ELEC-E8421 Components of Power Electronics Thyristors
Thyristors Turn on and the di/dt rating At turn on the gate current goes to cathode only at the small region near the gate. The initial turn on area is quite small and increases only about 0,02 0,1 mm/μs Thyristor may be destroyed if the anode current increases more than its di/dt rating. Typical di/dt ratings: 50 A 1000 A / µs Current paths in the beginning of the turn-on Recommendations: Gate pulse magnitude 1 2 A Increase rate of gate current 1 2 A/μs
Methods used to improve thyristor di/dt rating Gate Cathode surface Interdigitated gate The area initially conducting at turn-on can be increased by extending the gate in the cathode area With a dense gate pattern also the time required to fully turn on the thyristor is minimized The drawback is naturally high gate current
Methods used to improve thyristor di/dt rating Amplifying gate consists of small pilot thyristor that feeds the current to the interdigitated main gate. Thus a large thyristor can be turned on with small gate current pulse. Equivalent circuit Structure
Thyristor's du/dt rating and recovery The reverse recovery is similar to the diode recovery When entering to forward blocking state the trapped charges near gate may turn on the thyristor The du/dt rating defines how fast the voltage may rise without turning on the thyristor The du/dt rating depends on temperature, hold-off time t H and voltage during hold-off time The hold-off time has to be longer than the recovery time t Q of the thyristor depletion region charge carrier cloud
Methods to improve thyristor du/dt gate emitter shorts cathode cathode emitter short gate anode Emitter shorts allow the trapped charges to flow from gate to cathode without crossing p-n junction Drawback is slower turn on Typical du/dt ratings: 100 V 1000 V/µs
Different thyristor types 50/60 Hz thyristors: t Q 150 400 µs Triac Integrated Gate Commutated Thyristors (IGCT) Light triggered thyristors Fast thyristors: t Q 4 70 µs Asymmetric fast thyristors Reverse conducting fast thyristors Gate Assisted Turn Off (GATO) thyristors Gate Turn Off (GTO) thyristors NOT USED ANYMORE IN NEW PRODUCTS
Asymmetric thyristor n+ buffer layer at the anode side of middle n- region increases forward blocking voltage but at the same time decreases reverse blocking voltage to about 30 50 V Asymmetric fast thyristors were used in 80's in applications where thyristors had anti-parallel diodes and thus never had high reverse voltages (for example in force commutated frequency converters) Circuit symbols
Reverse conducting thyristors Used at the end of 80's in some applications to replace asymmetric thyristor and its antiparallel diode. Diode section Circuit symbol Internal structure
Light triggered thyristor (LTT) Used in high voltage DC transmission (HVDC) applications. Fired by laser light pulse via fiber-optic cable Multiple pilot thyristors are used to amplify the gate current Often over voltage protection functions are also integrated (BOD = Break Over Diode)
Gate turn-off thyristor (GTO) GTO can be turned off by a negative gate current pulse that draws holes out of the gatecathode junction. Typically required gate current is 20 % of anode current. Cathode is divided in small islands that allow gate to efficiently draw holes out at turn-off High voltage up to 4500 V, maximum turn off current up to 5 ka. GTO replaced fast thyristors at the 80's but was itself replaced by IGBTs and IGCTs in 90's GTO wafer from cathode side
GTO turn-off Most critical moment is when anode current is falling: the anode cathode voltage should not exceed about 600 V level Voltage rise limitation requires quite big snubber capacitor Anode cathode voltage Anode cathode current Gate cathode current Current distribution in cathode island Gate current
Integrated Gate Commutated Thyristor (IGCT) The GTO turn off problem and snubbers can be avoided if gate current is very fast increased to be greater than anode current. Then cathode current is zero and does not cause local hot spots Requires high but short gate current pulse Special flange contact to gate is needed as low inductance is essential for fast gate current increase Usually sold with integrated gate driver Replaced GTOs at the end of 90s Advantages Low conduction losses Snubber circuit is not needed Connection in series is possible Disadvantages Short circuit current may exceed turn off capability High switching losses limit pulse frequency to about 300 Hz
Triac Equivalent to two thyristors but has only one gate. Thus very simple to use. Negative gate pulse turns on the triac regardless of the polarity of the voltage U 21 between main terminals Also positive gate pulse turns the triac on, but this is not always recommended if U 21 < 0 Used in dimmers, vacuum cleaners, hand drills, etc for stepless control of the device Lamp dimmer
Triak with inductive load problems with turn-off Triak circuit diagram with inductive load Voltage and current curves For older triacs maximum allowed du/dt is only 5 10 V/μs when di/dt is nominal Many new triacs survive even without snubber, but snubber is recommended due to EMC
Voltage and current ratings of thyristors (2015) a) 50/60 Hz thyristors with light control b) Normal 50/60 Hz thyristors c) GTO d) IGCT 100000 U DRM / V 10000 1000 e) Triacs (Current value is RMS) 100 e) 100 1000 10000 c) b) d) I TAV / A a)
Electrical properties listed in a thyristor data sheet Most of the symbols are the same than with diodes However, subscript T is used instead of F, for example rated average current of thyristor is I TAV For thyristors some special parameters are given in addition to di/dt, du/dt, t Q and forward blocking ratings, for example: Latching current I LAT (anode current needed to keep thyristor conducting after turn-on) Holding current I H (anode current below which the thyristor starts itself turn off) Both latching and holding currents are high for GTOs and IGCTs, several tens of amperes. These require continuous gate current in order to keep all cathode islands conducting
Current voltage curve of the gate Manufacturers give diagrams that show the spread in the characteristica As the turn on is more difficult in low temperatures the diagram shows areas where the turn on may not be succesfull. Gate circuit characteristica