02-03-2013, 11:47 AM
Gate Turn On Turn Off Thyristors
[attachment=52392]
What is a thyristor?
Thyristors are power semiconductor devices used in power electronic circuits
They are operated as bistable switches operating from non-conducting to conducting states.
They are made of 4 layers and 3 pn junctions.
Thyristors are three terminal devices the terminals being anode, cathode and gate.
How is a thyristor different from transistor?
Thyristors have lower on state conduction losses.
They also have higher power handling capacity.
On the other hand transistors have superior switching performances
Transistors also have better switching speed and lesser switching losses
Working of a thyristor
When anode voltage is positive with respect to cathode junctions J1 and J3 are forward biased and junction J2 is reverse biased and the thyristor is off. This state is called forward blocking state.
When the voltage is increased the reverse junction breaks and it is called forward breakdown voltage.
In this state a large anode current flows and the thyristor is said to be on or in conducting state
WHAT IS A GTO?
GTO stands for Gate Turn On Turn Off Thyristor.
It is a four layered pnpn device.
It three terminals just like all thyristors.
GTO is similar to SCR s and can be built with current and voltage ratings similar to that of an SCR.
It was developed in the late 1960 s.
HOW IS A GTO DIFFERENT FROM OTHER THYRISTORS?
Conventional thyristors have only gate – controlled turn on capabilities.
They can recover from conducting state to a non-conducting state only when the current is brought down to zero by other means.
GTO s on the other hand have gate controlled turn off capacity.
They can be switched on or off by applying short positive or negative pulses respectively.
CONSTRUCTION OF GTO
Compared to a conventional thyristor it has an additional n+ layer near the anode .
In order to obtain high emitter efficiency the n+ cathode layer is highly doped.
This forms the turn off circuit between the gate and the cathode.
The doping level of the p type gate region is highly varied.
GTO – TURN ON
The turn on process for GTO is similar to a conventional thyristor.
GTO has no regenerative state and hence it requires a large initial gate trigger pulse for turning on.
If the anode current is low a longer period of gate pulse is required.
GTO TURN OFF
The turn off characteristics is greatly influenced by the gate turn off circuit and it must match the device requirements.
This process involves the extraction of gate charge, gate avalanche period and anode current decay.
The charge extraction is not significantly affected by external circuit conditions
The turn off time and peak turn off current depend on the external circuit components.
FET Controlled Thyristors
Combines a MOSFET & a thyristor in parallel as shown.
High switching speeds & high di/dt & dv/dt.
Turned on like conventional thyristors.
Cannot be turned off by gate control.
Application of these are where optical firing is to be used.
MOS-Controlled Thyristor
New device that has become commercially available.
Basically a thyristor with two MOSFETs built in the gate structure.
One MOSFET for turning ON the MCT and the other to turn OFF the MCT.
Features
Low on-state losses & large current capabilities.
Low switching losses.
High switching speeds achieved due to fast turn-on & turn-off.
Low reverse blocking capability.
Gate controlled possible if current is less than peak controllable current.
Gate pulse width not critical for smaller device currents.
Gate pulse width critical for turn-off for larger currents.
[attachment=52392]
What is a thyristor?
Thyristors are power semiconductor devices used in power electronic circuits
They are operated as bistable switches operating from non-conducting to conducting states.
They are made of 4 layers and 3 pn junctions.
Thyristors are three terminal devices the terminals being anode, cathode and gate.
How is a thyristor different from transistor?
Thyristors have lower on state conduction losses.
They also have higher power handling capacity.
On the other hand transistors have superior switching performances
Transistors also have better switching speed and lesser switching losses
Working of a thyristor
When anode voltage is positive with respect to cathode junctions J1 and J3 are forward biased and junction J2 is reverse biased and the thyristor is off. This state is called forward blocking state.
When the voltage is increased the reverse junction breaks and it is called forward breakdown voltage.
In this state a large anode current flows and the thyristor is said to be on or in conducting state
WHAT IS A GTO?
GTO stands for Gate Turn On Turn Off Thyristor.
It is a four layered pnpn device.
It three terminals just like all thyristors.
GTO is similar to SCR s and can be built with current and voltage ratings similar to that of an SCR.
It was developed in the late 1960 s.
HOW IS A GTO DIFFERENT FROM OTHER THYRISTORS?
Conventional thyristors have only gate – controlled turn on capabilities.
They can recover from conducting state to a non-conducting state only when the current is brought down to zero by other means.
GTO s on the other hand have gate controlled turn off capacity.
They can be switched on or off by applying short positive or negative pulses respectively.
CONSTRUCTION OF GTO
Compared to a conventional thyristor it has an additional n+ layer near the anode .
In order to obtain high emitter efficiency the n+ cathode layer is highly doped.
This forms the turn off circuit between the gate and the cathode.
The doping level of the p type gate region is highly varied.
GTO – TURN ON
The turn on process for GTO is similar to a conventional thyristor.
GTO has no regenerative state and hence it requires a large initial gate trigger pulse for turning on.
If the anode current is low a longer period of gate pulse is required.
GTO TURN OFF
The turn off characteristics is greatly influenced by the gate turn off circuit and it must match the device requirements.
This process involves the extraction of gate charge, gate avalanche period and anode current decay.
The charge extraction is not significantly affected by external circuit conditions
The turn off time and peak turn off current depend on the external circuit components.
FET Controlled Thyristors
Combines a MOSFET & a thyristor in parallel as shown.
High switching speeds & high di/dt & dv/dt.
Turned on like conventional thyristors.
Cannot be turned off by gate control.
Application of these are where optical firing is to be used.
MOS-Controlled Thyristor
New device that has become commercially available.
Basically a thyristor with two MOSFETs built in the gate structure.
One MOSFET for turning ON the MCT and the other to turn OFF the MCT.
Features
Low on-state losses & large current capabilities.
Low switching losses.
High switching speeds achieved due to fast turn-on & turn-off.
Low reverse blocking capability.
Gate controlled possible if current is less than peak controllable current.
Gate pulse width not critical for smaller device currents.
Gate pulse width critical for turn-off for larger currents.