Gate Turn Off Thyristor(GTO)
What is GTO? How does GTO work?
The Gate turn off thyristor (GTO) is a four layer PNPN power semiconductor switching device that can be turned on by a short pulse of gate current and can be turned off by a reverse gate pulse.
- This reverse gate current amplitude is dependent on the anode current to be turned off.
- There is no need for an external commutation circuit to turn it off. So inverter circuits built by this device are compact and low-cost.
- The device is turned on by a positive gate current and it is turned off by a negative gate cathode voltage.
GTO symbol :
As shown the Symbol has three terminals namely Anode(A), Cathode(K) and Gate(G).
The two-way arrow convention on the gate lead distinguishes the GTO from the conventional thyristor.
GTOs are of two types
- The Asymmetrical type GTOs are the most common type on the market.
- This type of GTOs are normally used with a anti-parallel diode.
- They do not have high reverse blocking capability.
- They are used in Voltage Fed Converters.
- The symmetrical type GTOs have an equal forward and reverse blocking capability.
- They are used in Current Fed Converters.
Turn OFF Current Gain:
- The Turn Off Current Gain of a GTO is defined as the ratio of anode current prior to turn off to the negative gate current required for turn off. It is typically very low (4 or 5).
- It means a 6000A rating GTO requires 1500A gate current pulse. However, the gate pulse duration and the power loss due to the gate pulse is small. It can be supplied by low voltage power MOSFETs.
- This gate turn off capability is advantageous because it provides increased flexibility in circuit application. Now it becomes possible to control power in DC circuits without use of elaborated commutation circuitry.
Applications of GTO:
They are used in motor drives, static VAR compensators (SVCs) and AC/DC power supplies with high power ratings.
What are the disadvantages of GTO?
Compared to a conventional SCR, the device has the following disadvantages
- Magnitude of latching, holding currents is more. The latching current of the GTO is several times more as compared to conventional thyristors of the same rating.
- On state voltage drop and the associated loss is more.
- Due to multicathode structure of GTO, triggering gate current is higher than that required for normal SCR.
- Gate drive circuit losses are more. Its reverse voltage blocking capability is less than the forward voltage blocking capability.
What are the advantages of GTO?
The prime design goal of GTO devices are to achieve fast turn off time and high current turn off capability and to enhance the safe operating area during turn off. The GTO's turn off occurs by removal of excess holes in the cathode base region by reversing the current through the gate terminal. Compare to BJT the GTO has the following advantages:
- High blocking voltage capabilities
- High over current capabilities
- exhibits low gate currents
- fast and efficient turn off
- better static and dynamic dv/dt capabilities
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