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Electronic Troubleshooting
Chapter 9Regulated Power Supplies
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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
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Regulated Power Supplies• Overview
• Types of Regulated power supplies Covered• Troubleshooting Series Regulators• Single Chip Regulators • Switching Regulators• Other Switching Regulator Modes
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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
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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
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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
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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
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Zener Diode Regulators• Zener Diode Power Supply
• 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
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Zener Diode Regulators• Zener Diode Power Supply
• 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
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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
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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
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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
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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
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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
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Series Regulators• Operation
• Calculation of Percent Regulation• For V Out
• Example problem 9-3 on page 234
100Re% xV
VVgulation
loadfull
loadfullloadno
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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
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Adjustable Voltage Regulator• Characteristics
• Output level adjustment
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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
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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
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Single Chip Regulators• Characteristics
• Internal circuitry is at least as sophisticated as Current limiting circuit cover before
• Typical packages
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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
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Single Chip Regulators• Single Chip Regulated Adjustable PS
•
• Example Problem 9-5 on page 240
21
RR
VVV regregO
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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
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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
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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
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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
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Switching Regulators• Characteristics of Switching Regulators
• 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)
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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.
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Switching Regulators• Characteristics of Switching Regulators
• 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
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Switching Regulators• Characteristics of Switching Regulators
• Sample Waveforms for a Pulse Width Switching PS
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Switching Regulators• Characteristics of Switching Regulators
• 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
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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
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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
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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
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Inverting Switcher