Transcript Electronic Troubleshooting

Electronic Troubleshooting
Chapter 9
Regulated 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
See Example
Problem 9-1
on page 228
• 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
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
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
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
PZ  VZ  I Z
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
Pd  ( E  VO ) I L
• Formulas
VO  VZ  VBE
IB 
IC

IZ  IS  IB
I L  VL
I S  ( E  VZ )
Example Problem
9-2 on page 232
RL
RS
Equation in
textbook is
wrong
Series Regulators
• Operation
• Conventional Representation
• Pass Transistor Power
Pd  ( E  VO ) I L
• Formulas
VO  VZ  VBE
IB 
IC

IZ  IS  IB
I L  VL
I S  ( E  VZ )
Example Problem
9-2 on page 232
RL
RS
Equation in
textbook is
wrong
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
% Re gulation 
Vnoload  V fullload
V fullload
• Example problem 9-3 on page 234
x100
Adjustable Voltage Regulator
• Characteristics
• Adds components for better regulation and some for
adjustment of the output
• Better 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
• Thus the output stays at a more constant level and higher
percent regulation
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
Example
Problem
on 236
• Q1 conducts much
less
I ShortCircuit 
0.7V
RSC
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
•
VO  Vreg 
Vreg
R1
R2
• Example Problem 9-5 on page 240
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
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
•
•
I1 
R2
I reg
R1
I L  I1  I reg
• Example Problem
9-6 on page 241
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
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 out
of 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.
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
Switching Regulators
• Characteristics of Switching Regulators
• Sample Waveforms for a Pulse Width Switching PS
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
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