Modern power thyristors lockable

Introduction

Establishment of semiconductor devices for power electronics began in 1953 when it became possible to produce high purity silicon and the formation of large disks of silicon. In 1955 was first established semiconductor controlled device having a structure and a four-called "SCR".

It involves feeding a control pulse to the electrode at a positive voltage between the anode and cathode. Turning off the thyristor is provided by reduction of flow through a direct current to zero, which developed a number of schemes inductive-capacitive switching circuits. They not only add value preobrazovaelya, but also impair its weight and size parameters, reduce reliability.

Therefore, simultaneously with the creation of a thyristor began research to ensure it is turned off on the control electrode. The main problem was to ensure rapid dispersal of the charge carriers in the base areas.

The first such thyristors appeared in 1960 in the United States. They got the name of Gate Turn Off (GTO). In our country, they are known as lockable or power thyristors.

In the mid-90s was designed lockable with a ring terminal thyristor control electrode. He was named Gate Commutated Thyristor (GCT) and was further development of the GTO-technology.

GTO thyristors

Device

Lockable SCR - fully controlled semiconductor device, based on the classic four-layer structure. Turned on and off its supply of positive and negative current pulses to the electrode control. In Fig. Figure 1 shows the symbol (a) and block diagram (b) turns off the thyristor. Like usual it has thyristor cathode K, anode A, the control electrode G. Differences in the structures of devices is in a different arrangement of horizontal and vertical layers of n-and p-conductivities.

In Fig. A. Lockable thyristor: a-symbol; b-block diagram

Undergone the greatest change in the device cathode layer n. It is divided into hundreds of unit cells are uniformly distributed over the area and connected in parallel. This performance is due to a desire to ensure a uniform decrease in current over the entire area of ​​the semiconductor structure when you turn off the device.

The base layer of p, even though that is designed as a whole, has a large number of contacts the control electrode (cathode approximately equal to the number of cells) are also uniformly distributed over the area and connected in parallel. The base layer is made n similar to the corresponding layer of conventional thyristor.

The anode layer p has shunts (zones n), connecting the n-base contact with the anode through a small distributed resistance. Anode shunt thyristors used in non-reverse blocking capability. They are designed to reduce time off the device by improving the recovery of charges from the base region of n.

Basic version GTO thyristor tablet with a four-silicon plate sandwiched by termokompensiruyuschie molybdenum disks between two copper bases possessing high thermal and electrical conductivity. Since the silicon plate is contacted control electrode, which has concluded in a ceramic housing. The device is clamped contact surfaces between the two halves of the coolers, insulated from each other and having a structure defined by the type of cooling system.

Principle of operation

In a series of GTO thyristors are four phases: the inclusion, the conducting state, shutting down and locking state.

In the schematic sectional thyristor structure (Fig. 1, b) the lower end of the anode structure. The anode is in contact with a layer of p.Zatem bottom-up is followed by a base layer of n, the base layer of p (which has the output of the control electrode), a layer of n, direct contact with the cathode terminal. Four layers form three pn junction: j1 between layers p and n; j2 between layers n and p; j3 between layers p and n.

Phase 1 - inclusion. The transition structure of the thyristor blocking state to the conducting (inclusion) is only possible by applying a direct voltage between the anode and cathode. Transitions j1 and j3 move forward and do not hinder the passage of charge carriers. All voltage is applied to the average transition j2, which moves in the opposite direction. About j2 transition zone is formed, depleted of charge carriers, known as a region of space charge. To turn on the thyristor GTO, to the control electrode and the cathode of the control circuit voltage is applied with positive polarity U _G (O "+" to the layer of p). As a result, a circuit switching current I _G.

Lockable thyristors impose strict requirements on the steepness of the front dIG / dt and amplitude of the current management of IGM. After the transition j3, except for the leakage current begins to flow switching current I _G. Creating this current the electrons are injected from the n layer in the layer p. Further, some of them will be tossing the electric field of the underlying transition j2 in the layer n.

At the same time increase the counter of the hole injection layer in the layer of p n, and later in the layer p, ie will increase the current generated minority carriers.

The total current passing through the base transition j2, exceeds the switching current, the thyristor is opened, after which the charge carriers are free to navigate through all four of his field.

Phase 2 - the conducting state. In the course of the direct current is not necessary to control current I _G, if the anode current in the circuit exceeds the holding current. However, in practice, to ensure that all structures were constantly when turned off thyristor conduction, is still necessary to maintain the current, provided for a given temperature. Thus, all the time on and the conductive state of the control system generates a current pulse of positive polarity.

In the superconducting state, all of the semiconductor structure provides a uniform motion of charge carriers (electrons from the cathode to the anode, the hole - in the opposite direction). Through transitions j1, j2 anode current flows through the transition j3 - the total current of the anode and control electrode.

