Protection Circuit in Regulators

Protection Circuit in Regulators

The series regulator using transistors has no short circuit protection. If the load terminals are shorted accidently then,

1. Large amount of load current will flow.

2. The pass transistor Q₁ will get destroyed.

3. The diodes used in unregulated power supply, supplying the input voltage V_in to regulator circuit may get destroyed.

To avoid all such possibilities some sort of current limiting must be provided to the series regulator.

The various protection circuits used for the series voltage regulators are,

1. Constant current limiting (Short circuit protection).

2. Fold back current limiting. 

3. Overvoltage protection.

4. Thermal shutdown.

1. Constant Current Limiting Circuit

A block of series regulation using simple current limiting circuit is shown in the Fig. 5.9.1. It is also called short circuit protection circuit as it provides protection against short circuiting. The resistance R₄ is added m series with the pass transistor Q₁ and the load which is called current sensing resistor.

The drop across the current sensing resistance R₄ is applied to the base-emitter of Q₃. Under normal working condition and rated load current, this drop is insufficient to turn on the transistor Q₃ and hence pass transistor Q₁ continues to supply the rated load current.

When current increases due to over loading or short circuit conditions then drop across R₄ increases more than 0.6 V. The 0.6 V is sufficient to turn on the transistor Q₃. The collector current of Q₃ flows through R₃ and decreases the base voltage of Q₁. This decreases the load voltage. This decrease in output voltage prevents the large load current.

The value of R₄ can be selected to adjust the rated current of the circuit. As drop across R₄ should be less than 0.6 V under normal working condition, for rated current of 1 A, R₄ can be selected as 0.7 9. For a rated current of 7 A, R₄ must be selected as 0.1 2 and so on.

Let us study the variation of load current against load voltage with simple current limiting.

When R₁ = ∞ i.e. output terminals open, the output voltage is V_o current is zero.

When load increases, the load resistance decreases and the load current increases. The load current can increase till the drop across R₄ is not equal to 0.6 V. This load resistance at which drop across R₄ is 0.6 V is say Rmin. When load current further increases and R₁ becomes zero (short circuit), the load voltage decreases and the load current gets maintained at a value less than rated current. After R_L = R_min the regulation is lost and load voltage decreases as R_L decreases. This is shown in the 5.9.2.

Let I_SL = Load current when load terminals shorted.

In such a case, drop across R₄ is equal to V_BE to turn on the transistor Q₃.

V_BE = I_SL R₄

I_SL = V_BE / R₄  ... (5.9.1)

The minimum load resistance R_min  below which regulation is lost can be calculated as,

R_min = V_reg / I_SL ... (5.9.2)

The practical value of R_min will be slightly greater or less than this. 

2. Disadvantage of Simple Current Limiting

The disadvantage of constant current limiting is relatively large power dissipation in the series pass transistor when load terminals are shorted. Thus a large power rating transistor is required.

The power dissipation in the series pass transistor is given by,

P_D = (V_in - V_BE) I_SL         ...(5.9.3)

where V_BE = Base-emitter voltage of Q₁

The circuit which is used in practice which eliminates the disadvantage of simple current limiting is called foldback current limiting circuit.

Such protection is provided in IC 723 with the help of resistance R_SC connected to CL (current limiting) pin. The drop across R_SC is applied across pins CL and CS. When this voltage becomes more than 0.6 V, it turns on internal current limiting transistor Q₃. The calculations of RSC for IC 723 are discussed earlier while discussing the applications of IC 723.

3. Foldback Current Limiting

The disadvantage of constant current limit is relatively large power dissipation in the series pass transistor when the load terminals are shorted. Thus a large power rating transistor is required. The foldback limiting technique allows us to provide the necessary load current at rated voltage but reducing the short circuit current. Thus the series pass transistor gets utilised efficiently. The basic circuit for foldback limiting is shown in the Fig. 5.9.3.

All the voltages are measured with respect to a common point.

Let the voltage at point A be V_A and the current flowing through R₄ is almost I_L.

V_A = I_L R₄ + V_o    ... (5.9.4)

Neglecting the base current of Q₃, the cmrent flowing through R₅ and R₆ is same as I = V_A / R₅ + R₆ ... (5.9.5)

Hence the voltage at the base of Q₃ is the voltage across R₆

Thus if the output terminals are shorted, the output voltage V_o reduce to zero.

Hence we get from the equation (5.9.9)

The rated load current IL is also called I_knee known as knee current. 

It can be observed from the equation (5.9.11) that the rated load current is more than the short circuit current.

Key Point Thus when the output terminals are shorted, the current decreases rather than increasing. Thus the series pass transistor Q₁ gets protected automatically.

The ratio of rated load current and the short circuit current can be adjusted by selecting proper value of k. Typically k is selected to produce a maximum load current of 2 to 3 times the short circuit load current. The foldback characteristics which is the graph of the load voltage against load current is shown in the Fig. 5.9.4.

As seen from the characteristics, beyond the rated value, if the load increases, the voltage folds backward and finally becomes zero at Isc which is less than the rated load current. Hence the protection circuit is called foldback protection circuit.

The foldback current limiting circuit design with IC 723 is explained in the example below :

Example 5.9.1 Design a regulator using IC 723, to have V_o = 6V and the load current of 1 A. Use the foldback protection to get I_sc of 250 mA.

Solution : It is a low voltage less than 7 V and high current more than 150 mA, regulator. The circuit is shown in the Fig. 5.9.5.

Now to calculate foldback circuit components R₃, R₄ and R_SC.

Review Questions

1. Explain the use of current limiting technique.

Dec.-04, 05, 06, 10, May-07, Marks 4

2. Briefly explain protection circuits in voltage regulators.

Dec.-08, 09, Marks 8

3. Explain fold back characteristic of 723 IC regulator.

Dec.-15, Marks 8