Modern Power Lockable Thyristors
The creation of semiconductor devices for power electronics began in 1953 when it became possible to obtain high-purity silicon and the formation of large silicon disks. In 1955, a semiconductor controlled device was first created, which has a four-layer structure and is called a thyristor.
It was switched on by applying a pulse to the control electrode at a positive voltage between the anode and cathode. Turning off the thyristor is ensured 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 converter, but also worsen its weight and size indicators, and reduce reliability.
Therefore, simultaneously with the creation of the thyristor, research began aimed at ensuring its shutdown by 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 turn-off thyristors.
In the mid-90s, a lockable thyristor with a ring output of a control electrode was developed. It was called Gate Commutated Thyristor (GCT) and was a further development of GTO technology.
Lockable thyristor is a fully controllable semiconductor device based on a classic 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 structural diagram (b) of the thyristor to be switched off. Like a conventional thyristor, it has a cathode K, anode A, and a control electrode G. The differences in the structures of the devices lie in a different arrangement of horizontal and vertical layers with n- and p-conductivities.
Fig. 1. Lockable thyristor:
The cathode layer n has undergone the greatest change. It is divided into several hundred unit 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 being made as a whole, has a large number of contacts of the control electrode (approximately equal to the number of cathode cells), 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 reverse blocking ability. They are designed to reduce the turn-off time of the device by improving the conditions for extracting charges from the base region n.
The main version of GTO thyristors is tablet with a four-layer silicon wafer sandwiched through heat-compensating molybdenum disks between two copper bases with high thermal and electrical conductivity. A control electrode in contact with the silicon wafer has a lead in a ceramic case. The device is clamped by the contact surfaces between the two halves of the coolers, isolated from each other and having a design determined by the type of cooling system.
Four phases are distinguished in the GTO thyristor cycle: on, conductive state, off and blocking state.
On the schematic section of the thyristor structure (Fig. 1, b), the lower terminal of the structure is anode. The anode is in contact with the layer p. Then, from bottom to top, there follows: a base layer n, a base layer p (having a lead of the control electrode), a layer n directly in 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 - inclusion. The transition of the thyristor structure from the blocking state to the conducting state (switching on) is possible only when a direct voltage is applied between the anode and cathode. Transitions j1 and j3 are shifted in the forward direction and do not impede the passage of charge carriers. All voltage is applied to the middle j2 junction, which is biased in the opposite direction. Near j2 transition, a zone depleted in charge carriers is formed, which is called the space charge region. To turn on the thyristor GTO, a positive polarity voltage U G is applied to the control electrode and the cathode along the control circuit (terminal “+” to the layer p). As a result, the switching current I G flows through the circuit.
Lockable thyristors impose stringent requirements on the slope dIG / dt and the amplitude IGM of the control current. In addition to the leakage current j3, the switching current I G starts flowing through j3. The electrons generating this current will be injected from layer n into layer p. Further, some of them will be transferred by the electric field of the basic transition j2 to layer n.
At the same time, counter injection of holes from layer p to layer n and then to layer p will increase, i.e. there will be an increase in current created by minority charge carriers.
The total current passing through the base j2 transition exceeds the switching current, the thyristor opens, after which the charge carriers will freely pass through all four of its regions.
Phase 2 is a 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 structures of the switched-off thyristor to be constantly in a conducting state, it is still necessary to maintain the current provided for a given temperature regime. Thus, the control system generates a current pulse of positive polarity all the time of switching on and conducting state.
In the conducting state, all regions of the semiconductor structure provide uniform motion of charge carriers (electrons from the cathode to the anode, holes in the opposite direction). Anode current flows through junctions j1, j2, and the total current of the anode and control electrode passes through j3.
