Chapter 6Chapter 6

Voltage RegulatorsVoltage Regulators

- Part 2-- Part 2-

Control element for voltage regulator normally has four types of circuitry:

• Linear Series • Linear Shunt• Series switching• Shunt switching

Transistor Shunt Regulator

Fig. 6.8 Transistor shunt voltage regulator.

The control element is a transistor, in parallel with the load. While, the resistor, RS, is in series with the load.

The operation of the transistor shunt regulator is similarsimilar to that of the transistor series regulator, except that regulation is achieved by controlling the current through the parallel transistor.

The output voltage to the load is

This explains that the voltage across the load is set by the Zener diode voltage and the transistor base-emitter voltage. If the load resistance decreases, the load current will be larger at a value of:

BEZLo VVVV

(6-5)

L

LL R

VI (6-6)

The increase in load current causes the collector current shunted by the transistor is to be less as expressed in the following formula:

The current through RS is calculated as follows:

LSC III (6-9)

S

LiS R

VVI

(6-

10)

Shunt Regulator Shunt Regulator

Fig. 6.9 Block diagram of shunt regulator.

VIN

R1

VOUT

Controlelement(shunt)

Samplecircuit

Errordetector

Referencevoltage

Basic op-amp Shunt Regulator with load resistor

Fig. 6.10 Transistor shunt voltage regulator.

VIN

VOUT

R2

Q1

–

+

D1

R4

R3RL

VREF

Error detectorControlelement

Samplecircuit

R1

Shunt regulators use a parallel transistor for the control element. If the output voltage changes, the op-amp senses the change and corrects the bias on Q1 to follow.

The control element maintains a constant output voltage by varying the collector current in the transistor.

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.

VIN

VOUT

R2

Q1

–

+

D1

R4

R3RL

VREF

Error detectorControlelement

Samplecircuit

R1

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.

VIN

VOUT

R2

Q1

–

+

D1

R4

R3RL

VREF

Error detectorControlelement

Samplecircuit

R1

With IL and VOUT constant, a change in the input voltage produces a change in shunt current (Is)

as follow:

Maximum Load current:

REFout VR

RV

3

21

1(max) R

VI INL

1R

VI INS

(6-11)

(6-12)

(6-13)

Switching Regulator Switching Regulator

The switching regulator is a type of regulator circuit which its efficient transfer of power to the load is greater than series and shunt regulators because the transistor is not always conducting.

The switching regulator passes voltage to the load in pulses, which then filtered to provide a smooth dc voltage.

Fig. 6.11 Block representation of three-terminal voltage regulator.

A voltage regulator in which the control element operates as a switch.

+VIN

–VOUT

R1

Q1

R3

R2 RL

D1

D2

Variablepulse-widthoscillator

L

+

–

C

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.

Switching Regulator Switching Regulator

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.

VC

ton toff ton toff ton toff tonon/off control

VOUT

INon

out VT

tV

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

offon ttT

Tton /

(6-14)

(6-15)

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.

VIN

VOUT

R1

Q1

R3

R2

RLD1

–

+

D2

Variablepulse-widthoscillator

VREF

L

C

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.

+

L reverses polarity

off

VINVOUT

R1Q1

R3

R2

RL

D1 C

D2

Variablepulse-widthoscillator

L

++

+–

C discharges

+

on

offL field builds

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.

VINVOUT

R1Q1

R3

R2

RL

D1 C

D2

Variablepulse-width

oscillator

L

++

+–

on

off

C charges

+

L field collapses

IC VOLTAGE REGULATORS 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:(1)reference source, (2)comparator amplifier, (3)control device, and; (4)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.

Fixed Voltage Regulator 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 RegulatorFixed-Positive Voltage RegulatorThe series 78XX regulators are the three-terminal devices that provide a fixed positive output voltage.

Fig. 6.12 (a) Standard configuration of the series 78XX regulator and (b) typical packages

(a)(b)

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).

Fig. 6.12 Standard configuration of a 7812 voltage regulator.

IC PartIC Part Output Output Voltage (V)Voltage (V)

Minimum VMinimum Vii (V)(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

TABLE 6-1: Positive-Voltage Regulators in the 78XX Series.

Fixed-Negative Voltage RegulatorFixed-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. 6.14 Standard configuration.

IC PartIC Part Output Output Voltage (V)Voltage (V)

Minimum VMinimum Vii (V)(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

TABLE 6-2: Negative-Voltage Regulators in the 79XX Series.

Adjustable-Voltage RegulatorAdjustable-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. 6.15 Connection of LM317 adjustable-voltage regulator.

The LM317 is a adjustable positive output IC regulator. There is a fixed reference voltage of +1.25 V between the output and adjustment terminals. There is no ground pin.

The output voltage is calculated by:

2OUT REF ADJ 2

1

1R

V V I RR

AdjustmentR1

R2

C1

C2

C3

Positiveinput

Positiveoutput

(1)

(3) (2)LM317

What is VOUT? (Assume IADJ = 50 A.)

= 16.8 V

R1

R2

C1

C2

C3

(1)

(3) (2)LM317

OUT

2 kΩ1.25 V 1 50 μA 2 k

150 V

2 k

150

+20 V +16.8 V

150

2 k

EndEnd

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