Phase 3 - off. To turn off the GTO thyristor with a constant polarity of the voltage U _T (see Fig. 3) to the control electrode and the cathode of the control circuit voltage of negative polarity is applied UGR. It causes the current turn-off, which leads to the occurrence of resorption of the main charge carriers (holes) in the base layer of p. In other words, there is a recombination of holes, placed at the p layer of the base layer n, and the electrons collected in the same layer on the control electrode.

With the release of their basic transition j2 thyristor starts to lock up. This process is characterized by a sharp decrease in the direct current I _T thyristor in a short time to a small value of I _TQT (see Fig. 2). Immediately after closing the base of transition begins to close the transition j2 j3, but at the expense of energy stored in the inductance of the control circuit for some time, he is ajar condition.

In Fig. Two. Graphs of anode current (iT) and the control electrode (iG)

After all the energy stored in the inductance of the control circuit will be used up, the transition j3 from the cathode side is completely locked. Since then, the current through the thyristor is leakage current, which flows from the anode to the cathode through the chain of control electrode.

Recombination process and, therefore, lockable off thyristor depends on the steepness of the front dIGQ / dt and amplitude of the reverse current I _GQ management. To provide the necessary slope and amplitude of this current, the control electrode voltage is required to submit UG, which shall not exceed the value allowed for the transition j3.

Phase 4 - condition.Contact blocking mode locking state to the control electrode and the cathode is applied voltage of negative polarity U _GR from the control unit. In the control circuit takes the total current I _GR, consisting of a thyristor leakage current and reverse gate current flowing through the junction j3. J3 transition is shifted in the opposite direction. Thus, the thyristor GTO, located in the forward blocking state, two transitions (j2 and j3) are shifted in the opposite direction and are formed by two space-charge region.

All the time off and blocking the state of the control system forms the pulse of negative polarity.

The protective circuit

The use of thyristors GTO, requires special protection circuits. They increase the weight and size figures, the cost of the converter, and sometimes require additional cooling devices, but are necessary for normal functioning of the devices.

Назначение любой защитной цепи - ограничение скорости нарастания одного из двух параметров электрической энергии при коммутации полупроводникового прибора. При этом конденсаторы защитной цепи СВ (рис. 3) подключают параллельно защищаемому прибору Т. Они ограничивают скорость нарастания прямого напряжения dUT/dt при выключении тиристора.

Дроссели LE устанавливают последовательно с прибором Т. Они ограничивают скорость нарастания прямого тока dIT/dt при включении тиристора. Значения dUT/dt и dIТ/dt для каждого прибора нормированы, их указывают в справочниках и паспортных данных на приборы.

In Fig. Three. Схема защитной цепи

Кроме конденсаторов и дросселей, в защитных цепях используют дополнительные элементы, обеспечивающие разряд и заряд реактивных элементов. К ним относятся: диод DВ, который шунтирует резистор RВ при выключении тиристора Т и заряде конденсатора СВ, резистор RВ, ограничивающий ток разряда конденсатора СВ при включении тиристора Т.

Management system

Система управления (СУ) содержит следующие функциональные блоки: включающий контур, состоящий из схемы формирования отпирающего импульса и источника сигнала для поддержания тиристора в открытом состоянии; контур формирования запирающего сигнала; контур поддержания тиристора в закрытом состоянии.

Not all types of SU need all of these blocks, but the contours of the formation and unlock locking pulse must contain each SU. It is necessary to ensure the isolation control circuit and power circuit when turned off thyristor.

To control the operation when turned off thyristor used two major SU, differing ways of signal to the control electrode. In the case shown in Fig. 4, the signals generated by the logical unit St, subject to galvanic isolation (separation of potentials), followed by a presentation of their keys through the SE and SA on the gate electrode when turned off thyristor T. In the second case, the signals act on the first clues SE (included) and SA (off ) under the same potential as the SS, then through a galvanic isolation device UE and UA are fed to the control electrode.

Depending on the location of key SE and SA distinguish low-potential (NPSU) and high-potential (VPSU, Fig. 4) control circuit.

In Fig. 4. Option Chain Management

The control system NPSU structurally simpler than VPSU, but its capabilities are limited with regard to the formation of the control signals of long duration operating mode in the mode of flow through the thyristor forward current, as well as providing the steepness of the control pulses. For the generation of signals of long duration is necessary to use more expensive push-pull circuit.

In VPSU high toughness and increased the duration of the control signal is achieved easily. In addition, the control signal is used in full, while its value is limited NPSU device sharing capabilities (for example, a pulse transformer).

The information signal - a command to turn on or off - usually applied to the circuit through the optoelectronic converter.

