Silicon Controlled Rectifier (SCR)

Silicon Controlled Rectifier (SCR)

• The SCR is a four layer p‒n‒p‒n device where P and n layers are alternately arranged. The outer layers are heavily doped while inner layers are lightly doped.

• There are three p‒n junctions called J₁, J₂ and J₃.

• The outer p layer is called anode while outer n layer is called cathode. Middle p layer is called gate.

• The three terminals are taken out respectively from these three layers, as shown in Fig. 6.2.1.

• Fig. 6.2.1 Construction of SCR

• Anode must be positive with respect to cathode to forward bias the SCR.

• But this is not sufficient criterion to turn SCR ON. To make it ON, a current is to be passed through the gate terminal denoted as I_GT. Thus it is a current operated device.

• The I_GT is the gate trigger current required to make the SCR ON.

• The basic material used for the SCR fabrication is silicon.

 

1. Types of Construction

• Three types of constructions are used to manufacture SCR,

1) Planar type

2) Mesa type

3) Press pack type.

1. Planar type:

This construction is used for low current SCRs. In this type, all the p‒n junctions come to the same surface on the cathode side. This is shown in Fig. 6.2.3. All the junctions are diffused in this type of construction.

• The disadvantage of this type is more silicon per ampere current is required. The advantage is that the mass production is possible and large number of SCRs can be manufactured with uniform characteristics.

2. Mesa type:

• In this construction, the junction J₂ is diffused while the outer layers are alloyed to it. This is shown in Fig. 6.2.4.

• To handle the large currents, the molybdenum or tungsten plates are braced to p‒n‒p‒n silicon pellet. This provides the additional mechanical strength.

• In this construction, area around the gate is small hence this construction is not suitable for high di/dt ratings.

3. Press pack type:

• The construction is used for high power and center gate SCRS. A silicon wafer is used to make such a high power SCR.

• The entire circular area around the gate takes part in the initial conduction hence di/dt capability of such SCRS is large. This is shown in Fig. 6.2.5.

• This type of construction provides double sided cooling arrangement which is necessary for high power SCRs.

 

2. Working Principle

• The operation of SCR is divided into two categories,

i) When Gate is open and

ii) When Gate is closed.

1. When gate is open :

• Consider that the anode is positive with respect to cathode and gate is open.

• The junctions J₁ and J₃ are forward biased and junction J₂ is reverse biased. There is depletion region around J₂ and only leakage current flows which is negligibly small.

• Practically the SCR is said to be OFF. This is called forward blocking state of SCR and voltage applied to anode and cathode with anode positive is called forward voltage. This is shown in Fig. 6.2.6 (a).

• With gate open, if cathode is made positive with respect to anode, the junctions J₁, J₃ become reverse biased and J₂ forward biased. Still the current flowing is leakage current, which can be neglected as it is very small.

• The voltage applied to make cathode positive is called reverse voltage and SCR is said to be in reverse blocking state. This is shown in Fig. 6.2.6 (b).

• In forward blocking state, if the forward voltage is increased and made suficiently large, the reverse biased junction J₂ breaks down and SCR conducts heavily. This voltage is called forward breakover voltage V_BO of SCR. In such condition, SCR is said to be ON or triggered.

2. When gate is closed :

• Consider that the voltage is applied between gate and cathode when the SCR is in forward blocking state.

• The gate is made positive with respect to the cathode.

• The electrons from n‒type cathode which are majority in number, cross the junction J₃ to reach to positive of battery.

• While holes from p type move towards the negative of battery, this constitutes the gate current.

• This current increases the anode current as some of the electrons cross junction J₂. As anode current increases, more electrons cross the junction J₂ and the anode current further increases.

• Due to regenerative action, within short time, the junction J₂ breaks and SCR conducts heavily. The connections are shown in Fig. 6.2.7. The resistance R is required to limit the current.

• Once the SCR conducts, the gate loses its control.

 

3. Two Transistor Model

• The two transistor model of SCR is shown in Fig. 6.2.8.

    Fig. 6.2.8 Two transistor model of SCR

• The left half is a pnp transistor and right half is npn transistor.

• The collector current of T₁ becomes base current of T₂ and collector current of T₂ becomes base current of T₁.

