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_LR_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 T₂ as sensing element is shown. It has extra feature of control with help of potentiometer R₁ - R₂. It will be assumed that I is much greater than I_B2. Now, there is drop of V_L on (R₁R₂) and drop of (V_Z V_BE2) across R₂.

V_L/(V_Z + V_BE2) = (R₁ + R₂)/R₂ or V_L = ((R₁ + R₂)/R₂)(V_Z + V_BE2)

If potentiometer is adjusted so that R₂ 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 I₃ to be relatively constant (or decreasing only slightly), I_B1 is increased thereby decreasing terminal (collector-emitter) resistance of T₁. 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↓ V₂↑

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. T₁ is pass transistor and T₂ is feedback transistor whose job is to give ample output (i.e. load) voltage. It offsets any change in output voltage. As potentiometer R₃ 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 R₃ doesn't change suddenly.

Voltage at base of T₂ is 0.7V more positive than voltage at its emitter. Its emitter voltage and therefore base voltage can be changed with help of R₃. As base of T₂ 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 T₁ is equal to load current. R₂ prevents saturation of transistors whereas R₁ restricts current flowing through D.

Working of feedback transistor can be described as follows:

• As base voltage of T₂ is directly related to v_out it will change if V_out changes. Base and collector of T₂ 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 T₂ controls base of T₂. As base voltage of T₁ 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 R₂ - R₃ 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 T₁ 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 R₁ and transistor T₁ in parallel with load. In such a regulator, regulation is attained by controlling current through T₁.

Working:

When output voltage attempts to decrease because of change in either input voltage or load current or temperature, attempted decrease is sensed by R₃ and R₄ and applied to non-inverting input of op-amp. Resulting difference in voltage decreases op-amp's output, driving T₁ 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 R₁, 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 D₁ 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, D₂ 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 D₂ 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 D₂, 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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