Gate Turn-Off Thyristor (GTO)

Gate Turn-Off Thyristor

The Gate Turn-Off Thyristor is also a device from the Thyristor family. The ordinary thyristor is not a fully controlled switch.

It can be turned ON by gate but cannot turn OFF using a gate. Once the switch is ON, if you remove the gate pulses, it will not OFF. Hence there is no control to turn OFF the switch.

To turn OFF the switch, the primary current must be interrupted. Therefore, it is inconvenient to use in the application where the main must not interrupt. i.e., DC-DC and DC-AC conversion circuits.

A bulky and expensive commutation circuit had to be used to turn OFF the thyristor. The switching speed of the device is also slow.

The Gate Turn-Off Thyristor is designed to overcome these disadvantages of the standard thyristor. And it is abbreviated as GTO.

The GTO is a current-controlled device like a standard thyristor.

The turn OFF method of GTO is different compared to the conventional thyristor. The GTO can be turned OFF by giving a negative current to the gate terminal. A relatively high gate current needs to turn OFF the device.

On the other hand, the device behaves like a standard thyristor during conduction with a shallow ON-state voltage drop.

gate turn off thyristor GTO Symbol
gate turn off thyristor GTO Symbol

Construction of Gate Turn-Off Thyristor (GTO)

Similar to the thyristor, the GTO is also three junctions four-layer PNPN device.

The N+ layer is highly doped to obtain high emitter efficiency. This N+ layer provides a cathode terminal. As a result, the breakdown voltage of junction J3 is low. The typical value of the breakdown voltage is 20-40V.

gate turn off thyristor GTO structure
gate turn off thyristor GTO structure

The doping level of the P-layer should be low to maintain good emitter efficiency. But for good turn OFF property resistivity of this layer should be low, which requires a high doping level of this layer. Hence, the doping level of this layer is graded.

The gate cathode junction must be highly interdigitated to optimize current turn OFF capability. A 3000-amp GTO may be composed of 3000 individual cathode segments. A common contact accesses these segments.

The doping level and thickness of the N-layer will decide the forward blocking voltage. The doping level of this level was kept low to block the several KV forward voltage.

Operation of Gate Turn-Off Thyristor (GTO)

The operation of the Gate Turn-Off Thyristor is similar to the ordinary thyristor. The equivalent circuit is as shown in the below figure. It is a combination of PNP and NPN thyristors.

GTO PNP NPN transistor
GTO PNP NPN transistor

Forward Blocking Mode

In the forward blocking mode of operation, the device remains OFF until the avalanche breakdown occurs. In this mode, the potential is applied between the anode and cathode. But current will not flow until the voltage exceeds the breakdown voltage.

Once the voltage increased, the current will flow through the device due to the avalanche breakdown. For regular operation, the primary current will not flow because, in this mode, the gate current is zero.

Forward Conducting Mode

The gate current must be injected to turn ON the device. Here, the anode terminal is made positive with respect to the cathode and applies gate current. This results in the electron flow from the cathode to the anode.

Therefore, the hole flowing through the anode to the cathode. This injection of holes and electrons continues until the GTO comes in the forward conduction mode.

GTO has a narrow cathode element like thyristor. Hence the GTO comes into the conduction mode very quickly. This is the process to turn ON the GTO.

By making the gate negative with respect to the cathode, the reverse bias is applied to turn OFF the GTO. In this way, the holes from the P-layer are extracted, which suppress the flow of electrons from the cathode.

Therefore, the voltage drop across the P junction causes the reverse bias of the gate cathode junction, and the GTO is turned OFF.

The P base region gradually depleted during the hole extraction process so that the conduction area was minimized.

Because of this, the anode current flows through the remote area and forming the high current density filament. It causes a local hotspot which can damage the device.

These filaments were extinguished quickly to avoid damage. High negative gate voltage helps filament to be swiftly extinguished.

The anode to gate current continues to flow due to the N base region has stored some charges. This current is known as tail current.

Tail current decreased exponentially due to the excess charge carrier reduced by the recombination process.

The device remains in forwarding blocking mode once the tail current decreased compared to the leakage current.

VI characteristic

GTO VI
GTO VI

Turn ON characteristic is the same as the thyristor, and it is in the first quadrant. For a similar rating of thyristor and GTO, the latching current of GTO is very high. The forward leakage current is also high in the case of GTO.

If the gate current is insufficient to turn ON the device, it operates as a high voltage low gain transistor.

During forward blocking mode, when the gate is negatively biased with respect to the cathode, it can block rated forward voltage.

Between the gate cathode terminal, a low-value resistance must be connected. Increasing the value of this resistance reduces the forward blocking voltage of GTO.

Under certain conditions, asymmetric GTO may lead the device to operate in reverse avalanche due to small reverse breakdown voltage. If avalanche time and current are short, this condition is not dangerous for the GTO. The gate voltage remains negative during this condition.

Advantages

  • Excellent switching characteristic
  • No need for a commutation circuit
  • Maintenance-free operation
  • high capability of blocking voltage
  • more di/dt ratings at turn ON
  • high efficiency

Disadvantages

  • ON-state voltage drop and the associated loss is more
  • Gate drive circuit losses are more
  • the magnitude of latching and holding current is more
  • The triggering gate current is higher as compared to the current required for a conventional SCR.

Application

  • The control device in choppers and inverter
  • AC drives and DC drives (variable speed motor drives)
  • Traction
  • AC stabilizing power supply
  • DC circuit breaker
  • Induction heating

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