GCT thyristors

In the mid 90's firms "ABB" and "Mitsubishi" has developed a new type of thyristor Gate Commutated Thyristor (GCT). In fact, GCT is a further improvement of GTO, or upgrade. However, a fundamentally new design of the control electrode, and the markedly different processes that occur when you turn off the device, making it appropriate consideration.

GCT is designed as a unit, devoid of the shortcomings of the GTO, so you must first stop on the problems encountered when using GTO.

The main drawback is the large GTO energy losses in the circuit protective device when switching. Повышение частоты увеличивает потери, поэтому на практике тиристоры GTO коммутируются с частотой не более 250-300 Гц. Основные потери возникают в резисторе RВ (см. рис. 3) при выключении тиристора Т и, следовательно, разряде конденсатора СВ.

Конденсатор СВ предназначен для ограничения скорости нарастания прямого напряжения du/dt при выключении прибора. Сделав тиристор не чувствительным к эффекту du/dt, создали возможность отказаться от снабберной цепи (цепи формирования траектории переключения), что и было реализовано в конструкции GCT.

Особенность управления и конструкции

Основной особенностью тиристоров GCT, по сравнению с приборами GTO, является быстрое выключение, которое достигается как изменением принципа управления, так и совершенствованием конструкции прибора. Быстрое выключение реализуется превращением тиристорной структуры в транзисторную при запирании прибора, что делает прибор не чувствительным к эффекту du/dt.

GCT в фазах включения, проводящего и блокирующего состояния управляется также, как и GTO. При выключении управление GCT имеет две особенности:

• ток управления Ig равен или превосходит анодный ток Ia (для тиристоров GTO Ig меньше в 3 - 5 раз);
• управляющий электрод обладает низкой индуктивностью, что позволяет достичь скорости нарастания тока управления dig/dt, равной 3000 А/мкс и более (для тиристоров GTO значение dig/dt составляет 30-40 А/мкс).

In Fig. Five. Распределение токов в структуре тиристора GCT при выключении

In Fig. 5 показано распределение токов в структуре тиристора GCT при выключении прибора. Как указывалось, процесс включения подобен включению тиристоров GTO. Процесс выключения отличен. После подачи отрицательного импульса управления (-Ig) равного по амплитуде величине анодного тока (Ia), весь прямой ток, проходящий через прибор, отклоняется в систему управления и достигает катода, минуя переход j3 (между областями p и n). Переход j3 смещается в обратном направлении, и катодный транзистор npn закрывается. Дальнейшее выключение GCT аналогично выключению любого биполярного транзистора, что не требует внешнего ограничения скорости нарастания прямого напряжения du/dt и, следовательно, допускает отсутствие снабберной цепочки.

Изменение конструкции GCT связано с тем, что динамические процессы, возникающие в приборе при выключении, протекают на один - два порядка быстрее, чем в GTO. Так, если минимальное время выключения и блокирующего состояния для GTO составляет 100 мкс, для GCT эта величина не превышает 10 мкс. Скорость нарастания тока управления при выключении GCT составляет 3000 А/мкс, GTO - не превышает 40 А/мкс.

Чтобы обеспечить высокую динамику коммутационных процессов, изменили конструкцию вывода управляющего электрода и соединение прибора с формирователем импульсов системы управления. Вывод выполнен кольцевым, опоясывающим прибор по окружности. Кольцо проходит сквозь керамический корпус тиристора и контактирует: внутри с ячейками управляющего электрода; снаружи - с пластиной, соединяющей управляющий электрод с формирователем импульсов.

Сейчас тиристоры GTO производят несколько крупных фирм Японии и Европы: "Toshiba", "Hitachi", "Mitsubishi", "ABB", "Eupec". Параметры приборов по напряжению UDRM : 2500 В, 4500 В, 6000 В; по току ITGQM (максимальный повторяющийся запираемый ток): 1000 А, 2000 А, 2500 А, 3000 А, 4000 А, 6000 А.

Тиристоры GCT выпускают фирмы "Mitsubishi" и "ABB". Приборы рассчитаны на напряжение UDRM до 4500 В и ток ITGQM до 4000 А.

В настоящее время тиристоры GCT и GTO освоены на российском предприятии ОАО "Электровыпрямитель" (г. Саранск).Выпускаются тиристоры серий ТЗ-243, ТЗ-253, ТЗ-273, ЗТА-173, ЗТА-193, ЗТФ-193 (подобен GCT) и др. с диаметром кремниевой пластины до 125 мм и диапазоном напряжений UDRM 1200 - 6000 В и токов ITGQM 630 - 4000 А.

