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*Voltage Regulation:*

In unregulated power supply, output voltage changes whenever input supply voltage or load resistance changes. It is never constant. Change in voltage from no-load to full-load condition is known as voltage regulation. Aim of voltage regulator circuit is to decrease the variations to zero or, at least, to minimum possible value. Percentage regulation or, simply, regulation of power supply is provided by:

%regulation = (V_{max} - V_{min})/V_{max} X 100 minimum dc output voltage.

When we say that 10V regulated dc power supply has regulation of 0.005 per cent, it signifies that dc output voltage will differ within envelope 0.005 per cent of 10V.

Now 0.005% of 10V = 0.005X10/100

= 0.0005V = 0.5mV

Therefore, output voltage will differ by ±0.25mV. In general, %regulation = (V_{NL} - V_{FL})/V_{FL} X 100

Where, V_{NL} = no-load or open-circuit terminal voltage of supply and

V_{FL} = full-load terminal voltage of supply

In the ideal or perfectly regulated dc power supply, percentage voltage regulation is zero. This voltage regulation is also known as load regulation.

*Zener Diode Shunt Regulator:*

The simple shunt voltage regulating system utilizing Zener diode is shown below. Input voltage V_{in}, in fact, is unregulated output of the rectifier. This simple regulator limits output voltage variations within sensible limits around V_{z} in face of changing load current or changing input voltage. Obviously, Zener diode will regulate so long as it is kept in reverse conduction.

*Transistor Series Voltage Regulator:*

It is also known as emitter-follower regulator as voltage at emitter follows base voltage. In this set-up, transistor behaves like the variable resistor whose resistance is found out by base current. It is known as pass transistor as total current to be regulated passes through it.

Remembering polarities of different voltages, they are associated by equation derived from K_{VL}

V_{L} + V_{BE} - V_{Z} = 0

Therefore V_{BE} = V_{Z} - V_{L}

When current demand is increased by decreasing R_{L}, V_{L}, tends to decrease. It will increase V_{BE} as V_{Z} is fixed. This will increase forward bias of transistor thereby increasing its level of conduction. This, is turn, will lead to decrease in collector- emitter resistance of transistor that will slightly increase input current to compensate for decrease in R_{L} so that V_{L }= (I_{L}R_{R})will remain at constant value. Incidentally, R is utilized for restricting current passing through Zener diode.

*Controlled Transistor Series Regulator:*

Circuit using second transistor T_{2} as sensing element is shown. It has extra feature of control with help of potentiometer R_{1} - R_{2}. It will be assumed that I is much greater than I_{B}2. Now, there is drop of V_{L} on (R_{1}R_{2}) and drop of (V_{Z} V_{BE2}) across R_{2}.

V_{L}/(V_{Z} + V_{BE2}) = (R_{1} + R_{2})/R_{2} or V_{L} = ((R_{1} + R_{2})/R_{2})(V_{Z} + V_{BE2})

If potentiometer is adjusted so that R_{2} decreases, then V_{L} increases and vice versa. Assume R_{L} is decreased, then, I_{L} increases but V_{L} decreases. Decrease in V_{L} decreases I_{B2} and I_{C2}. Assuming I_{3} to be relatively constant (or decreasing only slightly), I_{B1} is increased thereby decreasing terminal (collector-emitter) resistance of T_{1}. This leads to decrease in V_{CE1} thereby offsetting decrease in V_{L} that is, thus, returned to original value. In sequential logic, we have

V_{L}↓ I_{B2}↓ I_{C2}↓ I_{B1}↑ V_{CE1}↓ V_{2}↑

*Transistor Shunt Voltage Regulator:*

It uses transistor in shunt configuration.

As path AB is in parallel across V_{L}, we have from Kirchhoff's Voltage Law

V_{L} - V_{Z} - V_{BE} = 0 or V_{BE} = V_{L }- V_{Z }(fixed)

Since V_{Z }is fixed, any decrease or increase in V_{L} will have corresponding effect on V_{BE}. Assume, V_{L} decreases, then as seen from relation, V_{BE} also decreases. Consequently, I_{B} decreases, therefore, I_{C}(=βI_{B}) decreases, thereby decreasing I and therefore VR(=IR). As a result, V_{L }increases since at all times

V_{in} = V_{R} + V_{L} or V_{L }= V_{in} - V_{R}

In sequential logic,

V_{z}↓ V_{BE}↓ I_{B}↓ I_{C}↓ I_{R}↓ V_{R}↓ V_{L}↑

Same line of logic applies in case V_{L} tries to increase.