Phase 3 - shutdown. To turn off the GTO thyristor with the voltage polarity U T unchanged (see Fig. 3), a negative polarity voltage UGR is applied to the control electrode and the cathode via the control circuit. It causes a turn-off current, the flow of which leads to the absorption of the main charge carriers (holes) in the base layer p. In other words, there is a recombination of holes entering the layer p from the base layer n and electrons entering 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 direct current I T of the thyristor over a short period of time to a small value of I TQT (see Fig. 2). Immediately after locking the base j2, the j3 junction begins to close, however, due to the energy stored in the inductance of the control circuits, it is still in an ajar state for some time.
Fig. 2. Graphs of changes in the current of the anode (iT) and the control electrode (iG)
After all the energy stored in the inductance of the control circuit is consumed, the transition j3 from the cathode side is completely locked. From this moment, 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 largely depends 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 is required to be applied to the control electrode, which must not exceed the value allowed for j3.
Phase 4 - blocking state. In the blocking state mode, the negative polarity U GR from the control unit remains applied to the control electrode and cathode. The total current I GR , which consists of the thyristor leakage current and the reverse control current passing through j3, flows through the control circuit. Transition j3 is shifted in the opposite direction. Thus, in a GTO thyristor in a direct blocking state, two transitions (j2 and j3) are biased in the opposite direction and two space charge regions are formed.
All the time of shutdown and blocking state, the control system generates a pulse of negative polarity.
The use of thyristors GTO requires the use of special protective circuits. They increase the weight and dimensions, the cost of the converter, sometimes require additional cooling devices, but are necessary for the normal functioning of the devices.
The purpose of any protective circuit is to limit the slew rate of one of two parameters of electrical energy when switching a semiconductor device. In this case, the capacitors of the CB protective circuit (Fig. 3) are connected in parallel with the protected device T. They limit the rate of increase of the forward voltage dUT / dt when the thyristor is turned off.
The chokes LE are installed in series with the device T. They limit the slew rate of the forward current dIT / dt when the thyristor is turned on. The dUT / dt and dIT / dt values for each device are normalized, they are indicated in the directories and passport data on the devices.
Fig. 3. Protective circuit diagram
In addition to capacitors and chokes, additional elements are used in the protective circuits that ensure the discharge and charge of the reactive elements. These include: a diode DB, which shunts the resistor RB when the thyristor T is turned off and the capacitor CB is charged, resistor RB, which limits the discharge current of the CB capacitor 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 for maintaining the thyristor in the open state; loop forming signal loop; closed circuit thyristor.
Not all of the listed blocks are needed for all types of control systems, but each control system must contain the contours of the formation of unlocking and locking pulses. In this case, it is necessary to provide galvanic isolation of the control circuit and the power circuit of the turn-off thyristor.
To control the operation of the switch-off thyristor, two main control systems are used, differing in the ways of supplying the signal to the control electrode. In the case presented 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 (off) ), which are under 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 distinguish between low-potential (NPSU) and high-potential (VPSU, Fig. 4) control circuits.
Fig. 4. Option control circuit
The control system of the NPSU is structurally simpler than the VPSU, however, its capabilities are limited in relation to the formation of control signals of long duration, operating in the mode in the flow mode through the thyristor direct current, as well as to ensure the steepness of the control pulses. For the formation of signals of long duration, 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 fully used, while in the NPSU its value is limited by a potential separation device (for example, a pulse transformer).
An information signal - a command to turn on or off - is usually fed to the circuit through an optoelectronic converter.
In the mid-90s, ABB and Mitsubishi developed a new type of Gate Commutated Thyristor (GCT) thyristor. Actually, GCT is a further improvement of GTO, or its modernization. However, the fundamentally new design of the control electrode, as well as the markedly different processes that occur when the device is turned off, make it advisable to consider it.
GCT was developed as a device devoid of the disadvantages characteristic of GTO, so first you need to dwell on the problems that arise during the operation of GTO.
The main disadvantage of GTO is the large energy loss in the protective circuits of the device when it is switched. Increasing the frequency increases losses, therefore, in practice, GTO thyristors are switched with a frequency of not more than 250-300 Hz. The main losses occur in the resistor RB (see Fig. 3) when the thyristor T is turned off and, consequently, the discharge of the capacitor CB.
