Gate Turn On Turn Off Thyristors

What is a thyristor? Thyristors are power semiconductor devices used in power electronic circuits They are operated as bistable switches operating from non-conducting to conducting states. They are made of 4 layers and 3 pn junctions. Thyristors are three terminal devices the terminals being anode, cathode and gate.

How is a thyristor different from transistor? Thyristors have lower on state conduction losses. They also have higher power handling capacity. On the other hand transistors have superior switching performances Transistors also have better switching speed and lesser switching losses

Symbol and cross-section of a thyristor

Working of a thyristor When anode voltage is positive with respect to cathode junctions J1 and J3 are forward biased and junction J2 is reverse biased and the thyristor is off. This state is called forward blocking state. When the voltage is increased the reverse junction breaks and it is called forward breakdown voltage. In this state a large anode current flows and the thyristor is said to be on or in conducting state

Latching and holding currents LATCHING CURRENT : It is defined as the minimum value of anode current required to maintain the thyristor in on state immediately after a thyristor has been turned on and the gate signal is removed. HOLDING CURRENT: It is defined as the minimum value of anode current to maintain the thyristor in conducting state.

V-I CHARACTERISTICS

WHAT IS A GTO? GTO stands for Gate Turn On Turn Off Thyristor. It is a four layered pnpn device. It three terminals just like all thyristors. GTO is similar to SCR s and can be built with current and voltage ratings similar to that of an SCR. It was developed in the late 1960 s.

HOW IS A GTO DIFFERENT FROM OTHER THYRISTORS? Conventional thyristors have only gate – controlled turn on capabilities. They can recover from conducting state to a non- conducting state only when the current is brought down to zero by other means. GTO s on the other hand have gate controlled turn off capacity. They can be switched on or off by applying short positive or negative pulses respectively.

SYMBOL AND CROSS-SECTION

CONSTRUCTION OF GTO Compared to a conventional thyristor it has an additional n+ layer near the anode. In order to obtain high emitter efficiency the n+ cathode layer is highly doped. This forms the turn off circuit between the gate and the cathode. The doping level of the p type gate region is highly varied.

GTO CONSTRUCTION Cont.. The gate cathode junction is highly interigited The maximum forward blocking voltage of the device is determined by the doping level and the thickness of the n type base region. In order to block several kv of forward voltage the doping level of this layer is kept relatively low while its thickness is made considerably higher (a few hundred microns).

CONSTRUCTION Cont The junction between the n base and p+ anode (J1) is called the “anode junction”. For good turn on properties the efficiency of this anode junction should be as high as possible. However, turn off capability of such a GTO will be poor with very low maximum turn off current and high losses.

CONSTRUCTION Cont In the first method, heavily doped n+ layers are introduced into the p+ anode layer. They make contact with the same anode metallic contact. This is the classic “anode shorted GTO structure”. In the other method, a moderately doped n type buffer layer is juxtaposed between the n- type base and the anode. This is called the “Transparent emitter structure”

GTO – TURN ON The turn on process for GTO is similar to a conventional thyristor. GTO has no regenerative state and hence it requires a large initial gate trigger pulse for turning on. If the anode current is low a longer period of gate pulse is required.

GTO-TURN ON Cont..  The turn on time for GTO consists of  Delay time  Rise time  Spread time  The turn on time can be decreased by increasing its forward gate current as in thyristors.

GTO ON STATE Once the GTO is turned on the forward gate current must be continued for the whole of the conduction period. This ensures that the device remains in conduction state. The device cannot remain in conduction state otherwise. The conduction gate current should be atleast 1% of the turn-on pulse to ensure that the gate does not unlatch.

TYPICAL TURN ON PULSE

GTO TURN OFF The turn off characteristics is greatly influenced by the gate turn off circuit and it must match the device requirements. This process involves the extraction of gate charge, gate avalanche period and anode current decay. The charge extraction is not significantly affected by external circuit conditions The turn off time and peak turn off current depend on the external circuit components.

GTO TURN OFF When a large pulse current is passed from cathode to gate, sufficient charge carriers are taken away from the cathode (emitter of npn transistor). This draws out the pnp transistor out of regenerative action. As the transistor Q1 (npn) turns off and leaves transistor Q2 (pnp) with an open base. Hence Q2 also turns off and the GTO returns to non conducting state.

GTO V-I CHARACTERISTICS

STATIC CHARACTERISTICS The first quadrant is similar to the V-I characteristics of a thyristor. A GTO can block rated forward voltage only when the gate is negatively biased with respect to the cathode during forward blocking state. A low value resistance must be connected across the gate cathode terminal.

GATE CHARACTERISTICS MinMax Ig Vg

GATE CHARACTERISTICS The zone between the min and max curves reflects parameter variation between individual GTOs. These characteristics are valid for DC and low frequency AC gate currents.

DYNAMIC CHARACTERISTICS

Dynamic characteristics are otherwise called as switching characteristics. When the GTO is off the anode current is zero and V AK =V d. To turn on the GTO, a positive gate current pulse is injected through the gate terminal. There is a delay between the application of the gate pulse and the fall of anode voltage, called the turn on delay time t d

DYNAMIC CHARACTERISTICS After this time the anode voltage starts falling while the anode current starts rising towards its steady value I L. Within a further time interval t r, they reach 10% of their initial value and 90% of their final value respectively. It should be noted that large value of ig (IgM) and di g \dt are required during t d and t r only. A minimum ON time period t ON (min) is required for homogeneous anode current conduction in the GTO.

APPLICATIONS GTO s replace thyristors in inverters and choppers where they are turned off by forced commutation. GTO eliminates the need for forced commutation circuitry and hence reduces the cost. They are widely used in voltage –source converters.

ADVANTAGES OVER SCR Elimination of forced commutation circuitry. Faster turn off giving rise to high switching frequencies Reduction in acoustic and electromagnetic noise Improved efficiency of converters

ADVANTAGES OVER TRANSISTOR Higher blocking voltage capacity High ratio of peak controllable current to average current High on state gain Pulsed gate signal of shorter duration

DISADVANTGES OF GTO Higher latching and holding currents Gate drive circuit is costlier On state voltage drop is high Triggering gate current is higher as compared to SCR.

FET Controlled Thyristors Combines a MOSFET & a thyristor in parallel as shown. High switching speeds & high di/dt & dv/dt. Turned on like conventional thyristors. Cannot be turned off by gate control. Application of these are where optical firing is to be used.

MOS-Controlled Thyristor New device that has become commercially available. Basically a thyristor with two MOSFETs built in the gate structure. One MOSFET for turning ON the MCT and the other to turn OFF the MCT.

Structure

Equivalent Circuit

Features Low on-state losses & large current capabilities. Low switching losses. High switching speeds achieved due to fast turn-on & turn- off. Low reverse blocking capability. Gate controlled possible if current is less than peak controllable current. Gate pulse width not critical for smaller device currents. Gate pulse width critical for turn-off for larger currents.

THANK YOU!