*Transistor Current Regulator:*

The main function of current regulator is to maintain fixed current through load despite variations in terminal voltage. Such a circuit using Zener diode and PNP transistor is shown. Assume, because of drop in V_{L}, current I_{L}(=I_{C})is decreased. This will decrease I_{E}(≈I_{C}). Therefore, drop across R_{E} i.e. V_{RE} will decrease. As per Kirchhoff's Voltage Law

-V_{RE} - V_{BE} + V_{Z} = 0 or V_{BE }= V_{Z} - V_{RE}

Therefore, a decrease in V_{RE} will increase V_{BE} and, therefore, conductivity of transistor thereby keeping I_{L }at fixed level. Similar logic applies when there is increase in V_{L}.

*Variable Feedback Regulator:*

Regulators give non-adjustable output voltage. This would be fine if only single value of regulated voltage is needed. A feedback regulator that gives different values of regulated dc voltage. T_{1} is pass transistor and T_{2} is feedback transistor whose job is to give ample output (i.e. load) voltage. It offsets any change in output voltage. As potentiometer R_{3} is connected in parallel with Zener diode D, it has Zener voltage V_{Z} applied across it. Voltage across wiper differs from 0 to V_{z}. Capacitor C makes sure that voltage across D and R_{3} doesn't change suddenly.

Voltage at base of T_{2 }is 0.7V more positive than voltage at its emitter. Its emitter voltage and therefore base voltage can be changed with help of R_{3}. As base of T_{2} is tied to output, it is liable for giving output or load voltage. Voltage V_{CE1} across pass transistor is given by difference of input voltage and output voltage. Current through T_{1} is equal to load current. R_{2 }prevents saturation of transistors whereas R_{1} restricts current flowing through D.

Working of feedback transistor can be described as follows:

- As base voltage of T
_{2}is directly related to v_{out}it will change if V_{out}changes. Base and collector of T_{2}are 180^{o}out of phase with each other. If base voltage increases because of increase in V_{out}collector voltage would decrease. Now, collector of T_{2 }controls base of T_{2}. As base voltage of T_{1}decreases, its collector-emitter resistance increases that lowers load current. This, in turn, lowers output voltage thereby offsetting attempted increases in V_{out}. Opposite of these steps gives the action of attempted decrease in output voltage.

*Basic Op-Amp Series Regulator:*

Its operations as follows:

Potentiometer R_{2} - R_{3} senses any change in out-put voltage V_{out}. When V_{out} tries to decrease due to decrease in V_{in} or due to increase in I_{L}, proportional voltage decrease is applied to inverting output of op-amp by potentiometer. As, other op-amp input is held by Zener voltage at fixed reference voltage V_{REF}, small difference voltage (known as error voltage) is developed across two inputs of op-amp. This difference voltage is amplified and op-amp's output voltage increases. This increase in voltage is applied to base of T_{1} causing emitter voltage (=V_{out}) to increase till voltage to inverting input again equals reference (Zener) voltage. This action offsets attempted decrease in output voltage therefore keeping it almost constant. Opposite action takes place if output voltage attempts to increase.

*Basic Op-Amp Shunt Regulator:*

Here, control element is the series resistor R_{1} and transistor T_{1} in parallel with load. In such a regulator, regulation is attained by controlling current through T_{1}.

Working:

When output voltage attempts to decrease because of change in either input voltage or load current or temperature, attempted decrease is sensed by R_{3} and R_{4} and applied to non-inverting input of op-amp. Resulting difference in voltage decreases op-amp's output, driving T_{1} less therefore decreasing its collector current(shunt current), and increasing its collector-to-emitter resistance. As collector-to-emitter resistance serves as voltage divider with R_{1}, this action offsets attempted decrease in output voltage and therefore, maintains it at constant value. Opposite action takes place when output voltage attempts to increase. Shunt regulator offers inherent short-circuit protection.

