Electronic Troubleshooting Chapter 9 Regulated Power Supplies

Electronic Troubleshooting

Chapter 9Regulated Power Supplies

Regulated Power Supplies• Overview

• Unregulated power supplies • Output voltages vary with loads- Higher loads – lower voltages

• Designs of many circuits assume stable power supplies for proper operation

• e.g., test equipment, digital circuits

• Types of Regulated power supplies Covered• Zener Diode Regulators

• Series Regulators

• Adjustable Voltage Regulator

• Current Limiters

Regulated Power Supplies• Overview

• Types of Regulated power supplies Covered• Troubleshooting Series Regulators• Single Chip Regulators • Switching Regulators• Other Switching Regulator Modes

Zener Diode Regulators• Characteristics

• One of the simplest types of regulated power supplies• However, Inefficient in High current applications• When Load currents are low high currents flow through the zener

• Requires a minimum unregulated voltage• Zener diode must always be in reverse bias and conducting for

regulation

• Zener characteristics are critical to the regulator operation

• Zener Diode Characteristics• Acts like a normal diode when forward biased

• Current increases rapidly when V Forward exceeds 0.7V

Zener Diode Regulators• Zener Diode Characteristics

• Reverse Biased characteristics• Due to doping of the semiconductor

material when the doped for reverse biased voltage (aka, Zener Voltage – VZ) is exceeded the zener diode conducts

• The heavier the zener conducts the greater the voltage drop across the power supplies internal resistance

• Represented as R Series (R S) » Thevenin Equivalent resistance

Zener Diode Regulators• Zener Diode Characteristics

• Reverse Biased characteristics• Zener Circuit Operation

• Voltage across RS = E –VZ • Source current is split between the zener and the load resistor

» IS = > IZ and IL

• If the load decreases (RL increases)

» IL decreases

» IZ increases enough

to keep the voltage across it constant

• If the load increases» IL increases

» IZ decreases

See Example Problem 9-1 on page 228

Zener Diode Regulators• Zener Diode Power Supply

• The zenrer circuit is feed by a simple unregulated PS

• VP is much higher than the zener voltage

• The head room allows for regulated output over a range of different loads

• Sample shown at the right• VO = Zener voltage

• VS(ave) =4V w/2V of ripple• The regulator eliminates ripple on

the output until the load gets to large


• Sample shown at the right• Continue - load gets to large

• The ripple voltage across C1 increases and VS(ave) decreases and the

• IS isn’t large enough for the load and to maintain a few milliamps through the zener, the Zener stops conducting – you have ripple on the output

» Limit before ripple shows in the out put depends upon zener rating


• Sample shown at the right• Continue - load gets to large

• If the increases more the zener will regulate even less of the out put

• Example Problem 9-1, page 228• Replacements

• Power rating is critical• Replace with equal or higher rating

• Typical ratings range from 1/4W to 10 W or higher• Power dissipated by a zener

ZZZ IVP

Series Regulators• Characteristics

• More efficient than a Zener regulated PS• The Pass transistor is placed in series

with the load and unregulated PS• Acts as a variable resistor that is adjusted to

maintain VO the same

• Operation• As long as the unregulated voltage (VCC)

is greater than VB the output voltage will be regulated

• Circuit is a basic emitter follower• Output voltage = VB - VBE

Series Regulators• Operation

• Conventional representation• The transistor is called the Pass Transistor • The Pass transistor must not go into saturation

• In saturation all regulation stops and the output is a scalar representation of the input

• Unlike the zener regulator, when load currents are low the regulators power dissipation decreases

• More efficient operation

Series Regulators

• Operation• Conventional representation

• Pass Transistor Power

• Formulas

LOd IVEP )(

BEZO VVV L

LL RVI

C

BII

S

ZS R

VEI )( Equation in textbook is

wrong

BSZ III Example Problem 9-2 on page 232

Series Regulators

• Operation• Conventional Representation

• Pass Transistor Power

• Formulas

LOd IVEP )(

BEZO VVV L

LL RVI

C

BII

S

ZS R

VEI )( Equation in textbook is

wrong

BSZ III Example Problem 9-2 on page 232

Series Regulators

• Operation• Real Circuit

• Figure 9-7 on page 233• Note the rating on the zener and the measured base voltage

on the transistor• Zener diodes have tolerances such as 5%, 10% or 20%

• Regulation isn’t perfect• Changes in load cause IB

changes which result

in VBE changes

• In the case of the graph to

the right VO would change

by 0.1 V

Series Regulators• Operation

• Calculation of Percent Regulation• For V Out

• Example problem 9-3 on page 234

100Re% xV

VVgulation

loadfull

loadfullloadno

Adjustable Voltage Regulator• Characteristics

• Adds components for better regulation and some for adjustment of the output

• Better Regulation

• Thus the output stays at a more constant level and higher percent regulation

• From the given condition and voltages

• Load increases, VA tends to decrease

• VB decreases, Q2 conducts less

• VD goes higher and then VA goes higher

Adjustable Voltage Regulator• Characteristics

• Output level adjustment

Current Limiters• Characteristics

• Provides a means to protect the PS from excessive loads or shorted outputs.

