power supplies linear voltage regulators · 4/05/2020 3 unregulated power supply •the circuit has...
TRANSCRIPT
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ECEN303
Power Supplies
Linear Voltage Regulators
What is a Power Supply?
• Electrical Definition of Power Supply
• Conversion of a voltage into a desired voltage
• Example: Car Battery (12 V) Microprocessor (5 V)
• Efficiency Example:
• PIN = (12 V) x (1 A) = 12 W
• POUT = (5 V) x (1 A) = 5 W
• The remaining 7W (12 W – 5 W) of power is lost as heat
• The efficiency η is:
Power Supply OutputInput Load1) LED
2) Micro
Source1) Battery (DC)
2) Wall Outlet (AC)
5 V1 A
12 V 1 A
%24 W12
W5
INP
OUTP
η Power supplies are not
100% efficient.
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Power Supply
• Transformer to step-down the voltage
• Rectifier – converts AC to DC; Has some ripple
• Filter- filters the AC component – provides raw
output (unregulated)
• Voltage regulator – regulates a DC output despite
variations in the supply side
230V
Transformers
230V
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Unregulated Power Supply
• The circuit has
–Poor line regulation – change in input voltage cause change in output voltage
–Poor load regulation – change in output voltage to change in output current
• A regulated power supply will provide a constant output voltage regardless of the changes in
input voltage or output current.
Voltage Regulation
• Two basic performance specifications 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
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Line Regulation
• Line regulation can be defined as the percentage change in the output
voltage for a given change in the input voltage.
Δ means “a change in”
• Line regulation can be calculated using the following formula:
IN
OUTOUT
V
VVregulationLine
%100/
Load Regulation
Load regulation: A change in load current (due to a varying RL) has practically
no effect on the output voltage of a regulator (within certain limits)
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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
%100
FL
FLNL
V
VVregulationLoad
Load Regulation
• Sometimes power supply manufacturers specify the equivalent output
resistance (Rout) instead of its load regulation.
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Types of Regulator
• Fundamental classes of voltage regulators are linear regulators and
switching regulators.
• Two basic types of a 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.
How to achieve good regulation?
Regulator - Zener diode regulator
• For low current power supplies - a simple voltage regulator can be made
with a resistor and a zener diode connected in reverse.
• Zener diodes are rated by their breakdown voltage Vz and maximum power
Pz (typically 500mW-1N52221B or 1.3W- 1N4728A, 1.5W, 3W, 5W etc.)
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Operation
• Balance between (series limiting R) RS, Zener and the
RL
–As load current increases, the Zener current
decreases so that the current through RS is
constant – hence output voltage is constant
–If input voltage increases more current will flow
through the Zener, the voltage drop across RS will
increase keeping load voltage constant
• Drawbacks
–Low efficiency for heavy load conditions
–Zener impedances are unpredictable
–Can’t handle large changes in load current or
input voltage
Conditions for proper operation of Zener
• Must operate in breakdown region
• Zener must not exceed maximum dissipation of power
• There is a limit on minimum RL dictated by the breakdown region.
• Example: Determine current through the resistance, minimum and maximum load currents and
Zener currents (Ans: IS = 75mA, IL(max) = 60mA (@200Ω), IZ(min) = 15mA, IZ(max) = 75mA)
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Voltage Regulation
• Zener can provide a good enough line regulation but provides poor load regulation.
• Zener can only be used for low current applications
• So better solution?
IC Voltage Regulators
• Regulation circuits in integrated circuit form are widely used.
• Their operation is no different but they are treated as a single device with
associated components.
• These are generally three terminal devices that provide a positive or
negative output.
• Some types have variable voltage outputs.
• A typical 7800 series voltage regulator is used for positive voltages.
• The 7900 series are negative voltage regulators.
• These voltage regulators when used with heatsinks can safely produce
current values of 1A and greater.
• The capacitors act as line filtration.
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IC Voltage Regulators
• Several types of both linear (series and shunt) and switching regulators are
available in integrated circuit (IC) form.
• Single IC regulators contain the circuitry for:
(1) reference source
(2) comparator amplifier
(3) control device
(4) overload protection
• Generally, the linear regulators are three-terminal devices that provides
either positive or negative output voltages that can be either fixed or
adjustable.
Regulator ICs
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Fixed Voltage Regulator
• The fixed voltage regulator has an unregulated dc input voltage Vi applied to
one input terminal, a regulated output dc voltage Vo from a second terminal,
and the third terminal connected to ground.
Fixed-Positive Voltage Regulator
• The series 78XX regulators are the three-terminal devices that provide a
fixed positive output voltage.
Fixed Voltage Regulator
• An unregulated input
voltage Vi is filtered by a
capacitor C1 and
connected to the IC’s IN
terminal.
• The IC’s OUT terminal
provides a regulated +12
V, which is filtered by
capacitor C2.
• The third IC terminal is
connected to ground
(GND)
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Positive-Voltage Regulators in the 78XX SeriesFixed Voltage Regulator
IC Part Output Voltage
(V)
Minimum Vi (V)
7805 +5 +7.3
7806 +6 +8.3
7808 +8 +10.5
7810 +10 +12.5
7812 +12 +14.5
7815 +15 +17.7
7818 +18 +21.0
7824 +24 +27.1
Fixed Voltage Regulator
Fixed-Negative Voltage Regulator
• The series 79XX regulators are the three-terminal IC regulators that provide
a fixed negative output voltage.
• This series has the same features and characteristics as the series 78XX
regulators except the pin numbers are different.
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Fixed Voltage Regulator
IC Part Output Voltage
(V)
Minimum Vi (V)
7905 -5 -7.3
7906 -6 -8.4
7908 -8 -10.5
7909 -9 -11.5
7912 -12 -14.6
7915 -15 -17.7
7918 -18 -20.8
7924 -24 -27.1
Negative-Voltage Regulators in the 79XX Series
Data Sheet
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7800s
LDO – TL750M12
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Adjustable-Voltage Regulator
• Voltage regulators are also available in circuit configurations that allow to
set the output voltage to a desired regulated value.
• The LM317 is an example of an adjustable-voltage regulator, can be
operated over the range of voltages from 1.25 to 37 V.
Example
• If LM317 is adjusted to provide +8V output, determine maximum allowable input voltage. (Ans:
48V)
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Summary
• Voltage regulators keep a constant dc output despite input voltage or load changes.
• The two basic categories of voltage regulators are linear and switching.
• The two types of linear voltage regulators are series and shunt.
• The three types of switching are step-up, step-down, and inverting.
• Switching regulators are more efficient than linear making them ideal for low voltage
high current applications.
• IC regulators are available with fixed positive or negative output voltages or variable
negative or positive output voltages.
• Both linear and switching type regulators are available in IC form.
• Current capacity of a voltage regulator can be increased with an external pass
transistor.