• Consider a positive voltage on the anode with respect to the cathode and gate current is zero.

• As gate current is zero, base current of T₂, I_B2 is zero and I_C2 is approximately I_CO.

• The base current of T₁, I_B1 = I_C2 = I_CO, is too small to turn T₁ ON. Both transistors are therefore in the "OFF" state, resulting in a high impedance between the collector and emitter of each transistor.

• The anode current is then just the sum of the leakage currents of the two transistors, I_CO1 + I_CO2.

• When we apply positive voltage from gate to cathode, holes are injected into the base of T₂. This forward biases the base emitter junction of T₂ increasing I_C2.

• This collector current is the base current for T₁, therefore increase in I_C2 (I_B1) will increase collector and emitter currents of T₁, resulting increase in base current of T₂.

• The increase in base current for T₂ will result in a further increase in I_C2.

• The net result is a regenerative increase in the collector current of each transistor. This regenerative process is continuous until both transistors are driven into saturation making all junctions forward biased.

• This results in large anode current which is limited only by the external circuit resistance and voltage.

• Let I_C1 and I_C2 are collector currents, I_E1 and I_E2 are emitter currents while I_B1 and I_B2 are base currents of transistors T₁ and T₂.

• Let both the transistors are operating in active region.

• From transistor analysis we can write,

 I_C1 = α₁I_E1 +I_C01 and I_C2 = α₂1_E2 + I_CO2

where I_CO  = Reverse current or leakage current

and

 α =  β / 1+β

Now I_E2 = I_C2 + I_B2

             …… (6.2.1)

I_A = Anode current = I_EI

I_K = Cathode current = I_E2,

I_G = Gate current

Now

I_K = I_A + I_G      …… (6.2.2)

I_E2 = I_A+I_G = I_C2+I_B2      …… (6.2.3)

But

 I_B2 = I_C1 + I_G      …… (6.2.4)

I_A+I_G = I_C2 + I_C1 + I_G      …… (6.2.5)

Substituting I_C1 and I_C2,

 I_A = α₁I_E1 + I_C01 + α₂I_E2 +I_C02

      …… (6.2.6)

Ι_Α = α₂ (I_A + I_G) + α₁I_A +I_C01 +I_C02

          ………Using (6.2.3)

 Ι_Α ‒ α₂I_A ‒ α₁I_A = α₂I_G +I_C01 +I_C02

 I_A = [ a₂I_G + I_C01 + I_C02 ] / [ 1‒(α₁+α₂) ]

      …… (6.2.7)

• In blocking state α₁ and α₂ are small. Thus I_A is small. As α₁+α₂ approaches unity, the SCR is ready to enter into conduction. Then due to positive gate current, the regenerative action takes place and SCR conducts.

 

4. Characteristics of SCR

• Fig. 6.2.9 shows the characteristics of SCR.

• The characteristics are divided into two sections:

1. Forward characteristics :

• It shows a forward blocking region, when I_G = 0. It also shows that when forward voltage increases upto V_BO, the SCR turns ON and high current results. The drop across SCR reduces suddenly which is now the ohmic drop in the four layers. The current must be limited only by the external resistance in series with the device.

• It also shows that, if gate bias is used then as gate current increases, less voltage is required to turn ON the SCR.

• If the forward current falls below the level of the holding current I_H, then depletion region begins to develop around J₂ and device goes into the forward blocking region.

• When SCR is turned ON from OFF state, the resulting forward current is called latching current I_L. The latching current is slightly higher than the holding current.

2. Reverse characteristics :

• If the anode to cathode voltage is reversed, then the device enters into the reverse blocking region. The current is negligibly small and practically neglected.

• If the reverse voltage is increased, similar to the diode, at a particular value avalanche breakdown occurs and a large current flows through the device. This is called reverse breakdown and the voltage at which this happens is called reverse breakdown voltage V_BR.

• The forward breakover voltage is greater than reverse breakover voltage.

 

5. SCR Parameters

1. Forward breakover voltage (V_BO):

It is the voltage above which the SCR enters the conduction region ('ON' state). The The forward breakdown voltage is dependent on the gate bias.

