Gate Turn-Off Thyristor (GTO)

Introduction and Motivation

Review: Conventional Thyristor Limitations

Key Operating Principles

  • Thyristors are excellent power switches with high voltage and current ratings

  • Turn-ON: Apply positive gate current \(I_g > 0\) between gate and cathode

  • Once ON, thyristor remains conducting until anode current drops below holding current \(I_H\)

Turn-OFF Methods

  • Natural commutation: AC supply naturally drives current to zero

  • Forced commutation: External LC circuits force current to zero

Critical Limitation

  • Cannot turn OFF using gate signal

  • Forced commutation circuits are bulky, complex, and expensive

Question: How can we overcome this limitation?
Answer: Gate Turn-Off Thyristor (GTO)

What is a GTO?

Definition and Basic Operation

  • GTO = Gate Turn-Off Thyristor

  • A modified thyristor that can be turned OFF by applying negative gate current

  • Turn-ON: Positive gate current \(I_g > 0\) (similar to conventional thyristor)

  • Turn-OFF: Negative gate current \(I_g < 0\) of sufficient magnitude

  • Self-commutating capability eliminates need for external commutation circuits

Key Applications

  • DC-AC inverters (UPS systems)

  • DC choppers for motor drives

  • Variable frequency drives (VFDs)

  • High-power switching applications

Basic Structure and Construction

GTO Structure and Construction

Basic Structure

  • Four-layer \(p\)-\(n\)-\(p\)-\(n\) semiconductor device

  • Three terminals: Anode (A), Cathode (K), Gate (G)

Key Structural Modifications

  • Highly interdigitated gate-cathode structure

  • Anode shorts: \(n^+\) regions penetrate \(p\)-type anode layer

  • Reduced \(p_1\) layer thickness for better turn-OFF

  • Shorter carrier lifetime in drift region (\(\tau = 1\,\mathrm{\mu s}\) to \(10\,\mathrm{\mu s}\))

Cross-sectional structure of GTO showing key modifications
Cross-sectional structure of GTO showing key modifications

Two-Transistor Model and Circuit Symbol

Two-Transistor Equivalent Circuit

  • \(T_1\): \(p\)-\(n\)-\(p\) transistor (upper)

  • \(T_2\): \(n\)-\(p\)-\(n\) transistor (lower)

  • Common base connection

  • Regenerative feedback loop

Current Gains

\[\begin{aligned} \alpha_1 &: \text{Common base gain of } T_1\\ \alpha_2 &: \text{Common base gain of } T_2 \end{aligned}\]

Circuit Symbol

  • Similar to thyristor

  • Two-way arrow on gate indicates bidirectional control

  • Distinguishes GTO from conventional thyristor

Two-transistor model and circuit symbol
Two-transistor model and circuit symbol

Current-Voltage Characteristics

I-V Characteristics of GTO

Forward Characteristics

  • Similar shape to conventional thyristor but with higher switching currents

  • Forward breakover voltage: \(V_{\mathrm{BO}} = 200\,\mathrm{V}\) to \(8000\,\mathrm{V}\)

  • Latching current: \(I_L = 2\,\mathrm{A}\) to \(10\,\mathrm{A}\)

  • Holding current: \(I_H = 200\,\mathrm{mA}\) to \(500\,\mathrm{mA}\)

  • Higher ON-state voltage drop: \(V_{\mathrm{TM}} = 2\,\mathrm{V}\) to \(4\,\mathrm{V}\)

Static I-V characteristics of GTO
Static I-V characteristics of GTO

Reverse Characteristics

  • Poor reverse blocking capability

  • Reverse breakdown voltage: Only \(20\,\mathrm{V}\) to \(30\,\mathrm{V}\)

  • Due to anode-short structure and heavy doping

  • Consequence: Requires anti-parallel diode for AC applications

Operating Principles

Forward Blocking Mode

Operating Conditions

Conditions: \(V_{\mathrm{AK}} < V_{\mathrm{BO}}\) and \(I_g = 0\)

Mathematical Analysis

Both transistors operate with small current gains: \(\alpha_1, \alpha_2 \ll 1\)

Condition for blocking: \(\alpha_1 + \alpha_2 < 1\)

Anode current equation:

\[I_A = \frac{\alpha_1 I_{\mathrm{CO1}} + \alpha_2 I_{\mathrm{CO2}} + I_g}{1-(\alpha_1 + \alpha_2)}\]

Parameter Definitions

  • \(I_{\mathrm{CO1}}, I_{\mathrm{CO2}}\): Reverse saturation currents of junctions \(J_1\) and \(J_3\)

  • \(I_g\): Gate current (zero in blocking mode)

  • Since \(\alpha_1 + \alpha_2 < 1\), denominator is positive and \(I_A\) remains small

Result: GTO blocks forward voltage with only small leakage current

Turn-ON Process

Turn-ON Mechanism

  • Apply positive gate current: \(I_g > I_{\mathrm{gt}}\) (gate trigger current)

