SILICON CONTROLLED RECTIFIER

• It  is  a  four  layered  PNPN  device  and  is  a  prominent  member  of  thyristorfamily.   It consists of   three diodes   connected   back   to   back   with   gate connection or two complementary transistor connected back to back.

• It  is  widely  used  as  switching  device  in  power  control  applications.  It  canswitch ON for variable length of time and delivers selected amount of power to load.

• It  can  control  loads,  by  switching  the  current  OFF  and  ON  up  to  manythousand  times  a  second.  Hence  it  possess  advantage  of  RHEOSTAT  and  a switch with none of their disadvantages

Construction

As  shown  in  figure  1  it  is  a  four  layered  three terminal  device, layers  being
alternately  P-type  and  N-type  silicon.  Junctions  are  marked  J1,  J2,  J3  whereas terminals are anode (A), cathode
(C) and gate
(G). The gate terminal is connected to inner P-type layer and it controls the firing or switching of 5CR.

Biasing

The  biasing of  SCR is  shown  in  figure  
1(a).  The  junction  J1  and  J3  become
forward biased while J2 is reverse biased. In figure 1 polarity is reversed. It is seen that now junction J1 and J3 become reverse biased and only J2 is forward biased.

Operation of SCR

• In  SCR  a  load  is  connected  in  series  with  anode  and  is  kept  at  positivepotential  with  respect  to  cathode  when  the  gate  is  open  i.e.,  no  voltage  is applied at the gate.

• Under this condition, junctions J1 and J3 are for ward biased and junction J2is reverse biased. Due to this, no current flows through RL and hence the 8CR is cutoff.

• However when the anode voltage is increased gradually to breakover voltage,then  breakdown  occurs  at  junction  J  due  to  this  charge  carriers  are  able  to flow from cathode to anode easily, hence SCR starts conducting and is said tobe in ON state.

• The SCR  offers  very  small forward  resistance so that  it  allows infinitely highcurrent.  The  current  flowing  through  the  8CR  is  limited  only  by  the  anode voltage and external resistance.

• If  the  battery  connections  of  the  applied  voltage  are  reversed  as  shown  infigure 2 the junction J1 and J3 are reverse biased. J2 is forward biased.

• If  the  applied  reverse voltage is  small  the  SCR is  OFF  and  hence no  currentflows through the device.

• If the reverse voltage is increased to reverse breakdown voltage, the junctionJ1 and J3 will breakdown due to avalanche effect. This causes current to flow through the SCR.

• From the above discussion we conclude that the SCR can be used to conduct only in forward direction. Therefore SCR is called as “unidirectional device”.

Vl Characteristics of SCR

The  “forward characteristics”   of SCR may be obtained using the figure 3.

The volt-ampere characteristics of a SCR for IG = 0 is shown in figure 3.

 

• As the applied anode to cathode voltage is increased above zero, very smallcurrent flows through the device, under this condition the 5CR is off. It will be continued  until;  the  applied  voltage  reaches  the  forward  Breakover  voltage (point A).

• if  the  anode-cathode  (applied)  voltage  exceeds  the  Breakover  voltage  itconducts heavily the SCR turns ON  and  anode to  cathode voltage decreases quickly  to  a  point  B  because,  under  this  condition  the  5CR  offers  very  low resistance hence it drops very low voltage across it.

• At this stage the 5CR allows more current to low through it. The amplitude ofthe   current   is   depending   upon   the   supply   voltage   and   load   resistance connected in the circuit.

• The current corresponding to the point ‘B’ is called the “holding current (IH)”It can be defined as the minimum value of anode current required to keep the     SCR  in  ON  State.  If  the  5CR  falls  below  this  holding  current  the  SCR  turns OFF.

• If the value of the gate current I is increased above zero, (‘G > O) the SCRturns ON even at lower Breakover voltage as shown in figure 3(b).

• The region lying between the points OA is called forward blocking region. Inthis  region  5CR  is  OFF’.  The  region  lying  between  the  points  BC  is  called forward conduction region. In this region SCR is ON.

• Once  the  SCR  is  switched  ON  then  the  gate  looses  all  the  control.  So  SOBcannot  be  turned  OFF  by  varying  the  gate  voltage.  It  is  possible  only  by reducing the applied voltage.

To obtain the “reverse characteristics”the following points are followed.

1.In  this  case  the  SCR  is  reverse  biased,  if  the  applied  reverse  voltage  is increased  above  zero,  hence  a  very  small  current  flows  through  the  SCR.

 Under  this  condition  the  SCR  is  OFF,  it  continues  till  the  applied  reverse voltage reaches breakdown voltage.

2.As the applied reverse voltage is increased above the breakdown voltage, the avalanche  breakdown  occurs  hence  5CR  starts  conducting  in  the  reverse direction. It is shown in curve DE. Suppose the applied voltage is increased to   a very high value, the device may get damaged.

