Chapter 6Chapter 6

Voltage RegulatorsVoltage Regulators

- Part 1-- Part 1-

POWER SUPPLIES (VOLTAGE POWER SUPPLIES (VOLTAGE REGULATORS)REGULATORS)

Fig. 6.1 Block diagram showing parts of a power supply.

Power supplyPower supply: a group of circuits that convert the standard ac voltage (120 V, 60 Hz) provided by the wall outlet to constant dc voltage.

Transformer Transformer : a device that step up or step down the ac voltage provided by the wall outlet to a desired amplitude through the action of a magnetic field.

Transformers convert AC electricity from one voltage to another with little loss of power.

Transformers work only with AC and this is one of the reasons why mains electricity is AC.

Step-up transformers increase voltage.

Step-down transformers reduce voltage.

RectifierRectifier: a diode circuits that converts the ac input voltage

to a pulsating dc voltage.

The pulsating dc voltage is only suitable to be used as a

battery charger, but not good enough to be used as a dc

power supply in a radio, stereo system, computer and so on.

There are two basic types of rectifier circuits:

1. Half-wave rectifier

2. Full-wave rectifier i) Center-tapped full-wave rectifier ii) Bridge rectifier

In summary, a full-wave rectified signal has less ripple than a

half-wave rectified signal and is thus better to apply to a filter.

FilterFilter: a circuit used to reduce the fluctuation in the rectified output voltage or ripple. This provides a steadiersteadier dc voltage.

RegulatorRegulator: a circuit used to produces a constantconstant dc output voltage by reducing the ripple to negligible amount.

VOLTAGE REGULATIONVOLTAGE REGULATION

Two basic categories of voltage regulation are:

line regulation;

load regulation.

The purpose of line regulationline regulation is to maintain a

nearly constant output voltage when the input input

voltagevoltage varies.

The purpose of load regulationload regulation is to maintain a

nearly constant output voltage when the loadload varies.

Figure 6–2 Line regulation. A change in input (line) voltage does not significantly affect the output voltage of a regulator (within certain limits).

Figure 6–3 Load regulation. A change in load current has practically no effect on the output voltage of a regulator (within certain limits).

Line regulation can be defined as the percentage change in the output voltage for a given change in the input voltage.

Line RegulationLine Regulation

Line regulation in %/V can be calculated using the following formula:

Δ means “a change in”.

%100xV

VregulationLine

IN

OUT

IN

OUTOUT

V

xVVregulationLine

%100/

(6-1)

(6-2)

Load regulation can be defined as the percentage change in the output voltage from no-load (NL) to full-load (FL).

where

VNL = the no-load output voltage VFL = the full-load output voltage

Load RegulationLoad Regulation

%100xV

VVregulationLoad

FL

FLNL

(6-3)

Sometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification.

RTH = ROUT

RLVTH = VNL

VOUT

Power Supply

In this case, VOUT can be found by applying the voltage divider rule:

LOUT NL

OUT L

RV V

R R

In terms of resistances, load regulation can be expressed as:

OUT

FL

Load regulation 100%R

R

Load RegulationLoad Regulation

Exercise Exercise ; ; 1) A power supply has an output resistance of 25 mW and a full load

current of 0.50 A to a 10.0 W load. (a)What is the load regulation? (b)What is the no load output voltage?

2) The input of certain regulator increase by 3.5V. As a result, the output voltage increase by 0.042V. The nominal output 20V. Determine the line regulation in both % and in %/V.

3) If a 5.0V power supply has an output resistance of 80mΩ and specific maximum output current of 1.0A, what is the load regulation in both % and in %/mA.

TYPES OF REGULATORTYPES OF REGULATOR

Two basic types of voltage regulator are the series regulator and the shunt regulator.

The series regulator is connected in series with the load and the shunt regulator is connected in parallel with the load.

Figure 6.4 Series and shunt regulators.

Series Regulator CircuitSeries Regulator Circuit

Figure 6.5 Block diagram of the basic connection of a series regulator circuit.

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

The output voltage is sampled by a circuit that provides a feedback voltage to be compared to a reference voltage.

Transistor Series RegulatorTransistor Series Regulator

Figure 6.6 Pass-transistor regulator.

The transistor series regulator is also called the pass-transistor regulator because the load current passes through the series transistor.

Equation ( ) explains the response of the pass-transistor to a change in load resistance as follows:

- If load resistance increases, load voltage also increases.

- Since the Zener voltage is constant, the increase in Vo causes VBE to decrease.

- The decrease in VBE reduces conduction through the pass- transistor, so load current decreases.

- This offsets the increase in load resistance, and a relatively constant load voltage is maintained.

Since Q1 is an npn transistor, Vo is found as

oZBE VVV (6-4)

Basic op-amp Series Regulator

Fig. 6.7 Op-amp series regulator circuit.

Control Element

Error DetectorSample Circuit

VREF

The resistor R1 and R2 sense a change in the output voltage and provide a feedback voltage. The error detector compares the feedback voltage with a Zener diode reference voltage.

Control Element

Error DetectorSample Circuit

VREF

The resulting difference voltage causes the transistor Q1 controls the conduction to compensate the variation of the output voltage. The output voltage will be maintained at a constant value of:

REFo VR

RV

2

11 (6-5)

The output voltage for the series regulator circuit is:

2OUT REF

3

1R

V VR

(a) What is the output voltage for the series regulator?(b) If the load current is 200 mA, what is the power dissipated by Q1?

2OUT REF

3

1

100 k1+ 3.9 V

47 k

RV V

R

(a)

(b)

VIN VOUT

R1Q1

D1

VREF+

– R2

R347 k

100 k3.9 V

18 V 4.7 k

P = VI = (18 V – 12.2 V)(0.2 A)

Exercise ; Exercise ;

Current limiting prevents excessive load current. Q2 will conduct when the current through R4 develops 0.7 V across Q2’s VBE. This reduces base current to Q1, limiting the load current.

VIN VOUT

R1

Q1

R3

R2

Q2

Current limiter

R4

+

–

The current limit is:

L(max)4

0.7 VI

R

For example, a 1.4 resistor, limits current to about 0.5 A.

Series Regulator with constant-current limiting

5R5 OUT

5 6

RV V

R R

An overload causes VR5 to drop because VOUT drops. This means that less current is needed to maintain conduction in Q2 and the load current drops.

+VIN VOUT

R1

Q1

+

–Q2

R4

D1

R3

R2

R6

R5

Fold-back current limiting drops the load current well below the peak during overload conditions. Q2 conducts when VR5 +VBE = VR4 and begins current limiting. VR5 is found by applying the voltage-divider rule:

Regulator with Fold-back current limiting

EndEnd

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