Параллельно с запираемыми тиристорами и для использования в комплекте с ними в ОАО "Электровыпрямитель" разработаны и освоены в серийном производстве быстровостанавливающиеся диоды для демпфирующих (снабберных) цепей и диоды обратного тока, а также мощный импульсный транзистор для выходных каскадов драйвера управления (система управления).

Тиристоры IGCT

Through the concept of strict control (fine regulation of doping profiles, mezatehnologiya, proton and electron irradiation for the creation of a special distribution of controlled recombination centers, the technology is the so-called clear or thin emitters, the use of a buffer layer in the n - base region, and others) managed to achieve significant performance improvement GTO when turned off. The next major advancement in technology tightly managed GTO (HD GTO) from the viewpoint of device management and application was the idea of ​​managed devices based on the new "locked thyristor with integrated control unit (driver)" (English Integrated Gate-Commutated Thyristor (IGCT)) . Thanks to the hard control technology enhances switching even safe operating area IGCT to the limits bounded by avalanche breakdown, ie, to the physical capabilities of silicon. No protective circuits against excess du / dt. The combination with improved power loss allowed us to find new applications in the kilohertz range. Power needed for control, reduced 5-fold compared with standard GTO, mainly due to the transparent anode structure. A new family of devices IGCT, with monolithic integrated high power diodes have been developed for use in the range 0.5 - 6 MB * A. With the current technical possibilities of serial and parallel connections IGCT devices can increase the power level of up to several hundred megavolt - ampere.

With the integrated control unit cathodic current decreases before the anode voltage begins to increase. This is achieved by a very low inductance of the control electrode, realized by a coaxial connection to the control electrode coupled with a multilayer board control unit. As a result, it became possible to achieve values ​​of speed when turned off current of 4 kA / ms. When the voltage control UGK = 20 V. When the cathode current becomes zero, the remainder of the anode current passes into the control unit, which has at this moment a low resistance. In this way energy consumption is minimized by the control unit.

Working with "hard" control, the thyristor turns on closing of the pnpn state in pnp mode for 1 ms. Turning off the transistor is in full mode, eliminating any possibility of a trigger effect.

Reducing the thickness of the device is achieved by using a buffer layer on the anode side. Buffer layer improves the characteristics of power semiconductors traditional elements by reducing their thickness by 30% during the same forward breakdown voltage. Main Advantages of thin elements - improved processing characteristics at low static and dynamic losses. Such a buffer layer in a four-unit requires removal of anode shorts, but it remains an effective release of electrons during the shutdown. The new device IGCT buffer layer combined with a transparent anode emitter. Transparent anode - a pn junction with a controllable current efficiency of the emitter.

For maximum noise immunity and the compactness of the control unit surrounds the IGCT, forming a single structure with cooling and contains only the part of the scheme, which is necessary to control directly IGCT. As a consequence, reduced the number of elements of the control unit, reduced scattering parameters of heat, electrical and thermal loads. Therefore, also significantly reduced the cost of the control unit and the failure rate. IGCT, with its integrated control unit, easily fixed in the module and just connected to the power source and the source of the control signal through an optical fiber. By simply opening the spring, thanks to elaborate the pressure contact system, is attached to the IGCT to calculate the clamping force, which creates an electrical and thermal contact. Thus, the maximum relief is achieved by the assembly and the highest reliability. When working without IGCT snubber, free-wheeling diode should also work without snubber. These requirements are a high power diode performs in the pressure housing with improved characteristics, produced using the process of irradiation in combination with classical processes. Opportunities for the di / dt of the diode are determined (see Fig. 6).

In Fig. 6. A simplified diagram of a three-phase inverter IGCT

The main producer of IGCT company "ABB". Options thyristor voltage U _DRM: 4500, 6000; current ITGQM: 3000 A and 4000 A.

Conclusion

The rapid development in the early 90's power transistor technology led to the emergence of a new class of devices - bipolar transistors with insulated gate (IGBT - Insulated Gate Bipolar Transistors). The main advantages are the high values ​​of the IGBT operating frequency, efficiency, simplicity and compactness of the control schemes (due to the smallness of the current administration).

The emergence in recent years the IGBT has an operating voltage of 4500 V and the ability to switch currents up to 1800 led to the ousting of a locked thyristor (GTO) in devices up to 1 MW and the voltage to 3.5 kV.

However, new devices IGCT, capable of operating at frequencies shift from 500 Hz to 2 kHz and having a higher specification compared with IGBT transistors, combine the optimum combination of proven technology of thyristors with their inherent low loss, and bessnabbernoy, high technology off by acting on the control electrode. The device IGCT today - the ideal solution for applications in power electronics, medium and high voltages.

Характеристики современных мощных силовых ключей с двусторонним теплоотводом приведены в табл. A.

Таблица 1. Характеристики современных мощных силовых ключей с двусторонним теплоотводом