TRIAC and GTO Thyristors: Operation and Advantages
TRIAC
- An SCR is a unidirectional device; it can conduct from anode to cathode only and not from cathode to anode.
A TRIAC can conduct in both directions.
TRIAC: Bidirectional thyristor with three terminals.
Extensively used for the control of power in AC circuits.
- A TRIAC is a bidirectional device, and its terminals can have various combinations of positive and negative voltages. There are four possible electrode potential combinations, as given below:
MT2 is positive with respect to MT1, G positive with respect to MT1.
MT2 is positive with respect to MT1, G negative with respect to MT1.
MT2 is negative with respect to MT1, G negative with respect to MT1.
MT2 is negative with respect to MT1, G positive with respect to MT1.
Mode 1 (MT2 is Positive with Respect to MT1, G Positive with Respect to MT1)
- TRIAC operates in the first quadrant.
- When MT2 is positive with respect to MT1, junctions P1N1 and P2N2 are forward biased.
- Junction N1P2 is reverse biased.
- When G is positive with respect to MT1, gate current flows mainly through P2N2.
- When gate current has injected sufficient charges into the P2 layer, the reverse-biased junction N1P2 breaks down.
- As a result, the TRIAC starts conducting through P1N1P2N2.
Mode 2 (MT2 is Positive with Respect to MT1, G Negative with Respect to MT1)
When G is negative with respect to MT1, gate current flows through P2N3.
N1P2 is forward biased.
Conduction through P1N1P2N3.
- With the above conduction, the voltage drop across this path falls, but P2N3 rises towards MT2.
A potential gradient exists across P2.
- The left-hand region becomes higher potential than the right-hand region.
Current (shown in the dotted line) is established.
Mode 3 (MT2 is Negative with Respect to MT1, G Negative with Respect to MT1)
- N3 acts as a remote gate.
- Gate current flows from P2 to N3.
- Reverse-biased junction N1P1 is broken.
- Finally, P2N1P1N4 is turned on completely.
- Operation is in the 3rd quadrant.
- The device is more sensitive under this condition.
Mode 4 (MT2 is Negative with Respect to MT1, G Positive with Respect to MT1)
Gate current forward biases junction P2N2.
N2 injects electrons into the P2 layer (shown by dotted arrows).
- As a result, the reverse-biased junction N1P1 breaks down.
- The structure P2N1P1N4 is completely turned on.
- Current after turn-on is limited by external load.
The device is less sensitive.
Operation is in the 3rd quadrant.
Triggering Circuit for TRIAC
GTO (Gate Turn-Off Thyristor)
- Conventional thyristors (CTs) are nearly ideal switches for their use in power electronic applications.
- These can be easily turned on by a positive gate current.
Once in the on state, the gate loses control.
- CTs can now be turned off by expensive and bulky commutation circuitry.
- This shortcoming of thyristors limits their use up to about 1KHz applications.
- These drawbacks in thyristors have led to the development of GTOs.
- GTO is a more versatile power semiconductor device.
- Like a CT but with added features.
- It can be easily turned off by a negative gate pulse of appropriate amplitude.
- GTO: A PNPN device that can be turned on by a positive gate current and turned off by a negative gate current at its gate-cathode terminal.
- The self-turn-off capability of GTO makes it the most suitable device for inverter and chopper circuits.
Comparison between GTO and Thyristor
Disadvantages
- The magnitude of latching and holding current is more.
- On-state voltage drop and associated loss are more.
- Triggering gate current required is high.
- Gate drive circuit losses are more.
- Reverse voltage blocking capability is less than forward voltage blocking capability.
Advantages
Faster switching speed.
- Surge current capability is comparable with an SCR.
- More di/dt rating at turn-on.
- GTO circuit configuration has low size and weight.
- Higher efficiency.
- Reduced acoustical and electromagnetic noise due to the elimination of commutation chokes.
Power MOSFET
