ch5 power supplies voltage regulator pp15a[1]

DENE 3353APPLIED ELECTRONICS

CHAPTER 5REGULATED POWER

SUPPLIES

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

OutlineOutline

• Introduction• Filter Voltage Regulation & Ripple• Capacitor Filter• RC Filter• Discrete Transistor Voltage Regulation• Integrated Circuit Regulator

22



Power Supply DiagramPower Supply Diagram

33

Fig. 15.1: Block Diagram Showing Parts of a Power Supply



Filter CircuitsFilter Circuits

• The output from the rectifier section is a pulsating DC.

• The filter circuit reduces the peak-to-peak pulses to a small ripple voltage.

44



Ripple FactorRipple Factor

After the filter circuit a small amount of AC is still remaining. The amount of ripple voltage can be rated in terms of ripple factorripple factor (r).

100V

)V

voltage dc

(rms) voltage ripple%r

dc

r(rms

55

Fig. 15.2: Filter Voltage Waveform Showing dc and Ripple Voltages.



Example 15.1Example 15.1

Using a dc and ac voltmeter to measure the output signal from a filter circuit, we obtain readings of 25V dc and 1.5 V rms. Calculate the ripple of the filter output voltage.

100V

)V

voltage dc

(rms) voltage ripple%r

dc

r(rms

66




A dc voltage supply provides 60V when the output is unloaded. When connected to a load, the output drops to 56V. Calculate the value of voltage regulation.

77

100V

V-V

voltageload full

voltageload full- voltageload-no%VR

FL

FLNL



Rectifier Ripple FactorRectifier Ripple Factor

mdc 0.636VV

%481000.636

0.308

100V

V%r

Vm

Vm

dc

r(rms)

mr(rms) 0.308VV

mdc 0.318VV

mr(rms) 0.385VV

121%1000.318V

0.385V

100V

V%r

m

m

dc

r(rms)

88

Half-WaveHalf-Wave

DC output:

AC ripple output:

Ripple factor:

Full-WaveFull-Wave

DC output:

AC ripple output:

Ripple factor:



Types of Filter CircuitsTypes of Filter Circuits

Capacitor FilterCapacitor Filter

RC FilterRC Filter

99



Capacitor FilterCapacitor Filter

The larger the capacitor the smaller the ripple voltage.

CR

2.4V

C

2.4I

fC34

IV

L

dcdcdcr(rms)

Ripple voltageRipple voltage

C

4.17IV

4fC

IVV dc

mdc

mdc

DC outputDC output

100RLC

2.4100

CV

2.4I100

V

V%r

dc

dc

dc

r(rms)

Ripple factorRipple factor

1010

Fig. 15.4: Capacitor Filter Operation




Calculate the ripple voltage of a full-wave rectifier with a 100µF filter capacitor connected to a load drawing 50mA.

1111

CR

2.4V

C

2.4I

fC34

IV

L

dcdcdcr(rms)




If the peak rectified voltage for the filter circuit of Example 15.3 is 30V, calculate the filter dc voltage.

1212

C

4.17IV

4fC

IVV dc

mdc

mdc




Calculate the ripple voltage of a capacitor filter for a peak rectified voltage of 30V, capacitor C = 50µF, and a load current of 50mA.

1313

100RLC

2.4100

CV

2.4I100

V

V%r

dc

dc

dc

r(rms)



Voltage Regulation CircuitsVoltage Regulation Circuits

There are two common types of circuitry for voltage regulation:

• Discrete Transistors• IC’s

1414



Discrete-Transistor RegulatorsDiscrete-Transistor Regulators

Series voltage regulatorSeries voltage regulatorCurrent-limiting circuitCurrent-limiting circuitShunt voltage regulatorShunt voltage regulator

1515



Series Voltage Regulator CircuitSeries Voltage Regulator Circuit

The series element controls the amount of the input voltage that gets to the output.

If the output voltage increases (or decreases), the comparator circuit provides a control signal to cause the series control element to decrease (or increase) the amount of the output voltage.

1616

Fig. 15.12: Series Regulator Block Diagram




Calculate the output voltage and the Zener current in the regulator circuit of Fig. 15.14 for RL =1kΩ.