The CB capacitor is designed to limit the slew rate of the forward voltage du / dt when the device is turned off. Having made the thyristor insensitive to the du / dt effect, we created the opportunity to abandon the snubber circuit (switching trajectory formation circuit), which was implemented in the GCT design.
Management and design feature
The main feature of GCT thyristors, in comparison with GTO devices, is a quick 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 one when the device is locked, which makes the device insensitive to the du / dt effect.
GCT in the phases of inclusion, conductive and blocking state is controlled in the same way as GTO. When turned off, the GCT control has two features:
- the control current Ig is equal to or greater than the anode current Ia (for thyristors GTO Ig is 3-5 times less);
- the control electrode has a low inductance, which makes it possible to achieve a slew rate of the dig / dt control current of 3000 A / μs or more (for GTO thyristors, the dig / dt value is 30-40 A / μs).
Fig. 5. Current distribution in the structure of the GCT thyristor when turned off
In fig. 5 shows the current distribution in the structure of the GCT thyristor when the device is turned off. As indicated, the turn-on process is similar to turning on the GTO thyristors. The shutdown process is different. After applying a negative control pulse (-Ig) equal in magnitude to the anode current (Ia), all the direct current passing through the device is deflected into the control system and reaches the cathode, bypassing j3 transition (between regions p and n). The j3 junction is biased in the opposite direction, and the cathode transistor npn is closed. 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 du / dt and therefore allows the absence of a snubber chain.
The change in the design of the GCT is due to the fact that the dynamic processes that occur in the device during shutdown occur one to two orders of magnitude faster than in GTO. So, if the minimum turn-off and blocking time for GTO is 100 μs, for GCT this value does not exceed 10 μs. The slew rate of the control current when the GCT is turned off is 3000 A / μs, GTO does not exceed 40 A / μs.
To ensure high dynamics of switching processes, we changed the design of the output of the control electrode and the connection of the device with the pulse shaper of the control system. The conclusion is made circular, encircling the device around the circumference. The ring passes through the ceramic thyristor case and contacts: inside with cells of the control electrode; outside, with a plate connecting the control electrode to the pulse shaper.
Now GTO thyristors are produced by several large companies in Japan and Europe: Toshiba, Hitachi, Mitsubishi, ABB, Eupec. UDRM voltage parameters: 2500 V, 4500 V, 6000 V; Current ITGQM (Maximum Repeatable Lockable Current): 1000 A, 2000 A, 2500 A, 3000 A, 4000 A, 6000 A.
GCT thyristors are manufactured by Mitsubishi and ABB. The devices are designed for voltage UDRM up to 4500 V and current ITGQM up to 4000 A.
At present, thyristors GCT and GTO have been developed at the Russian enterprise OAO Elektrovypryamitel (Saransk). Thyristors of the series TZ-243, TZ-253, TZ-273, ZTA-173, ZTA-193, ZTF-193 (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 currents ITGQM 630 - 4000 A.
In parallel with the lockable thyristors and for use in conjunction with them, OAO Elektrovypryamitel developed and mastered in mass production fast-mounted diodes for damping (snubber) circuits and reverse current diodes, as well as a powerful pulse transistor for the output stages of the control driver (control system).
Thanks to the concept of tight control (fine regulation of alloying profiles, mesotechnology, proton and electron irradiation to create a special distribution of controlled recombination centers, the technology of the so-called transparent or thin emitters, the use of a buffer layer in the n - base region, etc.), a significant improvement in GTO characteristics was achieved when turned off. The next major advance in hard-drive GTO (HD GTO) technology in terms of instrumentation, control and application was the idea of instrumentation based on the new “Lockable Thyristor with Integrated Control Unit (Driver)” (Integrated Gate-Commutated Thyristor (IGCT)) . Thanks to the hard control technology, uniform switching increases the IGCT safe operation area to the limits limited by avalanche breakdown, i.e. to the physical capabilities of silicon. No protective circuits against du / dt excess are required. The combination with improved indicators of power losses made it possible to find new applications in the kilohertz range. The power required for control is reduced by 5 times compared to standard GTOs, mainly due to the transparent design of the anode. The new IGCT family of devices, with monolithic integrated high power diodes, has been developed for use in the 0.5 - 6 MV * A range. With the existing technical feasibility of serial and parallel connection, IGCT devices allow to increase the power level to several hundred megavolts - amperes.
