TRIAC

• Two  thyristors  may be connected in inverse parallel,  but  at  moderate  powerlevels  the  two  antiparallel  thyristors  can  be  integrated  into  a  single  device structure, as shown.

• This  device  commonly  known  as  Triac  (triode a.c.  switch)  is  represented  bythe circuit symbol shown.

• Triac  is  the  word  derived  by  combining  the  capital  letters  from  the  wordsTRIode  &  a.c.  As  the  Triac  can  conduct  in  both  the  directions,  the  terms anode & cathode are not applicable to Triac.

• Its three terminals are usually designated as main terminals, MT1, MT2 & gateG, as in a thyristor.

• The  terminals  MT1is  the  reference  point  for  measurement  of  voltages  &currents  at  the  gate  terminal  &  at  the  terminal  MT2.  The  gate  is  near  to terminal MT1.

• The V-I characteristics of a Triac is shown. This characteristic of the Triac arebased on the terminal MT1 as the reference point.

• The first quadrant is the region wherein MT2 is positive with respect to MT1 &vice-versa for the third quadrant.

• The  peak  voltage  applied  across  the  device  in  either  direction  must  be  lessthan the break over voltage in order to retain control of the gate.

• A  gate  current  of  specified  amplitude  of  either  polarity  will  trigger  the Triacinto conduction in either quadrant, assuming that the device is in a blocking condition initially before the gate signal is applied.

• The characteristics of a Triac are similar to those of SCR, both in blocking &conducting states, expect for the fact that SCR conducts only in the forward direction, whereas the Triac conducts I both directions.

• Depending  upon  the  polarity  of  a  gate  pulse  &  biasing  conditions,  the  mainfour-layer structure that turns on by a regenerative process could be one of P1N1 P2N2, P1N1 P2N3, or P2N1 P1N4,

TRIGGERING MODES OF TRIAC

Triggering can be obtained from d.c., rectified a.c., or pulse sources such as
unijunction  transistors  &  switching  diodes  such  as  the  Diac,  silicon  bilateral  switch (SBS) & asymmetrical trigger switch.

1. MT2 positive, positive gate current (MODE 1)


When  the  gate  current  is  positive  with  respect  to  MT1  ,  gate  current  flows normally  from  the  gate  lead  to  the  terminal  MT1  through  the  P2  –  N2  junction  as shown. The  device  turns  on  in  the  conventional  manner  as  in  the  case  of  an  SCR. However, in the case of a Triac, the gate current requirement is higher for turn on at a  particular voltage.  Because  of  ohmic  contacts of  gate & MT1  terminals  on  the  P2- layer, some more gate current flows from the gate lead G to the main terminal MT1    through  the  semiconductor  P2  layer  without  passing through  the  P2N2 junction. The main  structure which  ultimately turns  on through  regenerative  action is  P1N1 P2N2.


The P2   layer is  flooded  with electrons  when the  gate current  flows  across  the P2N2 junction. These electrons diffuse to the  edge  of  the  junction J2, are collected by N1 layer. Therefore, the electrons build a space charge in the N1 region & more holes from P1 diffuse into N1 to neutralize the negative space charge. These holes arrive at    the junction J2. They produce a positive space charge in the P2 region which results
in more electrons being injected from N2 into P2. This results in positive regeneration & ultimately the structure P1N1 P2N2 conducts the external current.

2. MT2 positive, negative gate current (MODE 2)


A  cross  sectional  view  of  the  structure  is  shown.  When  the  terminal  MT2  is positive & gate terminal is negative with respect to terminal MT1, gate current flows through P2-N3 junction & this gate current IG forward biases the gate current P2-N3 of the auxiliary P1N1 P2N  3structure. As a result, Triac starts conducting through P1N1 P2N3 layers initially. With the conduction of P1N1 P2N3, the voltage drop across it falls but potential of layer between P2N3 rises towards the anode potential of MT2. As the right  hand  portion  of  P2  is  clamped  at  the  cathode  potential  of  MT1,  a  potential gradient  exists  across  layer  P2  from,  its  left  hand  region  begin  at  higher  potential than its right hand region. A current is thus established in layer P2 from left to right which  forward  biased  P2N2  junction  &  finally  the  main  structure  P1N1  P2N2,  may  be considered as a pilot SCR, while the structure OP1 N1 P2N2 may be regarded as the main SCR, both begin built in one common structure. The anode current of the pilot
SCR serves as the gate current for the main SCR. As compared with turn on process,
the  device  with  MT2 positive but  gate current negative is less sensitive & therefore
more gate current is required.

3. MT2 negative, positive gate current (MODE 3)


When terminal  MT2 is negative & terminal MT1 is  positive,  the device  can  be turned on  by  applying a  positive  voltage  between  the  gate &  terminal  MT1. In  this mode, the device operates in the third quadrant when it is triggered into conduction.    The turn on is initiated by remote gate control. The main structure that leads to turn on is P2N1 P1N4 with N2 acting as a remote gate as shown. The external gate current IG forward biases P2N2