• Added components – Q3 and RSC • RSC s sized so that normal operating currents will develop

much less than 0.7V across it and Q3 is off

• If IL is large enough

Q3 turns on • Point D is tied to

point A• Q1 conducts much

less

SCCircuitShort R

VI

7.0

Example Problem on 236

Troubleshooting Series Regulators• Characteristics

• Significant difficulty due to the interaction of many of the components

• Best approach may be isolate some parts of the circuit and test

• Sample walk through• Assume:

• VO is abnormal

• Adjusting RX doesn’t fix the problem

• Follow suggested flow chart on page 238

Single Chip Regulators• Characteristics

• Internal circuitry is at least as sophisticated as Current limiting circuit cover before

• Typical packages

Single Chip Regulators• Characteristics

• Typical part numbers• 78XX and 340XX - the XX are replaced by the rated voltage• Check data sheets for rated currents, min/max input voltages

• Typical Configuration

Single Chip Regulators• Single Chip Regulated Adjustable PS

•

• Example Problem 9-5 on page 240

21

RR

VVV regregO

Single Chip Regulators• When current demands exceed a signal chip

• You may find separate regulators on multiple circuit cards in a multi-card systems

• Outputs of multiple regulator should never be connected – Check Specs

• Provide an parallel higher current path• See the circuit on the next slide or Figure 9-18 on page 241 of the textbook• Operation

• On startup Q1 is off and the regulator starts delivering power

•

• Example Problem 9-5 on page 240

Single Chip Regulators• When current demands exceed a signal chip

• Provide an parallel higher current path• Operation

• Current through R2 biasing the base of Q2 (Note R2 is sized to match the emitter-base resistor) and Q2 turns on

• VR1 = 0.7V

•

•

• Example Problem

9-6 on page 241

regIR

RI

1

21

regL III 1

Switching Regulators• Why use

• Series regulated PS still can consume substantial power just operating

• Work Example Problem 9-7 on page 242

• Characteristics of Switching Regulators• Pass transistor isn’t always on

• It is switched on/off at a high rate to keep the output voltage at a desired value

• The power delivered by the Pass transistor depends upon the average value of the pulses that result from the on/off switching

• The pulses look like a rectangular digital waveform with varying duty cycles – aka, Pulse width modulation

• See Figure 9-19 on page 243 of the textbook

Switching Regulators• Characteristics of Switching Regulators

• Filtering Circuit for Switching PS• Switch subs for the Pass Transistor• The inductor/choke is critical to operation

• Act to keeps current through the load constant

• The Cap helps smooth out ripple voltages– the larger the better

• Closed switch• Current flows through S, L, & RL • Counter emf (voltage) developed across L to prevent load current from changing too rapidly


• Complete PS• Sampler tests the output voltage• Compare/Control

• Compares the sample to a reference• Changes the pulse width of the pulses outof the Pass Transistor

» The greater the difference between the sample and reference the greater the pulse width

• The complete sampling, comparing, and control circuitry is available in a monolithic IC – e.g., SG 1524 (Pulse Width Modulation)

Switching Regulators

• Characteristics of Switching Regulators• Complete PS

• See the example circuit on page 245 that uses the SG 1524 for PWM

• It shows a simplified view of the IC’s circuitry» 5V reference, comparator, Sawtooth oscillator, error

amplifier, and other components» Note the OSC typically operate at 5-100kHz

• Vs comes from a pot Rx

• Difference Amp feed by Vs and the reference voltage V3 multiplies the difference between them by RF/R1.


• Complete PS• See the example circuit on page 245 that uses the SG

1524 for PWM• The amplified error signal feeds the + input of the comparator and

the the sawtooth OSC feeds the - input

» When more positive than the sawtooth OSC output Q2 and Q1 are turned on. Else Q2 and Q1 are off

» Note: Additional not shown control circuits prevent the voltage on the base of Q1 at zero if the Ramp voltage is less than +!V

• Q1 is switched on/off with longer/shorter duty cycles, as needed to maintain a constant output voltage (See Fig 9-23 on page 246)

» Thus the pulse width is directly related to the magnitude of the differences between the sampled output voltage and reference

» During conduction Q1 is in saturation, very low voltage drop, thus low power loss, In cut-off zero current flows and zero power


• Sample Waveforms for a Pulse Width Switching PS


• The Fairchild µA78S40 is similar but not a pulse width• It outputs fixed width pulses when the comparator indicates

a need for more energy in the output filtering circuit• Otherwise – NO PULSES

• Sample circuit using the IC and a simplified view of its internal circuitry – at the bottom of page 248

• Internal timing signals at the top of Page 248• When the sampled output Vs falls below Vref the And-Gate is

enabled and Q outputs a square wave, else it is off• Which turns Q1 and Q2 on/off until Vs is greater than Vref

• Typical output signals at the top of page 249• Oscillator output compared to output voltages under light,

medium and heavy loads

Other Switching Regulators• Key Factors

• Three basic types of switching PS

• Step down• Vo less than the

input» Just covered -

previous section• Step Up

• Vo greater than the input

• Inverting• Vo opposite polarity

than the input

Other Switching Regulators• Step-Up Switched PS

• With the switch closed• A significant current is

established in the inductor

• When the switch opens• A cemf voltage develops

across the inductor• The cemp adds to E and

charges the output Cap• Cap maintains the voltage on

the load when the switch closes again

Other Switching Regulators• Inverting Switched PS

• With the switch closed• A significant current is

established in the inductor• Diode ids reversedbiased

• When the switch opens• A cemf voltage develops

across the inductor• The cemp charges the output

Cap• Cap maintains the voltage on

the load when the switch closes again

Inverting Switcher

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Electronic Troubleshooting Chapter 9 Regulated Power Supplies