodes, has been developed for use in the range of 0.5 - 6 MV * A. With the existing technical capability of serial and parallel connection, IGCT devices allow increasing the power level up to several hundred megavolt - ampere.
With an integrated control unit, the cathode current decreases before the anode voltage begins to increase. This is achieved due to the very low inductance of the control electrode circuit, realized through the coaxial connection of the control electrode in combination with a multilayer board of the control unit. As a result, it became possible to achieve a switch-off current of 4 kA / µs. When the control voltage UGK = 20 V. when the cathode current becomes zero, the remaining anode current goes into the control unit, which has at this moment low resistance. Due to this, the energy consumption of the control unit is minimized.
Working with a "hard" control, the thyristor switches to the pnp mode when the state is locked from the pnpn state for 1 µs. Switching off occurs completely in the transistor mode, eliminating any possibility of a trigger effect.
Reducing the thickness of the device is achieved by using a buffer layer on the side of the anode. The buffer layer of power semiconductors improves the characteristics of traditional elements by reducing their thickness by 30% with the same direct breakdown voltage. The main advantage of thin elements is the improvement of technological characteristics with low static and dynamic losses. Such a buffer layer in a four-layer instrument requires the elimination of anode short circuits, but at the same time, the effective release of electrons during the shutdown is retained. The new IGCT device combines a buffer layer with a transparent anode emitter. The transparent anode is a pn junction with current-controlled emitter efficiency.
For maximum noise immunity and compactness, the control unit surrounds the IGCT, forming a single design with a chiller, and contains only that part of the circuit that is necessary to control the IGCT directly. As a result, the number of elements of the control unit is reduced, the parameters of heat dissipation, electrical and thermal overloads are reduced. Therefore, the cost of the control unit and the failure rate are also significantly reduced. The IGCT, with its integrated control unit, is easily fixed in the module and precisely connected to the power supply and control signal source via optical fiber. By simply opening the spring, thanks to a well-developed pressure contact system, a properly calculated pressure force is applied to the IGCT, creating electrical and thermal contact. Thus, maximum ease of assembly and maximum reliability is achieved. When operating IGCT without snubber, the reverse diode should also work without snubber. These requirements are fulfilled by a high-power diode in a pressure housing with improved characteristics, produced using an irradiation process in combination with classical processes. The possibilities for providing di / dt are determined by the operation of the diode (see Fig. 6).
Fig. 6. Simplified Three Phase Inverter on IGCT
The main manufacturer IGCT firm "ABB". The parameters of the thyristors for voltage U DRM : 4500 V, 6000 V; on current ITGQM: 3000 A, 4000 A.
The rapid development in the early 90s of power transistor technology led to the emergence of a new class of devices - Insulated Gate Bipolar Transistors (IGBT). The main advantages of IGBT are high values of the operating frequency, efficiency, simplicity and compactness of control circuits (due to the smallness of the control current).
The emergence in recent years of IGBT with an operating voltage up to 4500 V and the ability to switch currents up to 1800 A led to the displacement of lockable thyristors (GTO) in devices up to 1 MW and voltage up to 3.5 kV.
However, the new IGCT devices, capable of operating with switching frequencies from 500 Hz to 2 kHz and having higher parameters than IGBT transistors, combine the optimal combination of proven thyristor technology with their inherent low losses, and unsupported, highly efficient shutdown technology by affecting control electrode. The IGCT device today is the ideal solution for applications in the field of medium and high voltage power electronics.
Characteristics of modern powerful power switches with double-sided heat sink are given in Table. one.
Table 1. Characteristics of modern powerful power switches with double-sided heatsink
|Device type||Benefits||disadvantages||Areas of use|
|Traditional thyristor (SCR)||The lowest loss in the on state. The highest overload capacity. High reliability. Easy to connect in parallel and in series.||Not capable of forced locking on the control electrode. Low operating frequency.||DC drive; powerful power supplies; welding; melting and heating; static compensators; ac keys|
|GTO||The ability to controlled locking. Relatively high overload capacity. The possibility of serial connection. Operating frequencies up to 250 Hz at voltages up to 4 kV.||High losses in the on state. Very large losses in the control system. Sophisticated control and power supply systems for potential. Large switching losses.||Electric drive; static compensators; reactive power; uninterruptible power systems; induction heating|
|IGCT||The ability to controlled locking. The overload capacity is the same as that of the GTO. Low losses in the switched on state. Operating frequency - up to units, kHz. Built-in control unit (driver). The possibility of serial connection.||Not identified due to lack of operating experience.||Powerful power sources (inverter and rectifier substations of DC transmission lines); electric drive (voltage inverters for frequency converters and electric drives for various purposes)|
|IGBT||The ability to controlled locking. Highest operating frequency (up to 10 kHz). Simple non-power management system. Built-in driver.||Very high losses in the on state.||Electric drive (choppers); uninterruptible power systems; static compensators and active filters; key power sources|