Voltage Regulators

Voltage Regulators:

An ideal power supply maintains a constant voltage at its output terminals under all operating conditions. The output voltage of a practical power supply changes with load generally dropping as load current increases as shown in fig. 1.

Fig. 1

The terminal voltage when full load current is drawn is called full load voltage (V_FL). The no load voltage is the terminal voltage when zero current is drawn from the supply, that is, the open circuit terminal voltage.

Power supply performance is measured in terms of percent voltage regulation, which indicates its ability to maintain a constant voltage. It is defined as

The Thevenin's equivalent of a power supply is shown in fig. 2. The Thevenin voltage is the no-load voltage V_NL and the Thevenin resistance is called the output resistance R_o. Let the full load current be I_FL. Therefore, the full load resistance R_FL is given by

Fig. 2

From the equivalent circuit, we have

and the voltage regulation is given by

It is clear that the ideal power supply has zero outut resistance.

Example-1

A power supply having output resistance 1.5Ω supplies a full load current of 500mA to a 50Ω load. Determine:

1. percent voltage regulation of the supply
2. no load output voltage.

Solution:

(a). Full load output voltage V_FL = (500mA ) (50Ω) = 25V.

Therefore,   

(b). The no load voltage 

An unregulated power supply consists of a transformer (step down), a rectifier and a filter. These power supplies are not good for some applications where constant voltage is required irrespective of external disturbances. The main disturbances are:

1. As the load current varies, the output voltage also varies because of its poor regulation.
2. The dc output voltage varies directly with ac input supply. The input voltage may vary over a wide range thus dc voltage also changes.
3. The dc output voltage varies with the temperature if semiconductor devices are used.

An electronic voltage regulator is essentially a controller used along with unregulated power supply to stabilize the output dc voltage against three major disturbances

1. Load current (I_L)
2. Supply voltage (V_i)
3. Temperature (T)

Fig. 3, shows the basic block diagram of voltage regulator. where

V_i = unregulated dc voltage.

V_o = regulated dc voltage.

Fig. 3

Since the output dc voltage V_Lo depends on the input unregulated dc voltage V_i, load current I_L and the temperature t, then the change ΔV_o in output voltage of a power supply can be expressed as follows

V_O = V_O(V_i, I_L, T)

Take partial derivative of V_O, we get,

S_V gives variation in output voltage only due to unregulated dc voltage. R_O gives the output voltage variation only due to load current. S_T gives the variation in output voltage only due to temperature.

The smaller the value of the three coefficients, the better the regulations of power supply. The input voltage variation is either due to input supply fluctuations or presence of ripples due to inadequate filtering.

 Voltage Regulator:  

A voltage regulator is a device designed to maintain the output voltage of power supply nearly constant. It can be regarded as a closed loop system because it monitors the output voltage and generates the control signal to increase or decrease the supply voltage as necessary to compensate for any change in the output voltage. Thus the purpose of voltage regulator is to eliminate any output voltage variation that might occur because of changes in load, changes in supply voltage or changes in temperature.

Zener Voltage Regulator:

The regulated power supply may use zener diode as the voltage controlling device as shown in fig. 4. The output voltage is determined by the reverse breakdown voltage of the zener diode. This is nearly constant for a wide range of currents. The load voltage can be maintained constant by controlling the current through zener.

Fig. 4

The zener diode regulator has limitations of range. The load current range for which regulation is maintained, is the difference between maximum allowable zener current and minimum current required for the zener to operate in breakdown region. For example, if zener diode requires a minimum current of 10 mA and is limi­ted to a maximum of 1A (to prevent excessive dissipation), the range is 1 - 0.01 = 0.99A. If the load current variation exceeds 0.99A, regulation may be lost.

Emitter Follower Regulator:

To obtain better voltage regulation in shunt regulator, the zener diode can be connected to the base circuit of a power transistor as shown in fig. 5. This amplifies the zener current range. It is also known as emitter follower regulation.

Fig. 5

This configuration reduces the current flow in the diode. The power transistor used in this configuration is known as pass transistor. The purpose of C_L is to ensure that the variations in one of the regulated power supply loads will not be fed to other loads. That is, the capacitor effectively shorts out high-frequency variations.

Because of the current amplifying property of the transistor, the current in the zenor dioide is small. Hence there is little voltage drop across the diode resistance, and the zener approximates an ideal constant voltage source.

Operation of the circuit:

The current through resistor R is the sum of zener current I_Z and the transistor base current I_B ( = I_L / β ).

I_L = I_Z + I_B

The output voltage across R_L resistance is given by

V_O = V_Z – V_BE

Where V_BE » 0.7 V

Therefore, V_O= constant.

The emitter current is same as load current. The current I_R is assumed to be constant for a given supply voltage. Therefore, if I_L increases, it needs more base currents, to increase base current I_zdecreases. The difference in this regulator with zener regulator is that in later case the zener current decreases (increase) by same amount by which the load current increases (decreases). Thus the current range is less, while in the shunt regulators, if I_L increases by ΔI_L then I_B should increase by ΔI_L / β or I_Z should decrease by ΔI_L / β. Therefore the current range control is more for the same rating zener.

The simplified circuit of the shunt regulator is shown in fig. 6.

Fig. 6

In a power supply the power regulation is basically, because of its high internal impedance. In the circuit discussed, the unregulated supply has resistance R_S of the order of 100 ohm. The use of emitter follower is to reduce the output resistance and it becomes approximately.

R_O = ( R_z + h_ie ) / (1 + h_fe)

Where R_Z represents the dynamic zener resistance. The voltage stabilization ratio S_V is approximately

S_V = ∂ V_o / ∂ V_I = R_z / (R_z + R)

S_V can be improved by increasing R. This increases V_CE and power dissipated in the transistor. Other disadvantages of the circuit are.

1. No provision for varying the output voltage since it is almost equal to the zener voltage.
2. Change in V_BEand V_z due to temperature variations appear at the output since the transistor is connected in series with load, it is called series regulator and transistor is allow series pass transistor.