A thyristor is a semiconductor component made up of four or more semiconductor layers with alternating doping . Thyristors are switchable components, which means that they are non-conductive in the initial state and can be switched on by a small current at 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.
Structure and functionality
In the basic state, the thyristor is blocking in both directions. In the forward direction, it blocks up to a certain ignition voltage ( zero breakover voltage for a gate-cathode voltage of 0 V ). A positive current pulse at the gate can also switch it to the conductive state below the ignition voltage. In the reverse direction, it blocks the current like a normal diode.
There are several ignition options:
- Control current (a positive current or current pulse on the gate),
- Light ignition ( photo thyristor )
Unconventional, mostly inadmissible
- Exceeding the zero breakover voltage (overhead ignition or breakover). Only permissible with the so-called dynistor , a special type of thyristor that allows overhead ignition and is the successor component of the former Shockley diode .
- Exceeding the permissible rate of voltage rise
- Temperature increase
In practice, the thyristor is used as a controllable diode.
The thyristor can be ignited (switched conductive) by current injection into the third layer (control at the gate). The prerequisite for this is a positive voltage between anode and cathode and a minimum current through the middle barrier layer. A characteristic of the thyristor switch-on process is that the process is supported by positive feedback . In contrast to other power semiconductors , the speed of the switch-on process can not be influenced via the gate. The current density in the third layer is problematic during the ignition process. When the electrons are injected, the layer at the point of entry becomes conductive. Until the entire silicon surface is conductive, the current is concentrated on the already conductive area, in which the entire power loss is converted. The power loss density can exceed the permissible value and lead to local temperature increases above the diffusion temperature or even the melting temperature (1683 K) of the silicon. It is therefore important that the rate of current rise (critical current rate of rise) does not exceed a certain value, which in most cases is ensured by inductance in the load and the lines. If a capacitive load is to be switched, the rate of current rise may have to be limited by additional measures. In the case of strongly inductive loads, on the other hand, the increase in current lags behind the increase in voltage. It can therefore happen that the so-called latching current is not yet reached immediately after the ignition pulse has been extinguished.This is understood as the minimum value of the current that must flow through the thyristor so that it remains conductive when switched on even without gate current. This leads to undefined switching processes, which are also known from AC dimmers ( phase-angle control ) (working with triacs ) , and lamps controlled in this way can often flicker in the lower load range. To avoid this effect, a snubber network is used, which means an RC element (series connection of a resistor and a capacitor, typical values: 470 Ω and 100 nF), which is connected between the anode and cathode of the thyristor section. When igniting, the capacitor discharges through the resistor and the thyristor and thus provides a small current for a short time in order to exceed the latching current. A series-connected choke coil for radio interference suppression is often found in thyristor circuits.
(That is in the off-state offset) deleted is the thyristor, either by falling below the holding current (engl. Holding Current ,) which is generally at shutdown or reversal of the voltage in the load circuit or in a current zero crossing (z. B. in the rectifier) of the load circuit happening , or by reversing the polarity in the reverse direction. The speed of this process is limited by the release time t q , which is necessary for the thyristor to regain its full control and blocking capability after the conduction phase has ended. This is only achieved again when the relevant middle barrier layer has been cleared through recombination of charge carriers . The release time is a component property and is specified in the data sheet. Depending on the type, it can be 10 to 400 µs. When inductive loads are switched off, the release time requires a limitation of the rate of voltage rise; this is also done by the aforementioned snubber network. Otherwise (the inductance still carries the holding current) spontaneous re-ignition ("overhead ignition") can occur. Newer thyristors (“snubberless” types) are able to cope with this voltage increase even without an RC element.
Please note: The holding current is the current that must at least flow through the conductive thyristor so that it remains conductive. Meanwhile, the latching current is understood to be that which has to flow immediately after the gate pulse has been extinguished so that the thyristor does not immediately fall back into the blocking state. Both currents are component characteristics and are listed in the data sheets, sometimes you only find the holding current. The latching current is always slightly higher than the holding current, but both are of the same order of magnitude (typically below 100 mA for small-power thyristors, some 100 mA for large disc thyristors).
Specially designed variants ( GTO thyristors ) can also be switched to the blocking state by a negative current pulse at the gate. The required current intensity of the negative extinguishing pulse is, however, orders of magnitude higher than that of the ignition pulse. A charged capacitor is frequently connected to the gate connection to provide the erase pulse.
