Thyristor

Thyristor is the name of a component in electrical engineering. It is a portmanteau of the two names thyratron and transistor.

A thyristor is a semiconductor device which is composed of four or more semiconductor layers of alternating doping. Thyristors are switchable components, that is, they are in the initial state non-conductive and can be turned on by a small current on the gate electrode. After switching on the thyristor remains conductive even without gate current. It is switched off by falling below a minimum current, the so-called holding current.

• 5.1 Small power
• 5.2 Average power
• 5.3 High performance

Design and operation

General

The thyristor has three pn-junctions in the pnpn sequence. It has as a diode, an anode and a cathode opposed to the diode is not a gate terminal to do so.

In the ground state of the thyristor in both directions is blocking. In the forward direction, it blocks up to a certain ignition voltage ( Nullkippspannung for a gate -to-cathode voltage of 0 V). It can also be connected below the ignition voltage to the conductive state by a positive current pulse to the gate. In the reverse direction, it blocks the current like a normal diode.

There are several ways of ignition:

• Conventional Control current ( positive current or current pulse to the gate ),
• Light ignition ( photothyristor )

• Exceeding the Nullkippspannung ( overhead ignition or breakover ). Only allowed in the so-called Dynistor, a special design of thyristor, which allows the overhead ignition and is the successor of the former component Shockley diode.
• Exceeding the permissible voltage rise rate
• Temperature increase

In practice, the thyristor is used as a controllable diode.

Turn on

By current injection to the third layer ( at the gate of driving ) of the thyristor can be (switched conductive) ignited. This requires a positive voltage between the anode and cathode and a minimum current through the middle barrier layer. Characteristic for the switch of the thyristor is that the process is supported by a positive feedback. The sequence of the switching-on is therefore - as opposed to other power semiconductors, - as not to affect on the gate in speed. Problem, the current density in the third layer is in the ignition. Upon injection of the electrons, the conductive layer is at the entry point. Until the entire silicon surface conductive, the current is concentrated on the already conducting region in which the total power dissipation is implemented. In this case, the power loss density exceed the permissible value and lead to local increases in temperature on the diffusion temperature or even the melting temperature (1683 K) of silicon addition. Therefore, it is important that the current increase rate (critical current gradient ) does not exceed a certain value, but this is secured in most cases by inductances of the load and the line. A capacitive load is to be switched, the current slew rate must be limited by additional measures as necessary. For highly inductive loads, however, lags the current rise after power is applied. It may therefore happen that is not reached immediately after the expiry of the ignition pulse of the so-called latch-up, this refers to the minimum value of the current which must flow through the thyristor so that it remains conductive at power without gate current. This leads to undefined switching operations, which are also known from ( working with triac ) AC dimmers ( phase control ) ago, while one can often observe a flicker of such controlled lamps in the lower load range. In order to avoid the effect, a snubber network is used, this refers to a RC circuit ( series circuit of a resistor and a capacitor, typical values ​​: 470Ω and 100nF ), which is connected between the anode and cathode of the Thyristorstrecke. Upon ignition, the capacitor discharges through the resistor and the thyristor and thus provides for a short time a small power available to pass safely the Latching Current. There often is a thyristor connected in series inductor for radio interference suppression.

Switch off

Cleared ( put in the blocking state ), the thyristor by falling below the holding current (English Holding Current), generally by turning off or reversing the voltage in the load circuit, or in the current zero-crossing of the load circuit (eg in the rectifier ). The speed of this process is limited by the tq recovery time that is required so that the thyristor after the end of the power line phase regains its full control and blocking capability. This will only be recovered if the authoritative for middle barrier layer has been cleared by recombination of charge carriers. The recovery time is a component property, and is specified in the data sheet. Depending on the type, they can be 10 to 400 microseconds. The recovery time required at the time of extinction at inductive loads, a limitation of the slew rate, which is done also by the above-mentioned snubber network. Otherwise ( the inductance leads nor the holding current ) it can lead to spontaneous re-ignition ( "upside - igniting " ) can occur. Newer thyristors ( " snubberless " types ) are able to cope with this increase in voltage without RC element.