SCR rating

The SCR rating are defined as follows

1)  “Forward  Breakover  voltage”:It   is   the   voltage   at   which   the   5CR  is switched  from  its  OFF  position  to  ON  position.  Its:  values  are maximum  for zero gate current, its values lie in the range of 50 to l200volts

2)  “Holding  current”:  it  is  the  minimum  value  of  anode  current  required  to keep the SCR in ON position.

3)   “Gate  triggering  current”:    It  is  the  value  of  anode  current  necessary  to switch  5CR from  OFF to  ON  position under specified condition. It  is  typically   about 4OmA.

4)   “Average forward current”:   It is the maximum value of anode current at which the 5CR can handle in its ON position. Its value lies in the range of 1 to 1800Amps.

5)   “Reverse breakdown voltage”:   It is the value of reverse voltage between cathode to anode at which the avalanche breakdown occurs.

6)  “Turn ON time – TON”: It can be defined as, the time required to switch it from OFF to ON state when triggering signal is applied. TON decreases if the trigger voltage is increased; TON is increases when anode current increases.

7)   “Turn OFF Time – TOFF”: It is the time required to switch it  from ON to OFF  state  by  dropping  anode  voltage.  TOFF  is  small  if  anode  voltage  in reverse direction and increases with temperature and anode current.

8)  “Gate  Recovery  time”:    It  is  the  time  required  for  which  anode  voltage  is reduced to VH to turn OFF SCR.

Basic Operation of an SCR

The operation of a PNPN device can best be visualized as a specially coupled
pair of transistors as shown in Figure

The  connections  between the  two transistors  are  such that  regenerative  action  can occur when a proper gate signal is applied to the base of the NPN transistor. Normal leakage  current  is  so  low  that  the  combined  hFE  of  the  specially  coupled  two- transistor  feedback  amplifier  is  less  than  unity,  thus  keeping  the  circuit  in  an  off- state  condition.  A  momentary  positive  pulse  applied  to  the  gate  will  bias  the  NPN transistor into conduction  which, in turn,  biases  the  PNP transistor into conduction.    The  effective  hFE  momentarily  becomes  greater  than  unity  so  that  the  specially coupled  transistors  saturate.  Once  saturated,  current  through  the  transistors  is enough to keep the combined hFE greater than unity. The circuit will remain “on” until    it  is  “turned  off”  by  reducing  the  anode-to-cathode  current  (IT)  such  that  the combined  hFE  is  less  than  unity  and  regeneration  ceases;  this  threshold  anode current is the holding current of the SCR.

Electrical Characteristic Curves of Thyristors V-I Characteristics of SCR Device

Methods of Switching on Thyristors There are three general ways to switch thyristors to on-state condition:

• Application of Gate Signal

• Static dv/dt Turn-On

• Voltage Breakover Turn-On

Listed below is a brief description of each method.

Application Of Gate Signal

For an SCR (unilateral device), this signal must be positive with respect to the cathode polarity. A triac (bilateral device) can be turned on with gate signal of either polarity;  however,  different  polarities  have  different  requirements  of  IGT  and  VGT which must be satisfied. Since a diac does not have a gate, this method of turn-on is     not applicable to diacs; in fact, the single major application of diacs is to switch-on   triacs.

Static dv/dt Turn-On

Comes  from  a  fast  rising  voltage  applied  across  the  anode  and  cathode terminals of an SCR or the main terminals of a triac. Due to the nature of thyristor construction,  a  small  junction  capacitor  is  formed  across  each  PN  junction.  Figure 14.14  shows  how  typical  internal  capacitors  are  linked  in  gated  thyristors.  When voltage  is  impressed  suddenly  across  a  PN  junction,  a  charging  current  will  flow which is equal to:

i = C (dv/dt)

When   c   (dv/dt)   becomes   greater   or  equal  to  thyristor  IGT,  the thyristor  switches  on.  Normally,  this  type  of  turn  on  does  not  damage  or  hurt the device providing the surge current is limited. Generally, thyristor application circuits are designed with static dv/dt snubber networks if fast rising voltages are
anticipated.

Voltage Breakover Turn-On

Is the method used to switch on diacs. However, exceeding voltage break over of SCRs and triacs is definitely not recommended as a turn-on method. In the case of   SCRs  and  triacs,  the  leakage  current  increases  until  it  exceeds  the  gate  current required  to  turn-on  these  gated  thyristors  in  a  small  localized  point.  When  turn-on occurs by this method, there is localized heating in a small area which may melt the    silicon  or  damage  the  device  if  di/dt  of  the  increasing  current  is  not  sufficiently limited.  Diacs  used  in  typical  phase  control  circuits  are  basically  protected  against excessive  current  at  breakover  as  long  as  the  firing  capacitor  is  not  excessively large. When diacs are used in a zener function, current limiting is necessary.