The first thyristors were developed at General Electric (GE) in 1957 after William B. Shockley , Jewell James Ebers and John Lewis Moll did the preliminary work at Bell Laboratories . The component was initially referred to by GE as SCR (from English silicon controlled rectifier , German controlled silicon rectifier ). Westinghouse produced similar components a little later and referred to them as trinistors. The AEG called their components first controllable silicon cell . The term thyristor first caught on in the 1960s, but SCR is still used in English-speaking countries.
The thyristor was the first controllable power semiconductor component for high performance and quickly opened up a wide range of applications. In the meantime, thyristors have been replaced by other power semiconductors in many applications, but still have a large market share due to their high switching capacity and robustness, especially in the field of high-current applications. New types with improved parameters are still being developed, e.g. B. with lower ignition currents, improved switch-off behavior or robustness against steep voltage increases when the holding current is torn off at inductive loads, which otherwise make a relief circuit ( English snubber ) necessary.
- Mains thyristor: Such thyristors are primarily optimized for forward and blocking properties and have release times of more than 100 µs. This makes them suitable for applications at line frequency.
- Frequency thyristor: Thyristor with release time between 8 µs and 100 µs for use with extinguishing circuits or in load-controlled inverters. In addition, frequency thyristors have special gate structures that quickly switch through a large area and thus allow a rapid increase in the load current.
- GTO thyristor ( Gate Turn Off ): It is asymmetrically doped and can not only be ignited at the control electrode , but also extinguished by a negative pulse. The erase pulse must be relatively strong. On average, 30% of the load current must be applied briefly as an extinguishing current. GTOs require a shutdown relief network.
- GCT ( Gate Commutated Thyristor ): Further development of the GTO with lower switching losses and for operation without a switch-off relief network. A gate current equal to the load current is required for switching off.
- IGCT ( Integrated Gate Commutated Thyristor ): GCT with permanently attached driver stage
- Thyristor tetrode : It has an electrode on the second and third layer . It can be ignited and extinguished on both electrodes or on each one individually, each with a positive or negative pulse.
- Photothyristor : It is not ignited by an electrical impulse, but with the help of light. Small power photothyristors are used as integrated components in optocouplers .
- LTT ( Light Triggered Thyristor ): High-performance component that is ignited with light like a photothyristor. It is ideally suited for use in systems for high-voltage direct current transmission .
- ITR ( Integrated Thyristor / Rectifier ) or RCT ( Reverse Conducting Thyristor ): A component which, in addition to a thyristor, contains a monolithically integrated diode connected in anti-parallel to it.
- Four- layer diode (also dinistor for diode thyristor or BOD for breakover device ): thyristor without control electrode. The component ignites when a defined breakdown voltage is reached. In contrast to the diac, the four-layer diode is only permeable in one direction.
In addition to these desired components, the alternating doping of the n-channel and p-channel field effect transistors in CMOS semiconductor components can result in undesirable, so-called “parasitic thyristors”. If these thyristors are triggered by short voltage peaks at the inputs of a CMOS stage ( latch-up effect ), the CMOS component can be destroyed.
Housing designs and performance ranges
- Plastic housing: Thyristors for currents up to 25 A and voltages up to 1600 V are usually manufactured in plastic housings, as they are also common for power transistors, such as TO-220 or TO-247. The cooling flag is at the anode potential; with TO-247 the cooling surface can also be insulated.
- Screw housing: metal housing with screw bolts and hexagon for currents up to several 100 A. This type of construction is only used to a limited extent today.
- Flat bottom housing: metal housing similar to the screw housing, but without bolts and hexagon. This design is also rarely used.
- Module housing: Consists of a metallic base plate and a plastic injection-molded housing. In contrast to the housings described so far, the cooling surface (base plate) is electrically isolated from the connections of the component. Usually several thyristors or combinations of thyristors and diodes are housed in a common housing. They are interconnected to form a half bridge, a full bridge or a three-phase bridge. Currents up to 800 A and voltages up to 3600 V are possible.
- Disc cell: Housing recognizable by two plane-parallel metal surfaces for anode and cathode as well as an insulating part made of ceramic or plastic. The thyristor element, a silicon wafer with a diameter of up to 12 cm, is located between the electrodes. Currents of up to 6 kA and voltages of up to 8 kV can be achieved. For operation, disk cells are clamped between heat sinks with forces of up to 130 kN in order to achieve good electrical and thermal contact with the heat sink, but also internally in the component.
The picture below shows the internal components of thyristors without a housing. The silicon wafers are soldered to tungsten plates, the polished bases of which are pressed onto the heat sink. The upper side is vapor-coated with gold and has spring contact so that the crystal cannot be destroyed by thermal expansion. The ignition contact can be seen in the middle of the SCR discs.