Take note: the holding current (holding current) is that current which must flow at least through the conducting thyristor, so that it remains conductive. Meanwhile we mean by the latching current ( Latching Current) those that must flow directly after eradication of the gate pulse, so that the thyristor is not instantly falls back into the blocking state. Both streams are component characteristics and are given in the data sheets, sometimes you find only the holding current. The latching current is always slightly higher than the holding current, but both are of the same order ( for Kleinleistungsthyristoren typically below 100 mA, for large disk-type some 100 mA).

Specially designed versions ( GTOs ) can be put into the blocking state by a negative current pulse at the gate. The required current intensity of the negative erasing pulse, however, is orders of magnitude higher than that of the ignition pulse. Often a charged capacitor is connected to the gate terminal to provide the erase pulse.

History

The first thyristors in 1957 at General Electric ( GE) developed after Shockley, Jewell James Ebers and John Lewis had minor laid the groundwork at Bell Laboratories. The component was developed by GE first as SCR called (of silicon controlled rectifier English, German silicon controlled rectifier ). Westinghouse put forth a little later similar components, which he described as Trinistor. The AEG calls its components, first silicon controlled cell. The term thyristor sat only in the 1960s by, but in the English language is still in use SCR.

The thyristor was the first controllable power semiconductor component for high power and quickly opened up a wide range of application areas. Meanwhile, thyristors have been replaced in many applications but other power semiconductors. Nevertheless, new Thyristortypen are still developing, and the market size grows. Cause is their unmatched shifting performance and the improvement of parameters such as ignition or particularly low robustness against steep voltage increases when tearing the holding current of inductive loads (English snubberless ).

Variants

• Netzthyristor: Such thyristors are primarily optimized for passage and barrier properties and have free recovery times of more than 100 microseconds. This makes them suitable for applications in grid frequency.
• Frequenzthyristor: thyristor turn-off time of 8 microseconds and 100 microseconds for use with erase circuits or load-commutated inverters. In addition Frequenzthyristoren have special gate structures, the fast switch through a large area and thus allow a rapid increase of the load current.
• GTO ( Gate Turn Off ): It is endowed asymmetric and can not only ignited at the gate, but also by a negative pulse will be deleted. The erase pulse must be relatively strong. On average, 30 % of the load current must be briefly applied as a clear stream. GTOs need a Ausschaltentlastungsnetzwerk.
• GCT ( Gate Commutated Thyristor): Further development of the GTO with lower switching losses and for operation without Ausschaltentlastungsnetzwerk. To turn off a gate current equal to the load current is required.
• IGCT (Integrated Gate Commutated Thyristor): GCT with fixed-mounted driver stage
• Thyristortetrode: It has an electrode at the second and the third layer. They can be ignited, and removed at both electrodes or at each individual, each with a positive or negative pulse.
• Photothyristor: He is not ignited by an electrical pulse, but with the help of light. Fotothyristoren low power are used as components in integrated optical couplers.
• LTT (Light -triggered Thyristor): High- power component which is ignited like a photothyristor with light. It is ideal for use in installations for high- voltage direct current transmission, but there is used only in the most recent systems, because, until recently, these thyristors for the required high power could not be established.
• Diac
• Triac
• ITR (Integrated Thyristor / Rectifier) ​​or RCT (reverse Conducting thyristor): a component which contains an anti-parallel to it, in addition to a monolithic integrated diode thyristor.
• Four-layer diode (also called " Dinistor " for diode - thyristor or BOD for breakover device): thyristor without control electrode. The component fires on reaching a defined breakdown voltage. In contrast to the four-layer diode diac is capable of transmitting in one direction only.

In addition to these desired components can by alternating doping of the n- channel and p -channel field effect transistors undesirable in CMOS semiconductor components, forming so-called " parasitic thyristors ". Ignition of these thyristors by short voltage spikes at the inputs of a CMOS level ( latch-up effect) may destroy the CMOS component.