1717




When the output increases:When the output increases:

1. The voltage at V2 and VBE of Q2 increases

2. The conduction of Q2 increases3. The conduction of Q1 decreases4. The output voltage decreases

• R1 and R2 act as the sampling circuit• Zener provides the reference voltage• Q2 controls the base current to Q1

• Q1 maintains the constant output voltage

When the output decreases:When the output decreases:

1. The voltage at V2 and VBE of Q2 decreases

2. The conduction of Q2 decreases3. The conduction of Q1 increases4. The output voltage increases

1818

Fig. 15.15: Improved Series Regulator Circuit



Example 15.9Example 15.9What regulated output voltage is provided by the circuit of Fig. 15.15 for the circuit elements R1=20kΩ, R2 = 30kΩ, and VZ = 8.3V?

1919

)(2

2

21BEZo VV

R

RRV




The op-amp compares the Zener diode voltage with the output voltage (at R1 and R2) and controls the conduction of Q1.

2020

Fig. 15.16: Op-amp Series Regulator Circuit




Calculate the regulated output voltage in the circuit of Fig. 15.17

2121

Zo VR

RV

2

11



Current-Limiting CircuitCurrent-Limiting Circuit

When IL increases:

• The voltage across RSC increases• The increasing voltage across RSC drives Q2 on• Conduction of Q2 reduces current for Q1 and the load

2222

Fig. 15.18: Current-Limiting Voltage Regulator

Short-circuit / overload protection



Example (Floyd)Example (Floyd)

Determine the maximum current that the regulator in Figure 15.18 can provide to a load.

2323

Fig. 15.18: Current-Limiting Voltage Regulator

scL R

I7.0

(max)



The load voltage is sampled and fed back to a comparator circuit. If the load voltage is too high, control circuitry shunts more current away from the load.

Shunt Voltage Regulator CircuitShunt Voltage Regulator Circuit

The shunt voltage regulator shunts current away from the load.

2424

Fig. 15.20: Block Diagram of Shunt Voltage Regulator




Determine the regulated voltage and circuit currents for the shunt regulator of Fig. 15.22

2525



When the output voltage increases:

• The Zener current increases• The conduction of Q2 increases• The voltage drop at Rs increases• The output voltage decreases


When the output voltage decreases:

• The Zener current decreases• The conduction of Q2 decreases• The voltage drop at Rs decreases• The output voltage increases

2626

Fig. 15.24: Improved Shunt Voltage Regulator Circuit




2727

Fig. 15.24: Shunt Voltage Regulator Using an Op-amp



Example of shunt regulatorExample of shunt regulator

2828

Fig. 15.24: Shunt Voltage Regulator Using an Op-amp

In Figure 15.24, what power rating must Rs have if the maximum input voltage is 12.5V?



IC Voltage RegulatorsIC Voltage Regulators

Regulator ICs contain:

• Comparator circuit• Reference voltage• Control circuitry• Overload protection

Types of three-terminal IC voltage regulators

• Fixed positive voltage regulator• Fixed negative voltage regulator• Adjustable voltage regulator

2929



Three-Terminal Voltage RegulatorsThree-Terminal Voltage Regulators

The specifications for this IC indicate:

• The range of input voltages that can be regulated for a specific range of output voltage and load current

• Load regulation—variation in output voltage with variations in load current

• Line regulation—variation in output voltage with variations in input voltage

3030

Fig. 15.25: Block Representation of Three-Terminal Voltage Regulator



Fixed Positive Voltage Regulator

These ICs provide a fixed positive output voltage.

3131

Fig. 15.26: Connection of a 7812 Voltage Regulator



These ICs output a fixed negative output voltage.

Fixed Negative Voltage RegulatorFixed Negative Voltage Regulator

3232



Quiz 4Quiz 4

Draw a schematic for voltage supply using a full-wave bridge rectifier, capacitor filter, and IC regulator to provide an output of +5V

3333




For a transformer output of 15V and a filter capacitor of 250µF, calculate the minimum input voltage when connected to a load drawing 400mA.

3434

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 14.6

7815 +15 17.7

7818 +18 21.0

7824 +24 27.1

Table 15.1: Positive-voltage regulators in the 7800 Series




Determine the maximum value of load current at which regulation is maintained for the circuit of Fig. 15.29.

3535



Adjustable Voltage RegulatorAdjustable Voltage Regulator

These regulators have adjustable output voltages.

The output voltage is commonly selected using a potentiometer.

3636

Fig. 15.30: Connection of LM317 adjustable-voltage regulator




Determine the regulated voltage in the circuit of Fig. 15.30 with R1 = 240Ω and R2 = 2.4kΩ.

3737




Determine the regulated output voltage of the circuit in Fig. 15.31

3838

ch5 power supplies voltage regulator pp15a[1]

Documents

upper saddle

voltage ripplerdcr

peak rectified

wave dc output

ac ripple

shunt voltage

regulated

output voltage