GTO - Gate Turn Off Thyristors

What does GTO stand for?

Gate Turn-Off Thyristor.

What is a GTO?

A GTO is a type of thyristor that can be turned on with a positive gate pulse and turned off with a large negative gate pulse.

How does a GTO differ from a standard SCR?

A standard SCR can only turn off when anode current drops below the holding current; a GTO can be turned off actively by a negative gate pulse.

Why was the GTO developed?

To eliminate the need for external commutation circuits required to turn off SCRs in DC applications.

What current pulse is needed to turn a GTO off?

A large negative gate pulse with amplitude about one-fourth to one-sixth of the anode current and lasting about 20-30 us.

What happens during GTO turn-off?

The negative gate pulse extracts carriers from the P-base region, removing stored charge and forcing the device into the blocking state.

Why does a GTO require a larger gate current for turn-on than an SCR?

Because the gate structure is subdivided into many small cathode regions that require higher current density to initiate conduction.

What are typical holding and latching current values for a GTO compared to an SCR?

The GTO's latching and holding currents are higher; for example, IL approximately 2 A for a GTO versus 100-500 mA for an SCR.

Why is a resistor often placed between gate and cathode in a GTO circuit?

To prevent false turn-on due to dv/dt-induced leakage current by maintaining a small reverse bias on the gate.

What is the purpose of connecting a diode anti-parallel with a GTO?

When the load is inductive, current cannot stop instantly — the diode conducts that current, preventing high voltage spikes across the GTO, allowing safe turn-off.

List common applications of GTOs.

High-power motor drives, traction control, inverters, and DC choppers operating above 1 kHz.

Describe the on-state and off-state regions of a GTO I-V curve.

The on-state region is a low-voltage, high-current area similar to an SCR; the off-state region shows small leakage current up to the forward-blocking voltage.

Describe the typical gate current waveform for a GTO.

The waveform has a large positive pulse for turn-on, then a larger negative pulse for turn-off; the negative pulse duration is about 20-30 us.

Describe the stages of the GTO turn-off process on a timing diagram.

1. Storage time (tₛ):
   Negative gate current removes charge carriers from the device; anode current remains nearly constant.

2. Fall time (t_f):
   Anode current rapidly decreases from 90% to 10% of its initial value as the GTO blocks conduction.

3. Tail time (t_t):
   Remaining carriers recombine; the anode voltage rises to full blocking level.

Total turn-off time = t_{s}+t_{t}+t_{f}