Housing types and power ranges

• Plastic Housing: thyristors for currents up to 25 A and voltages up to 1600 V are usually made ​​in plastic housings, which are common for power transistors, such as TO- 220 or TO- 247th The cooling fin is on anode potential; in TO-247 can also be isolated the cooling surface.
• Screw-in housing: metal housing with bolts and hex for currents up to several 100 A. This design is now used only to a limited extent.
• Flat-bottom housing: metal housing similar to the screw shell, but without bolts and hexagon. Also, this design is rarely used.
• Module Housing: Consists of a metallic base plate and plastic injection molded housing. In contrast to the cases previously described, the cooling surface ( bottom plate ) is electrically insulated from the terminals of the Baule Mentes. Usually more thyristors or combinations of thyristors and diodes are accommodated in a common housing. They are connected together to form a half-bridge, full bridge or three-phase bridge. Currents up to 800 A and voltages up to 3600 V are possible.
• Disc cell: housing identifiable by two plane-parallel metal surfaces of the anode and cathode, and an insulating part made ​​of ceramic or plastic. Is the thyristor element, a silicon wafer having a diameter of up to 12 cm between the electrodes. Currents of up to 6 kA and voltages up to 8 kV can be achieved. Disc cells are to operate between the cooling bodies with forces clamped up to 130 kN, in order to achieve good electrical and thermal contact to the heat sink, but also internally in the device.

In the picture below the " internal elements " of thyristors can be seen without housing. The silicon discs are soldered onto the tungsten plates, the polished floors are pressed to the heat sink. The top surface is vapor deposited with gold and is resiliently contacted therewith, the crystal in thermal expansion can not be destroyed. In the center of the discs to recognize the SCR firing contact.

Areas of application

Small power

Thyristors or triacs small power are used in household appliances to speed control of universal motors (vacuum cleaner, blender, hand drill ). Similarly, work -in dimmer for lighting control. In the late 1970s they were also used in the horizontal amplifiers and power supplies of TVs, later they were replaced by bipolar transistors or MOSFETs.

In conjunction with a Zener diode, the thyristor is in use clamping circuits. In normal operation block Zener diode and thyristor. If the zener voltage of the diode is exceeded by a defect in a transformer, for example, the thyristor is turned on and causes an intentional short circuit, causing the fuse of the power supply burns out immediately. This prevents that more expensive components be destroyed by a too high output voltage of the connected device.

Average power

In the power range above 2 kW find thyristors in many areas of industrial applications. While usually circuits are used for operation with three-phase current. Thyristor allow as Softstarter the starting of squirrel-cage induction motors with controlled starting currents and torques. Also with thyristor controllers, the output voltage of high-current rectifiers, such as for electroplating, or of high voltage rectifiers, such as the supply of electrostatic precipitators, are regulated. The thyristor is arranged on the primary side of the transformer, while used on the secondary side for rectifying power diodes. Thyristor for AC and AC are equal in structure to the thyristor. The power control is carried out here, however, not phase angle, but on the variation of the pulse-pause ratio. They are therefore suitable only for the control of loads with a large time constant, such as heating elements.

Thyristor rectifier were used for speed control of DC motors. But also in many modern frequency converters for variable-speed operation of three-phase motors work thyristors in the rectifier input, to allow a controlled charging of the DC voltage intermediate circuit.

Systems for induction hardening, with frequencies 5-20 kHz were previously built with Frequenzthyristoren. In this application, thyristors were replaced early on by IGBTs.

High performance

Frequenzthyristoren high performance are also still used today in load-commutated inverters in the megawatt range. In a brushless DC motor, a load- commutated inverter works with a synchronous machine and enables the variable speed operation of centrifugal compressors. Also systems for induction melting are still still running at high power and operating frequencies up to 1 kHz with Frequenzthyristoren.

Variable-speed drives with high output at the three-phase system can also be performed with direct ac at low speed. Here are several thyristor rectifier are connected and controlled so that the output side creates a three-phase system with frequencies up to 20 Hz.

In electric railways pulse inverter thyristors are used with both the drive vehicles as well as in stationary installations. In locomotives the pulse inverter allows the use of the squirrel-cage induction motor. Together with the network side, also working as a pulse inverter power converter, referred to as four-quadrant, regenerative energy is thus possible into the grid during braking. The power converter of the first three-phase locomotives series 120 or powerheads ICE 1 ( the first 40 power heads, but now converted to IGBT) are still running with Frequenzthyristoren and deletion circles, while in later series GTO thyristors were used. Meanwhile, thyristors have been largely supplanted by IGBTs. In fixed pulse inverters are used with GTO and IGCT for coupling the rail network to the national grid.

High power thyristor can be used for the aluminum and chlorine electrolysis.

In systems of high voltage direct current transmission, but also in systems for reactive power compensation thyristors are used in power transmission and distribution.

Thyristors have almost completely replaced controllable mercury vapor rectifier as thyratrons, ignitrons and Excitrons.