Download - EE 21-Lecture 3.5 Power Supply Design
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Power Supply DesignA little more diode applications…
1
E E 2 1 S l i d e s ( A A M S )
Topics
• Parts of a power supply
• Ripple
•
Voltage Regulation• Capacitor – input filter
• RMS ripple and ripple voltage
• DC voltage output
• Effect of capacitor sizing
2
E E 2 1 S l i d e s ( A A M S )
Block diagram of a power
supply
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E E 2 1 S l i d e s ( A A M S )
Ripple Voltage
• A rectified AC signal will have a DC (steady) component
and an AC (varying) component known as the ripple.
• Ripple is given by the equation
4
%100 voltagedc
ms)voltage(rripple )( x
V
Vr r
dc
rms
E E 2 1 S l i d e s ( A A M S )
Example
• The following readings were obtained in
measuring the output signal of a filter circuit:
• DC voltmeter: 25 volts DC
• AC voltmeter: 1.5 volts RMS
• Compute for the ripple of the filter output.
• Ans: 6% 5
E E 2 1 S l i d e s ( A A M S )
Voltage Regulation
• Describes the amount of change between the no-load
and load conditions of a power supply.
• Given by the following equation:
• Where
%V.R. – voltage regulation
VNL = DC voltage at no-load
VFL = DC voltage at full-load 6
%100..% xV
V V RV
FL
FL NL
E E 2 1 S l i d e s ( A A M S )
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Example
• A DC voltage supply provides 60 V when the
output is unloaded. When connected to a load,
the measured voltage drops to 56 V. Calculatethe voltage regulation.
• Ans. 7.1428%
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E E 2 1 S l i d e s ( A A M S )
Ripple factor of a half-wave
rectified signal
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mdc
V V 318.0
mrmsr V V 385.0)(
%121%100318.0
385.0)( x
V
V
V
V r
m
m
dc
rmsr E E 2 1 S l i d e s ( A A M S )
Ripple factor of a full-wave
rectified signal• For half-wave rectified signal:
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mdc V V 636.0
mrmsr V V 308.0)(
%48%100636.0
308.0)( x
V
V
V
V r
m
m
dc
rmsr E E 2 1 S l i d e s ( A A M S )
Capacitor Filter
• Capacitor develops voltage from the output of
the rectifier circuit (RC charging)
• The capacitor discharges during “down time”
of rectifier circuit cycle
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E E 2 1 S l i d e s ( A A M S )
Capacitor Filter Operation
• Normal rectifier
output
• With capacitor filter
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E E 2 1 S l i d e s ( A A M S )
Output Waveform Times
At T1:
-diodes conduct
-cap charges to Vm
At T2:
-rectifier V drops
-cap discharges
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E E 2 1 S l i d e s ( A A M S )
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Output Waveform Times
• Charge-discharge
cycle occurs every
half-cycle of rectifier
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• Filtered voltageincludes a dc level Vdc,as well as a ripple
voltage Vr(rms) causedby capacitor chargingand discharging.
E E 2 1 S l i d e s ( A A M S )
Ripple Voltage Vr (RMS)
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C RV
C I
fC I rmsVr
L
dcdcdc 4.24.234
)(
IMPORTANT!
Idc in milliamperes
C in microfarads
RL in kilohms
E E 2 1 S l i d e s ( A A M S )
Example
• Calculate the ripple voltage of a full-wave
rectifier with a 100-μF filter capacitor
connected to a load drawing 50mA.
• Ans. 1.2 V
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E E 2 1 S l i d e s ( A A M S )
DC Voltage Vdc
16
C
I V
fC
I V V dc
mdc
mdc
17.4
4
Where
Idc in milliamperes
C in microfaradsVm is the peak rectifiervoltage
E E 2 1 S l i d e s ( A A M S )
Example
• If the peak rectified voltage for the previous
filter circuit example is 30 Volts, calculate thefilter DC voltage.
• Ans. 27.915 Volts
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E E 2 1 S l i d e s ( A A M S )
Filter Capacitor Ripple
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%1004.2
%1004.2
%100)(
xC R
I x
CV
I x
V
rmsV r
L
dc
dc
dc
dc
r
Where
Idc in milliamperes
C in microfarads
Vdc in Volts
RL in kilohms
E E 2 1 S l i d e s ( A A M S )
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Example
• Calculate the ripple of a capacitor filter for a
peak rectified voltage of 30 Volts, a capacitor
C = 50μF, and a 50 mA load current.
• Ans. 4.3%
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E E 2 1 S l i d e s ( A A M S )
Diode Conduction Period and
Peak Diode Current • Larger capacitor = less ripple, higher Vdc
• However, capacitor size directly affects the
current drawn by the rectifying diodes
• Larger capacitor = larger peak current drawn
by rectifying diodes
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E E 2 1 S l i d e s ( A A M S )
Capacitor Filter Waveforms
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E E 2 1 S l i d e s ( A A M S )
Current calculations
• Where T1 = diodeconduction time
• T = rectifier period(note that it’s twicefor the fullwave)
• Idc = average current drawn from filter
• Ipeak = peak current through conducting
diodes22
peak dc I T
T I 1
dc peak I T
T I
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E E 2 1 S l i d e s ( A A M S )