Areas of application
Thyristors or triacs of low power are used in household appliances to control the speed of universal motors (vacuum cleaners, mixers, hand drills). Dimmers for lighting control work in a similar way . At the end of the 1970s, they were also used in the horizontal output stages and power supplies of televisions, later they were replaced by bipolar transistors or MOSFETs .
In connection with a Zener diode , the thyristor is used in clamping circuits . In normal operation, the Zener diode and thyristor block. If the zener voltage of the diode z. B. is exceeded by a defect in a transformer , the thyristor becomes conductive and causes an intentional short circuit, whereby the fuse of the power supply immediately burns out. This prevents more expensive components in the connected device from being destroyed by an output voltage that is too high.
In the power range above 2 kW, thyristors are used in numerous industrial applications. In most cases, circuits for operation with three-phase current are used. Thyristor allow as soft starter cranking of squirrel-cage induction motors with controlled start-up currents and torques. The output voltage of high-current rectifiers, e.g. for electroplating, or of high-voltage rectifiers, e.g. for supplying electrostatic precipitators , can also be regulated with thyristor controllers. The thyristor controller is arranged on the primary side of the transformer, while power diodes are used on the secondary side for rectification. Thyristor switches for alternating current and three-phase current have the same structure as the thyristor controllers. The power control does not take place here via phase control, but via the variation of the pulse-pause ratio . They are therefore only suitable for controlling loads with a large time constant, such as heating elements.
Thyristor rectifiers were used to control the speed of DC motors. But also in many modern frequency converters for the speed-variable operation of three-phase motors, thyristors work in the input rectifier to enable controlled charging of the DC voltage intermediate circuit .
High-power frequency thyristors are still used today in load-commutated inverters in the megawatt range. In the converter motor, a load-controlled inverter works together with a synchronous machine and thus enables the variable-speed operation of turbo compressors. Systems for inductive melting are still implemented with frequency thyristors with high power and working frequencies of up to 1 kHz.
Variable-speed drives with high performance on the three-phase network can also be implemented with direct converters at low speeds . Several thyristor rectifiers are interconnected and controlled in such a way that a three-phase system with frequencies of up to 20 Hz is created on the output side.
Pulse inverters with thyristors are used in electric railways in both traction vehicles and stationary systems. The pulse inverter enables the squirrel cage asynchronous motor to be used in traction vehicles. Together with the power converter on the line side , which also works as a pulse converter, here referred to as a four-quadrant converter, energy can be fed back into the grid when braking. The converters of the first three-phase locomotives series 120 or ICE 1 power cars (the first 40 power cars ; meanwhile, however, have been converted to IGBT ) are still designed with frequency thyristors and quenching circuits, while GTO thyristors were used in later series. In the meantime, thyristors have been largely replaced by IGBTs. Pulse-controlled inverters with GTOs and IGCTs are used in stationary systems to couple the rail network with the national network.
Thyristor rectifiers of high power are used for aluminum and chlorine electrolysis .
In systems of high-voltage direct current , but also in systems for reactive power compensation thyristors are used in power transmission and distribution.
- Dierk Schröder: Power electronic circuits: function, design and application . 2nd Edition. Springer, Berlin 2008, ISBN 3-540-69300-9 .
- Edward L. Owen: History - SCR is 50 Years Old . In: IEEE Industry Applications Magazine . tape 13 , no. 6 , 2007, p. 6-10 , doi : 10.1109 / MIA.2007.907204 .
- Friedrich-Karl Hinze: Controllable silicon cells from AEG . In: AEG communications . tape 53 , no. 3/4 , 1963.
- Joachim Specovius: Basic course in power electronics . Springer, Berlin 2008, ISBN 978-3-8348-0229-3 , pp. 73 ff .
- Thyristor relay on electronic. In: dieelektronikerseite.de. Retrieved October 29, 2008 .
- Thyristor (reverse blocking thyristor triode). In: Electronics Compendium. Retrieved October 29, 2008 .
- Thomas Schaerer: Thyristor Crowbar: Use a crowbar against too much voltage! In: Electronics Compendium. Retrieved October 29, 2008 .
- General Introduction to Thyristors and its Applications , accessed on December 18, 2011, engl.
- Hans-Joachim Fischer, Wolfgang E. Schlegel: transistor and circuit technology. 4th edition. Military publishing house of the GDR, Leipzig 1988, ISBN 3-327-00362-9 .