Ripple factor

Robert Boylestad
Digital Electronics
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Chapter 18:
Power Supplies
(Voltage Regulators)

Robert Boylestad
Digital Electronics
Power Supply Diagram

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Digital Electronics
Filter Circuits
The output from the rectifier section is a pulsating DC. The filter circuit reduces the peak-to-
peak pulses to a small ripple voltage.

Robert Boylestad
Digital Electronics
Ripple Factor
After the filter circuit a small amount of AC is still remaining. The amount of ripple voltage
can be rated in terms of Ripple Factor (r).
[Formula 18.1]100
Vdc
(rms)V
voltagedc
(rms)voltageripple
%r
r


Robert Boylestad
Digital Electronics
Half-wave Rectifier Ripple Factor
DC output:
AC Ripple output:
Ripple Factor:
(Note Vm is the peak rectifier output voltage.)
[Formula 18.3]
[Formula 18.4]
[Formula 18.5]
0.318VmVdc 
0.385Vm(rms)Vr 
121%100
0.318Vm
0.385Vm
100
Vdc
(rms)V
%r
r


Robert Boylestad
Digital Electronics
Full-wave Rectifier Ripple Factor
DC output:
AC Ripple output:
Ripple Factor:
The full-wave rectifier has a significantly lower ripple factor.
[Formula 18.6]
[Formula 18.7]
[Formula 18.8]
0.636VmVdc 
0.308Vm(rms)Vr 
%48100
0.636Vm
0.308Vm
100
Vdc
(rms)V
%r
r


Robert Boylestad
Digital Electronics
Types of Filter Circuits
•Capacitor Filter
•RC Filter

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Digital Electronics
Capacitor Filter

Robert Boylestad
Digital Electronics
Ripple Voltage with a Capacitor Filter
The larger the capacitor the smaller the ripple voltage.
A capacitor significantly reduces the AC content of the rectified signal.
Ripple Voltage: [Formula 18.9]
CR
2.4Vdc
C
2.4Idc
fC34
Idc
(rms)V
L
r 

Robert Boylestad
Digital Electronics
DC Output with a Capacitor Filter
A capacitor increases the DC output.
Dc Output:
Note: Vm = peak rectified voltage
IDC is the load current in mA.
[Formula 18.10]
C
4.17Idc
Vm
4fC
Idc
VmVdc 

Robert Boylestad
Digital Electronics
Ripple Factor with a Capacitor Filter
The capacitor reduces the ripple factor.
Ripple Factor: [Formula 18.11]100
CR
2.4
100
CVdc
2.4Idc
100
Vdc
(rms)V
%r
L
r


Robert Boylestad
Digital Electronics
Diode Ratings with Capacitor Filter
The size of the capacitor increases the current drawn through the diodes. The larger the
capacitance, the greater the amount of current.
Peak Current vs. Capacitance:
C = capacitance
V = change in capacitor voltage during charge/discharge
t = the charge/discharge time
A smaller capacitor will reduce the peak current through the diodes.
t
CV
I 

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Digital Electronics
RC Filter Circuit
Adding an RC section will further reduce the ripple voltage and decrease the surge current
through the diodes.

Robert Boylestad
Digital Electronics
Ripple Voltage in an RC Filter Circuit
The ripple voltage is significantly reduced by the addition of an RC circuit.
Ripple Voltage: [Formula 18.14]
Vr(rms) = ripple voltage after the RC filter
Vr(rms) = ripple voltage before the RC filter
R = resistor in the added RC filter
XC = reactance of the capacitor in the added RC filter
(rms)V
R
X
(rms)V r
C
r 

Robert Boylestad
Digital Electronics
Voltage Regulation
The amount of variation in DC output voltage due to varying loads from no-load to full-load
is called voltage regulation.
Voltage Regulation: [Formula 18.2]
VNL = no-load voltage
VFL = full-load voltage
100%
V
VV
%VR
FL
FLNL




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Digital Electronics
Voltage Regulation Circuits
There are two common types of circuitry for voltage regulation:
A.Discrete Transistors
B. IC’s

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Digital Electronics
A. Discrete Transistors
1. Series Voltage Regulation Circuit
2. Current Limiting Circuit
3. Shunt Voltage Regulation Circuit

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Digital Electronics
1. Series Voltage Regulation Circuit
The series element controls the amount of the input voltage that gets to the output.
If the output voltage increases (or decreases), the comparator circuit provides a control signal
to cause the series control element to decrease (or increase) the amount of the output voltage.

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Digital Electronics
Series Voltage Regulation Circuit
Sample Circuit
R1 and R2 act as the sampling circuit
~ Zener provides the reference voltage
~ Q2 controls the base current to Q1.
~ Q1 maintains the constant output voltage
If the output increases ~ The voltage through R1 and R2 increases at V2,
VBE of Q2 increases,
causing Q2 to conduct more,
and making less available to Q1,
which conducts less
~ output voltage decreases.
If the output voltage decreases ~ the opposite happens.

Robert Boylestad
Digital Electronics
Series Voltage Regulation Circuit
Sample Circuit
An op-amp is used as the comparator circuit
The op-amp compares the Zener diode voltage to the output voltage (via R1 and R2) and
controls the conduction of Q1.

Robert Boylestad
Digital Electronics
2. Current-Limiting Circuit
As IL increases,
the voltage drop across RSC (short circuit sensing R) increases.
When the voltage across RSC is large enough,
it drives Q2 on,
diverting current away from the base of Q1,
thereby reducing current through Q1 and the load.

Robert Boylestad
Digital Electronics
3. Shunt Voltage Regulation Circuit
The shunt voltage regulator shunts current away from the load.
The load voltage is sampled and fed back to a comparator circuit. If the load voltage is too
high, control circuitry shunts more current away from the load.

Robert Boylestad
Digital Electronics
3. Shunt Voltage Regulation
Sample Circuit
If the voltage across the load increases above the Zener voltage,
Q2 turns on,
which causes Q1 to conduct
and current is shunted away through Q1.

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Digital Electronics
3. Shunt Voltage Regulation
Sample Circuit
The op-amp acts as a comparator circuit.
If the load voltage exceeds the Zener voltage,
Q1 turns on
and shunts current away from the load.

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Digital Electronics
B. IC Voltage Regulators
Regulator ICs contain:
•Comparator circuit
•Reference Voltage
•Control circuitry
•Overload protection

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Digital Electronics
IC Voltage Regulator Types
Three-Terminal Voltage Regulators
1. Fixed Positive Voltage Regulator
2. Fixed Negative Voltage Regulator
3. Adjustable Voltage Regulator

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Digital Electronics
Three-Terminal Voltage Regulators
The specifications for this IC indicate
•the range of input voltages that can be regulated for a specific output voltage and load
current range
•load regulation: variation in output voltage with variations in load current
•line regulation: variation in output voltage with variations in input voltage

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Digital Electronics
1. Fixed Positive Voltage Regulator
These ICs provide a fixed positive output voltage.

Robert Boylestad
Digital Electronics
2. Fixed Negative Voltage Regulator
These ICs output a fixed negative output voltage.

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Digital Electronics
3. Adjustable Voltage Regulator
These regulators have adjustable output voltages. The output voltage is commonly selected
using a potentiometer.

Robert Boylestad
Digital Electronics
Practical Power Supplies
•DC Supply (Linear Power Supplies)
•Chopper Supply (Switching Power Supplies)
•TV Horizontal High Voltage Supply
•Battery Charger Circuits

Ripple factor

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