Chapter 6Voltage Regulators

- Part 1-

VOLTAGE REGULATION

Two basic categories of voltage regulation are:

line regulation;

load regulation.

The purpose of line regulation is to maintain a

nearly constant output voltage when the input

voltage varies.

The purpose of load regulation is to maintain a

nearly constant output voltage when the load 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).

Line Regulation

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

Line 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)

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

Load Regulation

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 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 Regulation

TYPES OF REGULATOR

The fundamental classes of voltage regulators are linear regulators and switching regulators.

Two basic types of linear 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.

Basic Linear Series Regulators

Series Regulator Circuit

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

• The control element is a pass transistor in series with the load between the input and output.

• The error detector compares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage.

Basic Op-amp Series Regulator

Figure 6.6 Basic op-amp series regulator.

• The resistor R2 and R3 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.

• 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.

REFOUT VR

RV

3

21 (6-5)

The closed-loop voltage gain is:

3

21R

RACL

Determine the output voltage for the regulator in Figure below.

EXAMPLE

Answer: VOUT = 10.2 V

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 Wresistor, 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