  • Gate current increases base current of transistor \(T_2\)

  • \(T_2\) starts conducting, increasing its collector current

  • Collector current of \(T_2\) becomes base current of \(T_1\)

  • \(T_1\) starts conducting, providing base current to \(T_2\)

  • Regenerative action: Both transistors saturate rapidly

Mathematical Condition for Turn-ON

\[\boxed{\alpha_1 + \alpha_2 = 1}\]
When this condition is met: \(I_A \to \infty\) (limited by external circuit)

Gate Current Requirements

  • Minimum gate current: \(I_{\mathrm{gt}} = 0.1\,\mathrm{A}\) to \(1\,\mathrm{A}\)

  • Gate trigger voltage: \(V_{\mathrm{gt}} = 2\,\mathrm{V}\) to \(5\,\mathrm{V}\)

Turn-OFF Process

Turn-OFF Mechanism

  • Apply negative gate voltage: \(V_{\mathrm{gk}} < 0\)

  • Negative gate current extracts holes from \(p_1\) base region

  • Base current of transistor \(T_2\) becomes negative

  • \(T_2\) exits saturation and current gain drops

  • Junction \(J_2\) becomes reverse biased

  • Electron injection from cathode stops

  • Stored charge recombines, causing tail current

  • GTO regains forward blocking capability

Turn-OFF Condition

For transistor \(T_2\) to exit saturation:

\[I_{\mathrm{B2}} < \frac{I_{\mathrm{C2}}}{\beta_2}\]

Turn-OFF Gain

\[\boxed{\beta_{\mathrm{off}} = \frac{I_A}{|I_g|} = \frac{\alpha_2}{\alpha_1 + \alpha_2 - 1}}\]
Typical values: \(\beta_{\mathrm{off}} = 3\) to \(10\)

Switching Characteristics

Turn-ON Characteristics

Gate Current Requirements

  • Peak gate current: \(I_{\mathrm{gp}} = 10\%\) to \(50\%\) of anode current

  • Rate of rise: \(\frac{\mathrm{d}i_g}{\mathrm{d}t} > 10\,\mathrm{A/\mu s}\)

  • Pulse width: \(t_g = 10\,\mathrm{\mu s}\) to \(50\,\mathrm{\mu s}\)

  • Back-porch current: Small current maintained during ON-state

Turn-ON Timing

  • Delay time: \(t_d = 0.5\,\mathrm{\mu s}\) to \(2\,\mathrm{\mu s}\)

  • Rise time: \(t_r = 1\,\mathrm{\mu s}\) to \(5\,\mathrm{\mu s}\)

  • Total turn-ON time: \(t_{\mathrm{on}} = t_d + t_r\)

Critical Design Considerations

  • All cathode segments must turn ON simultaneously

  • High \(\frac{\mathrm{d}i_g}{\mathrm{d}t}\) prevents current crowding

  • Low gate circuit inductance essential

Turn-OFF Characteristics

Gate Current Requirements

  • Peak reverse current: \(|I_g| = \frac{I_A}{\beta_{\mathrm{off}}}\)

  • Rate of fall: \(\left|\frac{\mathrm{d}i_g}{\mathrm{d}t}\right| > 100\,\mathrm{A/\mu s}\)

  • Gate resistance: \(R_g < 1\,\mathrm{\Omega}\) for fast turn-OFF

Turn-OFF Timing

  • Storage time: \(t_s = 5\,\mathrm{\mu s}\) to \(50\,\mathrm{\mu s}\)

  • Fall time: \(t_f = 5\,\mathrm{\mu s}\) to \(20\,\mathrm{\mu s}\)

  • Tail time: \(t_{\mathrm{tail}} = 50\,\mathrm{\mu s}\) to \(500\,\mathrm{\mu s}\)

  • Total: \(t_{\mathrm{off}} = t_s + t_f + t_{\mathrm{tail}}\)

Turn-OFF waveforms showing storage, fall, and tail times
Turn-OFF waveforms showing storage, fall, and tail times

Ratings and Specifications

Key GTO Ratings

Voltage Ratings

  • \(V_{\mathrm{DRM}}\): Peak repetitive forward blocking voltage
    ( \(1200\,\mathrm{V}\) to \(6000\,\mathrm{V}\))

  • \(V_{\mathrm{RRM}}\): Peak repetitive reverse voltage
    ( \(20\,\mathrm{V}\) to \(30\,\mathrm{V}\))

  • \(V_{\mathrm{TM}}\): Peak on-state voltage
    ( \(2\,\mathrm{V}\) to \(4\,\mathrm{V}\))

Current Ratings

  • \(I_{\mathrm{T(RMS)}}\): RMS on-state current
    ( \(50\,\mathrm{A}\) to \(6000\,\mathrm{A}\))

  • \(I_H\): Holding current
    ( \(200\,\mathrm{mA}\) to \(500\,\mathrm{mA}\))