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, which is realized due to 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 value of 4 kA / μs. At a control voltage of UGK = 20 V. when the cathode current becomes equal to zero, the remaining anode current passes to the control unit, which at this moment has a low resistance. Due to this, the energy consumption of the control unit is minimized.
When operating under "hard" control, the thyristor switches when it switches from pnpn state to pnp mode in 1 μs. Turning off occurs completely in transistor mode, eliminating any possibility of a trigger effect.
Reducing the thickness of the device is achieved through the use of 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% at 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 device requires the elimination of anode shorts, but at the same time, the effective release of electrons during shutdown is maintained. In the new IGCT, the buffer layer is combined with a transparent anode emitter. A 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 cooler, and contains only the part of the circuit that is needed 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. IGCT, with its integrated control unit, is easily fixed in the module and precisely connected to the power supply and the control signal source via fiber optic. By simply opening the spring, thanks to the well-developed pressure contact system, a correctly calculated pressure force is applied to the IGCT, creating electrical and thermal contact. Thus, maximum assembly facilitation and maximum reliability are achieved. When IGCT operates without snubber, the reverse diode must also work without snubber. These requirements are met by a high-power diode in a clamping case with improved characteristics, produced using the 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 circuit on IGCT
The main manufacturer of IGCT company "ABB". Parameters of thyristors for voltage U DRM : 4500 V, 6000 V; Current ITGQM: 3000 A, 4000 A.
The rapid development of power transistor technology in the early 90s led to the emergence of a new class of devices - Insulated Gate Bipolar Transistors (IGBTs). The main advantages of IGBT are high operating frequency, efficiency, simplicity and compactness of control circuits (due to the small control current).
The emergence in recent years of IGBT with an operating voltage of up to 4500 V and the ability to switch currents up to 1800 A has led to the displacement of lockable thyristors (GTO) in devices with a capacity of up to 1 MW and a voltage of 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 compared to IGBT transistors, combine the optimal combination of proven thyristor technologies with their inherent low losses, and a non-dial-up, highly efficient turn-off technology by influencing control electrode. The IGCT is today the ideal solution for medium and high voltage power electronics applications.
The characteristics of modern powerful power switches with double-sided heat sink are given in table. one.
Table 1. Characteristics of modern powerful power keys with double-sided heat sink
|Device type||Benefits||disadvantages||Areas of use|
|Traditional thyristor (SCR)||The lowest losses when turned on. Highest overload capacity. High reliability. Easily connected in parallel and in series.||Not capable of forced locking on the control electrode. Low operating frequency.||DC drive; powerful power sources; welding; melting and heating; static compensators; alternating current keys|
|GTO||The ability to controlled locking. Relatively high overload capacity. Possibility of serial connection. Operating frequencies up to 250 Hz with voltage up to 4 kV.||High losses when turned on. Very large losses in the control system. Sophisticated control and energy supply systems for potential. Large switching loss.||Electric drive; static compensators; reactive power; uninterruptible power systems; induction heating|
|IGCT||The ability to controlled locking. Overload capacity is the same as GTO. Low on-state losses on switching. Operating frequency - up to units, kHz. Built-in control unit (driver). Possibility of serial connection.||Not identified due to lack of operating experience||Powerful power sources (inverter and rectifier substations of direct current transmission lines); electric drive (voltage inverters for frequency converters and electric drives for various purposes)|
|IGBT||The ability to controlled locking. The highest operating frequency (up to 10 kHz). Simple non-energy-intensive control system. The built-in driver.||Very high losses when turned on.||Electric drive (choppers); uninterruptible power systems; static compensators and active filters; key power supplies|