2. Holding current (I_H):

It is that value of current below which the SCR switches from the conduction state (ON state) to the forward blocking state.

3. Latching current (I_L) :

This is the minimum current flowing from anode to cathode when SCR goes from OFF to ON state and remains in ON state even after gate bias is removed. It is greater than, but very close to holding current.

4. Reverse breakdown voltage (V_BR):

It is the reverse voltage (Anode‒negative and cathode‒positive) above which the reverse breakdown occurs, breaking J₁ and J₃ junctions.

5. Maximum on‒state voltage:

It is the maximum value of the voltage appearing across SCR during the conduction (on‒state). Typically, it is 1 V to 1.5 V.

6. Current rating:

It is the maximum current carrying ability of the SCR.

7. Minimum gate trigger current (I_GTmin)

The minimum value of gate current which can trigger SCR is defined as I_GTmin'.

8. Maximum gate current (I_GTmax):

It is the peak value of gate current which must not be exceeded to avoid damage to the SCR.

9. Gate power loss (P_G):

It is the mean power loss due to gate current between the gate and the main terminal.

10. Turn on time (t_on):

The time required by SCR to reach full conduction after triggering is called 'turn on time'.

The turn on time consists of: i) Time required for charging gate to cathode capacitance and ii) Time required for reaching latching current value.

Typically, turn on time of SCR is of the order of 2‒4 μsec.

11. Turn off time (t_off):

It is time required from the zero current point to the time when the SCR regains its full blocking voltage in positive direction after the application of reverse voltage across it.

Typically, the turn off time of SCR is of the order of 10‒50 μsec. For high frequency SCRS it is 10‒20 μsec.

12. Gate reverse voltage (V_GRM):

The maximum reverse voltage which gate can handle safely is called gate reverse voltage or maximum reverse gate voltage.

 

6. Merits of SCR

1. Very small amount of gate drive is required.

2. SCRS with high voltage and current ratings are available.

3. On state losses of SCR are less.

4. Can handle large power

5. Can be used as a switch.

6. Easy to turn on.

7. Can be easily protected with a fuse.

 

7. Demerits of SCR

1. Gate has no control, once SCR is turned on.

2. External circuits are required for turning it off.

3. Operating frequencies are low.

4. Additional protection circuits are required.

5. Conducts only in one direction hence controls power during only one half cycle of a.c. input.

 

8. Applications of SCR

1. Controlled rectifiers.

2. A.C. voltage stabilizers.

3. D.C. to D.C. converters called choppers.

4. D.C. to A.C. converters called inverters.

5. Dimmerstats to control light intensity.

6. For speed control schemes of d.c. and a.c. motors Ercalled drives.

7. As a switch.

8. Heater control circuit.

9. In protection circuits.

 

9. SCR Crowbar Circuit

• Crowbar circuits provide protection against over voltage conditions for entire circuit. Fig. 6.2.10 shows crowbar circuit using SCR.

• The zener diode in the circuit is selected such that at normal output voltage it acts as an open switch. This is because Vcc is less than the breakdown voltage of zener diode. Hence voltage across R is zero and SCR remains open.

• When output voltage of the supply increases than the normal supply voltage by any reason, the zener diode conducts and a voltage appears across R. This voltage is sufficient to turn ON the SCR.

• The conduction of SCR reduces the voltage drop across it and thus protects the circuit from large over voltage.

• As the turning ON of SCR is very fast, the instantaneous protection against the high voltage is provided to the load.

 

Review Questions

1. Explain the construction of SCR.

2. Enumerate the structure of a SCR, with a two transistor model derive an expression for anode current (I_A) Also indicate its V‒I characteristics.

3. Explain the working principle of SCR and its V‒I characteristics.

4. Draw and explain the operation of SCR using two transistor equivalent circuit.

5. Derive the expression for the anode current from the two transistor model of SCR.

6. Draw and explain the characteristics of SCR.

7. Define the following with respect to SCR:

i. Forward breakover voltage ii. Holding current iii. Latching current iv. Reverse breakdown voltage.

8. State the various specifications of SCR.

9. State the merits of SCR.

10. State the demerits of SCR.

11. State the various applications of SCR.

12. Explain the operation, characteristics and applications of SCR.