Lecture 9: Power Supplies

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th editionThomas L. Floyd

Lecture 9:

Power Supplies









Power Supply Regulation

An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions.

00 Current

Voltage

Ideal power supply

The output voltage of a real power supply changes under load as shown in the second plot. The output is also sensitive to input voltage changes.

00 Current

Voltage

Real power supply

VNL VNL

VFL



Line Regulation

Line regulation is a measure of how well a power supply is able to maintain the dc output voltage for a change in the ac input line voltage. The formula for line regulation is

OUT

IN

Line Regulation = 100%V

V

Line regulation can also be expressed in terms of percent change in VOUT per volt change on the VIN (%/V).

OUT OUT

IN

/ 100%Line Regulation =

V V

V



Load Regulation

When the amount of current through a load changes due to a varying load resistance, the voltage regulator must maintain a nearly constant output voltage across the load.



Load Regulation

Load regulation is a measure of how well a power supply is able to maintain the dc output voltage between no load and full load with the input voltage constant. It can be expressed as a percentage change in load voltage:

NL FL

FL

Load Regulation = 100%V V

V

Load regulation can also be expressed in terms of percent change in the output per mA change in load current (%/mA).

Sometimes a maximum error voltage is given in the specification.



Load Regulation

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

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?

OUT

FL

0.025 Load regulation 100% 100%

10.0

R

R

= 0.25%(a)

(b) By Ohm’s law, VOUT = 5.0 V.

OUTNL

L

OUT L

5.0 V10.0

0.025 + 10.0

VV

R

R R

= 5.013 V



Regulators

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

Both of these are available in integrated circuit form. Two basic types of linear regulator are:

the series regulator the shunt regulator.



Series Regulators

Series Regulator block diagram:

Basic series regulator circuit:

VIN VOUT

Errordetector

Samplecircuit

Controlelement

Referencevoltage

VIN VOUT

R1

Control element

Q1

D1

VREF+

– Error detectorR2

R3

Samplecircuit

The control element maintains a constant output voltage by varying the collector-emitter voltage across the transistor.



Series Regulators

The op-amp in the series regulator is actually connected as a noninverting amplifier where the reference voltage VREF is the input at the noninverting terminal, and the voltage divider R2/R3 forms the negative feedback circuit. The closed-loop voltage gain is



Series Regulators

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

R3

47 kW

100 kW3.9 V

18 V

4.7 kW

= 12.2 V

P = VI = (18 V – 12.2 V)(0.2 A)

= 1.16 W



Shunt Regulators

Shunt Regulator block diagram:

Basic shunt regulator circuit:

VIN

R1

VOUT

Controlelement(shunt)

Samplecircuit

Errordetector

Referencevoltage

VIN

VOUT

R2

Q1

–

+

D1

R4

R3RL

VREF

Error detectorControlelement

Samplecircuit

R1

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



Shunt Regulators

Summary

Although it is less efficient than the series regulator, the shunt regulator has inherent short-circuit protection. The maximum current when the output is shorted is VIN/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. For example, a decrease in output voltage causes a decrease in VB and an increase in VC. VIN

VOUT

R2

Q1

–

+

D1

R4

R3RL

VREF

Error detectorControlelement

Samplecircuit

R1



Shunt Regulators







Switching Regulators

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 tonton toff ton toff ton toff ton

VC

VC

ton toff ton toff ton toff tonon/off control

VOUT

An increase in the duty cycle increases the output voltage.A decrease in the duty cycle decreases the output voltage.



Buck Converter

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.



Buck Converter

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.

on

C charges

+ -VIN

VOUT

R1

Q1

R3

R2

RLD1

–

+

D2


VREF

L

C- +

L reverses polarityoff



Buck Converter

VC

ton toff ton toff ton toff tonton toff ton toff ton toff ton

VC

VC

ton toff ton toff ton toff tonon/off control

VOUT

An increase in the duty cycle increases the output voltage.A decrease in the duty cycle decreases the output voltage.



Buck Converter



VINVOUT

R1Q1

R3

R2

RL

D1 C

D2


L

++

+–

C discharges

+ -

on

offL field builds

Boost Converter

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


L

++

+–

on

off

C charges

- +

L field collapsesVIN

VOUT

R1Q1

R3

R2

RL

D1 C

D2


L

++

+–



Boost Converter

Lecture 9: Power Supplies

Documents

load voltage

load output voltage

load changes

load current

load regulation specification

load conditions

w load

floyd6 load regulation