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1. UNIT 1 SEMI CONDUCTOR SWITCHING CHARACTERISITCS

2. TRIAC TRIAC which belongs to the family of the thyristor can conduct in both directions as well as offer full control over the power supplied. Therefore, they are used for AC power regulation.

3. TRIAC - “ Tri ode for  A lternating  C urrent ”. It is a three-terminal bi-directional  switch  that conducts in both directions. It is made from the combination of two  SCRs  in anti-parallel with their gates joined together . The three terminals are Gate, A1 or MT1 and A2 or MT2. It does not have an anode and cathode like SCR because it can conduct in both directions

4. Construction of TRIAC TRAIC is a four-layer device that is made from a combination of two antiparallel SCRs having three terminals Gate, MT1 and MT2.

5. GTO A GTO thyristor , which stands for "Gate Turn-Off Thyristor ", is a three-terminal power semiconductor device that can be triggered in both on and off states using signals applied to its gate terminal ( gto gate turn off thyristor ).   GTO thyristors are also bipolar, voltage-triggered devices. 

6. GTO Symbol  The symbol of a GTO thyristor is quite similar to that of a conventional thyristor with a few differences ( gto thyristor symbol): It has three terminals - Anode, Cathode, and Gate. Double arrows are used at the gate terminal to signify bidirectional current flow during turn-on and turn-off actions.

7. Main Difference between Triac and GTO GTO Triac 1.Fully controlled switch. Triggered by the Turned on / off by gate gate signal Signal 2. Used in Power control AC power control and conservation systems applications. 3. Power handling: Used in both AC and DC Used in AC circuits 4. High power semi cond. Bipolar devices devices

8. Switching characteristics of a  Gate Turn Off Thyristor  or GTO comprises of dynamic characteristics during Turn ON and Turn OFF process. It basically represents the variation in anode voltage V a  and anode current I a  when either positive or negative gate signal I g  is applied. Turn On Characteristic of GTO: Turn on process in GTO is similar to that of  conventional thyristor . Gate turn on time of GTO is composed of delay time, rise time and spread time just like a normal thyristor . Besides, the turn on time can be decreased by increasing the forward gate current. Figure below shows the switching characteristics of a GTO.

9. A steep fronted gate pulse is applied in the above figure to turn on the GTO. It should be noted here that the gate drive can be removed once the anode current I a  reaches above the  latching current . However, some manufacturers suggests to not to remove the gate drive current to eliminate the possibility of unwanted turn off of gate turn off thyristor (GTO). Therefore, even when the GTO is turned on, a small amount of positive gate current is continuously applied. This small positive gate current is called “ Back Porch Current ”. This is shown in the above of I g  Vs. time (t) graph. Turn Off Characteristic of GTO: Unlike turn on characteristics, turn off characteristics of a GTO is different from an  SCR . Before the initiation of turn off process, GTO carries anode current I a  in forward direction. As soon as the negative gate current is applied at t=0, turn off process begins. The rate of rise of gate current depends on the circuit inductance and anode voltage. The very first step during turn off process is the removal of stored charges by the negative gate current. Stored charges here mean, the excess charges i.e. hole in p+ layers.  The time elapsed in removing the stored charges is called the  Storage Period ( t s ) . During this period, the anode voltage and current will remain unchanged.

10. Once the stored charges are removed, the anode current will fall rapidly and hence, the anode voltage starts rising. As can be seen from the figure that after storage time ( t s ) time, the anode current Ia starts to fall rapidly till a certain value and then changes its rate of fall abruptly. This time during which anode current falls rapidly is called the  Fall Time ( t f ) . This time is measured from the instant gate current is maximum negative to the instant anode current falls to its tail current. Kindly refer the figure to have the better understanding. The fall time is generally of the order of 1 micro second. At the end of the fall time ( t f ), there is a spike in the anode voltage due to abrupt change in rate of fall of anode current I a . After storage time and fall time, the anode voltage and current stars moving toward their turn off values i.e. rated anode voltage and zero respectively. The total time elapsed to reach anode voltage and current to their turn off values is called the  Tail Time ( t t ) . After tail time, anode current becomes zero but the anode voltage undergoes a  transient overshoot  due to the presence of resistance ( R s ) and capacitance (C s ) and then stabilizes to its off-state value i.e. source voltage applied to anode circuit. Here, R s  and C s  are the  Snubber Circuit  parameters. The duration of tail time ( t t ) depends on the characteristics of the device. From the above discussion, it may be said that the total turn off time of a GTO comprises of three times: Storage Time ( t s ), Fall Time ( t f ) and Tail Time ( t t ). So, turn off time ( t q ) of a Gate Turn Off Thyristor (GTO) may be written as t q  = t s  + t f  + t t

11. IGBT (Insulated Gate Bipolar Transistor)

12. Symbol and Structure The insulated gate bipolar transistor is A three terminal semiconductor device Gate, Emitter and Collector Emitter and Collector-Associated with a conductance path Gate terminal is associated with its control IGBT has MOSFET like input characteristics and Power BJT like output characteristics So its called as Voltage Controlled Device

13. Working When collector is at positive potential with respect to emitter and gate also at sufficient positive potential. With no voltage between gate and emitter, no current flow from collector to emitter and device is in off state. When gate is made positive with respect to emitter by voltage Vg, a channel is formed below the gate . Now the collector region injects electrons into n- region, and more number of electrons flow makes the device conduction( Ic ) The collector current Ic in IGBT constitutes of two components Ic = Ie + Ih Ie – Hole current due to injected holes from collector to drift region Ih – Electron current due to injected electrons from collector to drift region

14. Characteristics of IGBT

15. Transfer Characteristics The IGBT is in ON-state only after VGE is greater than a threshold value VGET.

16. Switching Characteristics of IGBT

17. Turn on time ton is composed of two components as usual, delay time ( tdn ) and rise time ( tr ) Delay Time Delay time is defined as the time in which collector current rises from leakage current ICE to 0.1 IC (final collector current) and collector emitter voltage falls from VCE to 0.9VCE Rise Time Rise time is defined as the time in which collector current rises from 0.1 IC to IC and collector emitter voltage falls from 0.9VCE to 0.1 VCE.

18. The turn off time toff consists of three components, delay time ( tdf ), initial fall time (tf1) and final fall time (tf2) Turn off Delay Time Delay time is defined as time when collector current falls from IC to 0.9 IC and VCE begins to rise Initial fall time Initial fall time is the time during which collector current falls from 0.9 IC to 0.2 IC and collector emitter voltage rises to 0.1 VCE Final fall time The final fall time is defined as time during which collector current falls from 0.2 IC to 0.1 IC and 0.1VCE rises to final value VCE

19. Triggering of SCR