1. Chapter 6
Voltage Regulators
- Part 2-

2. Basic Linear Shunt Regulators

3. Fig. 6.9 Block diagram of a basic components of a shunt regulator.

4. Fig. 6.10 Transistor shunt voltage regulator.
Basic op-amp Shunt Regulator with load resistor
Fig Transistor shunt voltage regulator.
In the basic shunt regulator, the control element is a transistor Q1, in parallel with the load.
A resistor R1, is in series with the load.

5. The operation of the circuit is similar to the series regulator, except that regulation is achieved by controlling the current through the parallel transistor Q1 When the output voltage tries to decrease due to a change in input voltage or load current caused by a change in load resistance, the decrease is sensed by R3 and R4.

6. A feedback voltage obtained from voltage divider R3 and R4 is applied to the op-amp’s non-inverting input and compared to the Zener voltage to control the drive current to the transistor.
The difference voltage reduces the op-amp’s output(VB), but increasing the collector voltage. This keeps the load output nearly constant.

7. (6-11)
With IL and VOUT constant, a change in the input voltage produces a change in shunt current (Is) as follow:
Maximum Load current:
(6-12)
(6-13)

8. Basic Switching Regulators

9. Basic Switching Regulators
The control element (transistor) operates as a switch.
The transistor switches on and off and dissipates power only when it is on.
The transistor operates only on the ends of the load line except during the very short switching time.
As a result, efficiencies can be greater than 90%.
Three basic configurations of switching regulators are:
Step-down
Step-up
Voltage-inverter

10. Fig. 6.11 Block representation of three-terminal voltage regulator.
Switching Regulator
Fig Block representation of three-terminal voltage regulator.

11. All switching regulators control the output voltage by rapidly switching the input voltage on and off with a duty cycle that depends on the load. Because they use high frequency switching, they tend to be electrically noisy.
on/off control
VOUT

12. The output voltage is expressed as :
The period in the sum of the on-time and the off-time:
The ratio is called the duty cycle
(6-14)
(6-15)

13. Step-Down Configuration
A step-down switching regulator controls the output voltage by controlling the duty cycle to a series transistor. The duty cycle changes depending on the load requirement.
Because the transistor is either ON or OFF on all switching regulators, the power dissipated in the transistor is very small and the regulator is very efficient. The pulses are smoothed by an LC filter.

14. Step-Up Configuration
In a step-up switching regulator, the control element operates as a rapidly pulsing switch to ground. The switch on and off times are controlled by the output voltage.
Step-up action is due to the fact the inductor changes polarity during switching and adds to VIN. Thus, the output voltage is larger than the input voltage.

15. Voltage-Inverter Configuration
In a voltage-inverter switching regulator, the output is the opposite polarity of the input. It can be used in conjunction with a positive regulator from the same input source.
Inversion occurs because the inductor reverses polarity when the diode conducts, charging the capacitor with the opposite polarity of the input.

16. IC VOLTAGE REGULATORS
Several types of both linear (series and shunt) and switching regulators are available in integrated circuit (IC) form.
Single IC regulators contain the circuitry for:
reference source,
comparator amplifier,
control device, and;
overload protection.
Generally, the linear regulators are three-terminal devices that provides either positive or negative output voltages that can be either fixed or adjustable.

17. Fixed Voltage Regulator
The fixed voltage regulator has an unregulated dc input voltage Vi applied to one input terminal, a regulated output dc voltage Vo from a second terminal, and the third terminal connected to ground.
Fixed-Positive Voltage Regulator
The series 78XX regulators are the three-terminal devices that provide a fixed positive output voltage.
(a)
(b)
Fig (a) Standard configuration of the series 78XX regulator and
(b) typical packages

18. The figure above shows:
Fig Standard configuration of a 7812 voltage regulator.
The figure above shows:
An unregulated input voltage Vi is filtered by a capacitor C1 and connected to the IC’s IN terminal.
The IC’s OUT terminal provides a regulated +12 V, which is filtered by capacitor C2.
The third IC terminal is connected to ground (GND).

19. TABLE 6-1: Positive-Voltage Regulators in the 78XX Series.
IC Part
Output Voltage (V)
Minimum Vi (V)
7805 +5
+7.3
7806 +6
+8.3
7808 +8
+10.5
7810
+10
+12.5
7812
+12
+14.5
7815
+15
+17.7
7818
+18
+21.0
7824
+24
+27.1

20. Fig. 6.14 Standard configuration.
Fixed-Negative Voltage Regulator
The series 79XX regulators are the three-terminal IC regulators that provide a fixed negative output voltage.
This series has the same features and characteristics as the series 78XX regulators except the pin numbers are different.
Fig Standard configuration.

21. TABLE 6-2: Negative-Voltage Regulators in the 79XX Series.
IC Part
Output Voltage (V)
Minimum Vi (V)
7905 -5
-7.3
7906 -6
-8.4
7908 -8
-10.5
7909 -9
-11.5
7912
-12
-14.6
7915
-15
-17.7
7918
-18
-20.8
7924
-24
-27.1

22. Adjustable-Voltage Regulator
Voltage regulators are also available in circuit configurations that allow to set the output voltage to a desired regulated value.
The LM317 is an example of an adjustable-voltage regulator, can be operated over the range of voltage from 1.2 to 37 V.
Fig Connection of LM317 adjustable-voltage regulator.

23. ~ End of Chapter 6 ~