Gate Turn-Off Thyristor

The Gate Turn-Off Thyristor is a PN PN device can be turned like an ordinary thyristor by a pulse of Positive gate current. It can be easily turned off by a negative gate pulse of approximate amplitude. As no forced commutation circuitry is required for GTO’s . These devices are compact and less cost. The negative gate current required to turn off a GTO is quite a large percentage (20 to 30%) of anode current 800A. GTO require a negative current Pulse of 200 A peak for turning it off.

Fig 1. GTO Symbol

GTO (Gate Turn-off Thyristor) Construction

Fig 2. GTO Structure

1. Unique features of the GTO.

• Highly interdigitated gatecathode structure (faster switching)
• Etched cathode islands (simplify electrical contacts)
• Anode shorts (speed up turn-off)

2. GTO has no reverse blocking capability because of anode shorts
3. Otherwise i-v characteristic the same as for standard SCR

GTO Basics

The Thyristors are not fully controllable switches (a "fully controllable switch" can be turned on and off at will). Thyristors can only be turned ON and cannot be turned OFF. Thyristors are switched ON by a gate signal, but even after the gate signal is removed, the thyristor remains in the ON-state until any turn-off condition occurs. Thus, a thyristor behaves like a normal semiconductor diode after it is turned on or "fired".

The GTO can be turned on by a gate signal, and can also be turned off by a gate signal of negative polarity.

Turn on is achieved by a "positive current" pulse between the gate and cathode terminals. As the gate-cathode behaves like PN junction, there will be some relatively small voltage between the terminals. The turn on phenomenon in GTO is however, not as reliable as an SCR (thyristor) and small positive gate current must be maintained even after turn on to improve reliability.

Turn off is achieved by a "negative voltage" pulse between the gate and cathode terminals. Some of the forward current is "stolen" and used to induce a cathode-gate voltage which in turn induces the forward current to fall and the GTO will switch off (transitioning to the 'blocking' state).

VI Characteristics:

1. The latching current for large power GTO’s is several amperes (2 A) as compared to 100-500 mA for conventional thyristors of the same rating.
2. If Gate current is not able to turn on the GTO, it behaves like a high voltage low gain transistor with considerable anode current.

Applications

1. Speed motor drives
2. High power inverters
3. Traction