Thyristor Devices GTO & SCR

Basics of Gate Turn-Off (GTO) Thyristor:

A Gate Turn off Thyristor or GTO is a three terminal, bipolar (current controlled) semiconductor switching device. Similar to conventional thyristor, the terminals are anode, cathode and gate as shown in figure below. As the name indicates, it has gate turn off capability. These are capable not only to turn ON the main current with a gate drive circuit, but also to turn it OFF. A small positive gate current triggers the GTO into conduction mode and also by a negative pulse on the gate, it is capable of being turned off. Observe in below figure that the gate has double arrows on it which distinguish the GTO from normal thyristor. This indicates the bidirectional current flow through the gate terminal.

SYMBOL:

CONSTRUCTION

It is also a four layer, three junction P-N-P-N device like a standard thyristor. In this, the n+ layer at the cathode end is highly doped to obtain high emitter efficiency. This result the breakdown voltage of the junction J3 is low which is typically in the range of 20 to 40 volts.
The doping level of the p type gate is highly graded because the doping level should be low to maintain high emitter efficiency, whereas for having a good turn OFF properties, doping of this region should be high. In addition, gate and cathodes should be highly inter-digited with various geometric forms to optimize the current turn off capability.

BLOCK DIAGRAM

PRINCIPLE OF OPERATION

TURN OFF

To turn OFF a conducting GTO, a reverse bias is applied at the gate by making the gate negative with respect to cathode. A part of the holes from the P base layer is extracted through the gate which suppress the injection of electrons from the cathode. In response to this, more hole current is extracted through the gate results more suppression of electrons from the cathode. Eventually, the voltage drop across the p base junction causes to reverse bias the gate cathode junction and hence the GTO is turned OFF. During the hole extraction process, the p-base region is gradually depleted so that the conduction area squeezed. As this process continuous, the anode current flows through remote areas forming high current density filaments. This causes local hot spots which can damage the device unless these filaments are extinguished quickly. By the application of high negative gate voltage these filaments are extinguished rapidly. Due to the N base region stored charge, the anode to gate current continues to flow even though the cathode current is ceased. This is called a tail current which decays exponentially as the excess charge carriers are reduced by the recombination process. Once the tail current reduced to a leakage current level, the device retains its forward blocking characteristics.

V-I CHARACTERISTICS CURVE

Symbol equivalent circuit MCT VI characteristics

Silicon Controlled Rectifiers (SCR)

Thyristor devices can convert and control large amounts of power in AC or DC systems while using very low power for control.
Thyristor family includes

1- Silicon controlled switch (SCR)

2- Gate-turnoff thyristor (GTO)

3- Triac

4- Diac

5- Silicon controlled switch (SCS)

6- Mos-controlled switch (MCT) 1

INTRODUCTION

SCR is most popular of thyristor family due to its Fast switching action, small size and high voltage and current ratings.
It is commonly used in power electronic applications.
SCR has 3 terminals (gate provides control)
SCR is turned on by applying +ve gate signal
when anode is +ve with repect to cathode.
SCR is turned off by interrupting anode current.

PNPN structure

TWO TRANSISTOR MODEL OF SCR

Gate requires small positive pulse for short duration to turn SCR on. Once the device is on, the gate signal serves no useful purpose and can be removed.

SCR CHARACTERISTIC CURVE

IDEAL CHARACTERISTIC OF SCR

CONDUCTION ANGLE

Duration for which SCR is on. It is measured as shown

Surge Current Rating

Peak anode current that SCR can handle for brief duration.

3- Latching current

Minimum anode current that must flow through the SCR in order for it to

stay on initially after gate signal is removed.

Holding Current

Minimum value of anode current, required to maintain SCR in conducting

state.

SCR VOLTAGE RATINGS

1- Peak repetitive forward blocking voltage

Maximum instantaneous voltage that SCR can block in forward direction.

2- Peak Repetitive Reverse Voltage

Maximum instantaneous voltage that SCR can withstand, without breakdown, in reverse direction.

3- Non-repetitive peak reverse voltage

Maximum transient reverse voltage that SCR can withstand.

SCRs are connected in series and parallel to extend voltage and current ratings.

For high-voltage, high-current applications, series-parallel combinations of SCRs are used.

DISADVANTAGE OF DC GATING SIGNALS

1. Constant DC gate signal causes gate power dissipation

2. DC gate signals are not used for firing SCRs in AC applications, because presence of positive gate signal during negative half cycle would increase the reverse anode current and possibly destroy the device.

PULSE SIGNALS

1. Instead of continuous DC signal, single pulse or train of pulses is generated.

2. It provides precise control of point at which SCR is fired.

3. It provides electrical isolation between SCR and gate-trigger circuit.