*Switching Regulators:*

In linear regulators considered so far, control element i.e. transistor conducts all the time, amount of conduction varying with changes in output voltage or current. Because of continuous power loss, efficiency of such a regulator is decreased to or 50% less.

The switching regulator is different as its control element operates like the switch i.e. either it is saturated (closed) or cut-off (open). Therefore, there is no needless wastage of power that results in higher efficiency of or more 90%. Switching regulators are of three kinds:

- Step-down regulator
- Step-up regulator and
- Inverting regulator

**Step-Down Switching Regulator:**

In this regulator, V_{out }is always less than V_{in}. Unregulated positive dc voltage is applied to collector of NPN transistor. The series of pulses from oscillator is sent to base of transistor T that gets saturated (closed) on each of positive pulses. It is so because a NPN transistor requires positive voltage pulse on its base to turn ON. The saturated transistor serves as closed switch, therefore it allows V_{in }to send current L through and charge C to value of output voltage during on-time (T_{ON}) of pulse. Diode D_{1} is reverse-biased at this point and therefore, doesn't conduct.

Ultimately when positive pulse turns to zero, T is cut-off and serves like open switch during off period (T_{OFF}) of pulse. Collapsing magnetic field of coil produces self-induced voltage and keeps current flowing by returning energy to circuit.

Value of output voltage depends on input voltage and pulse width that is on-time of transistor. When on-time is increased relative to off- time, C charges more therefore increases V_{out}. When T_{ON} is decreased, C discharges more therefore decreasing V_{out}. By adjusting duty cycle (T_{ON}/T) of transistor, V_{out }can be varied.

Thus V_{out} = V_{in}(T_{ON}/T)

Where T is period of ON-OFF cycle of transistor and is related to frequency by T= 1/f.

Also, T = T_{on} + T_{OFF} and ratio (T_{ON}/T) is known as duty cycle. Regulating action of circuit is as follows:

When V_{out} tries to decrease, on-time of transistor is increased causing an additional charge on capacitor C to offset attempted decrease. When V_{out} tries to increase, T_{ON} of transistor is decreased causing C to discharge enough to offset attempted increase.

**Step-Up Switching Regulator:**

When transistor T turns ON on arrival of positive pulse at its base, voltage across L increases rapidly to V_{out} - V_{CE}(sat) and magnetic field of L expands quickly. During on-time of transistor, V_{L }keeps decreasing from its initial maximum value. longer transistor is ON, the smaller V_{L} becomes.

Step-up Switching Regulator

When transistor turns OFF, magnetic field of L collapses and its polarity reverses so that its voltage adds to input voltage therefore producing output voltage greater than input voltage. During off-time of transistor, D_{2} is forward-biased and permits C to charge. Variations in V_{out} because of charging and discharging action are adequately smoothed by filtering action of L and C.

It may be noted that shorter on-time of transistor, greater the inductor voltage and therefore greater the output voltage (as greater V_{L }adds to V_{in}). On the other hand, longer on-time, the smaller the inductor voltage and therefore, lesser the output voltage (because smaller V_{L} adds to V_{in}). Regulating action can be explained as follows:

When V_{out} tries to decrease (due to either increasing load or decreasing V_{in}), transistor on time decreases thereby offsetting tried decrease in V_{out}. When V_{out} attempts to increase, on-time increases and attempted increase in V_{out} is offset.

- As seen, output voltage is inversely related to duty cycle.

Therefore V_{out} = V_{in}(T/T_{ON})

Inverting Switching Regulator

The basic diagram of such a regulator is shown. This regulator gives output voltage that is opposite in polarity to input voltage.

When transistor turns ON by positive pulse, inductor voltage V_{L} jumps to V_{in} - V_{CE(sat) }and magnetic field of inductor expands rapidly. When transistor is ON, diode D_{2} is reverse-biased and V_{L} decreases from its initial maximum value.

When transistor turns OFF, magnetic field collapses and inductor's polarity reverses. This forward-biases D_{2}, charges C and produces the negative output voltage. This repetitive ON-OFF action of transistor generates repetitive charging and discharging which is smoothed by LC filter action. As in case of step-up regulator, lesser the time for which transistor is ON, greater output voltage and vice versa.

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