  • \(I_L\): Latching current
    ( \(2\,\mathrm{A}\) to \(10\,\mathrm{A}\))

Switching Parameters

  • Turn-OFF gain: \(\beta_{\mathrm{off}} = 3\) to \(10\)

  • Maximum switching frequency: Up to \(10\,\mathrm{kHz}\)

  • \(\frac{\mathrm{d}i}{\mathrm{d}t}\) capability : \(100\,\mathrm{A/\mu s}\) to \(1000\,\mathrm{A/\mu s}\)

Thermal Ratings

  • Junction temperature: Up to \(125\,\mathrm{^\circ C}\)

  • Thermal resistance: \(0.1\,\mathrm{K/W}\) to \(1\,\mathrm{K/W}\)

  • Power dissipation: Critical for high-frequency operation

Comparison with Other Devices

GTO vs Conventional Thyristor

GTO Conventional Thyristor
Self turn-OFF capability Requires external commutation
Higher latching current (\(2\,\mathrm{A}\) to \(10\,\mathrm{A}\)) Lower latching current (\(50\,\mathrm{mA}\) to \(500\,\mathrm{mA}\))
Higher ON-state drop (\(2\,\mathrm{V}\) to \(4\,\mathrm{V}\)) Lower ON-state drop (\(1\,\mathrm{V}\) to \(2\,\mathrm{V}\))
Complex gate drive circuits Simple gate drive
Poor reverse blocking (\(20\,\mathrm{V}\) to \(30\,\mathrm{V}\)) Good reverse blocking (same as forward)
Faster switching (up to \(10\,\mathrm{kHz}\)) Slower switching (up to \(1\,\mathrm{kHz}\))
Compact circuit size Bulky commutation circuits
Higher gate drive losses Lower gate drive losses

Protection and Snubber Circuits

GTO Protection Requirements

Why Protection is Needed

  • High \(\frac{\mathrm{d}v}{\mathrm{d}t}\) during turn-OFF can cause false turn-ON

  • High \(\frac{\mathrm{d}i}{\mathrm{d}t}\) during turn-ON can damage the device

  • Gate-cathode junction needs protection from overvoltage

Protection Circuits

  • Turn-OFF snubber (RC circuit across anode-cathode):

    • Limits \(\frac{\mathrm{d}v}{\mathrm{d}t}\) during turn-OFF

    • Typical values: \(R = 10\,\mathrm{\Omega}\) to \(100\,\mathrm{\Omega}\), \(C = 0.1\,\mathrm{\mu F}\) to \(10\,\mathrm{\mu F}\)

  • Turn-ON snubber (series inductor):

    • Limits \(\frac{\mathrm{d}i}{\mathrm{d}t}\) during turn-ON

  • Gate protection: Zener diodes across gate-cathode

  • Overcurrent protection: Fast-acting fuses

Thermal Management

  • Adequate heat sinking required due to higher conduction losses

  • Junction temperature monitoring essential

Advantages and Applications

Advantages and Disadvantages of GTO

Advantages

  • Self turn-OFF capability

  • Eliminates commutation circuits

  • Faster switching than SCRs

  • Higher power density

  • Reduced electromagnetic interference

  • Simplified control circuits

  • Better dynamic response

Disadvantages

  • Complex gate drive circuits

  • Higher gate drive power

  • Poor reverse blocking

  • Higher conduction losses

  • Limited switching frequency

  • Requires anti-parallel diode

  • More expensive than SCRs

Trade-off: Higher complexity and cost vs. improved performance and circuit simplification

Applications of GTO

Major Application Areas

1. Variable Speed Drives:

  • DC motor drives (choppers)

  • AC motor drives (voltage source inverters)

  • High-performance industrial drives

2. Power System Applications:

  • Static VAR compensators (SVCs)

  • High-voltage DC transmission (HVDC)

  • Power quality improvement systems

3. Traction Systems:

  • Railway locomotive drives

  • Electric vehicle propulsion

4. Industrial Applications:

  • Uninterruptible power supplies (UPS)

  • Induction heating

  • Welding equipment

Conclusion

Summary and Key Points

Key Concepts Learned

  • GTO provides controllable turn-OFF capability through negative gate current

  • Structural modifications enable turn-OFF: anode shorts, interdigitated structure

  • Turn-OFF gain \(\beta_{\mathrm{off}} = I_A/|I_g|\) is typically 3–10

  • Trade-offs: Better controllability vs. higher complexity and losses

  • Requires protection circuits and anti-parallel diode

Design Considerations

  • Gate drive circuit design is critical

  • Snubber circuits are mandatory

  • Thermal management is important

  • Consider switching frequency limitations

Modern Perspective

  • GTOs paved the way for modern self-commutated devices

  • Being replaced by IGBTs and IGCTs in many applications

  • Still used in very high power applications (\(1\,\mathrm{MW}\) and above)