Ripple factor

Robert BoylestadDigital Electronics

Copyright ©2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Chapter 18:Power Supplies

(Voltage Regulators)

Slide 1




Power Supply Diagram

Slide 2




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.

Slide 3




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]100Vdc

(rms)V

voltagedc

(rms) voltageripple%r

r

Slide 4




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%1000.318Vm

0.385Vm100

Vdc

(rms)V%r

r

Slide 5




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

%481000.636Vm

0.308Vm100

Vdc

(rms)V%r

r

Slide 6




Types of Filter Circuits

•Capacitor Filter•RC Filter

Slide 7




Capacitor Filter

Slide 8




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

Lr

Slide 9




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

4.17IdcVm

4fC

IdcVmVdc

Slide 10




Ripple Factor with a Capacitor Filter

The capacitor reduces the ripple factor.

Ripple Factor: [Formula 18.11]100CR

2.4100

CVdc

2.4Idc100

Vdc

(rms)V%r

L

r

Slide 11




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 = capacitanceV = change in capacitor voltage during charge/discharget = the charge/discharge time

A smaller capacitor will reduce the peak current through the diodes.

t

CVI

Slide 12




RC Filter Circuit

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

Slide 13




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]

Vr(rms) = ripple voltage after the RC filterVr(rms) = ripple voltage before the RC filterR = resistor in the added RC filterXC = reactance of the capacitor in the added RC filter

(rms)VR

X(rms)V r

Cr

Slide 14




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 voltageVFL = full-load voltage

100%V

VV%VR

FL

FLNL

Slide 15




Voltage Regulation Circuits

There are two common types of circuitry for voltage regulation:

A.Discrete TransistorsB. IC’s

Slide 16




A. Discrete Transistors

1. Series Voltage Regulation Circuit2. Current Limiting Circuit3. Shunt Voltage Regulation Circuit

Slide 17




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.

Slide 18




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.

Slide 19




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.

Slide 20




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.

Slide 21




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.

Slide 22




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.

Slide 23




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.

Slide 24




B. IC Voltage Regulators

Regulator ICs contain:•Comparator circuit•Reference Voltage•Control circuitry•Overload protection

Slide 25




IC Voltage Regulator Types

Three-Terminal Voltage Regulators

1. Fixed Positive Voltage Regulator2. Fixed Negative Voltage Regulator3. Adjustable Voltage Regulator

Slide 26




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

Slide 27




1. Fixed Positive Voltage Regulator

These ICs provide a fixed positive output voltage.

Slide 28




2. Fixed Negative Voltage Regulator

These ICs output a fixed negative output voltage.

Slide 29




3. Adjustable Voltage Regulator

These regulators have adjustable output voltages. The output voltage is commonly selected using a potentiometer.

Slide 30




Practical Power Supplies

•DC Supply (Linear Power Supplies)•Chopper Supply (Switching Power Supplies)•TV Horizontal High Voltage Supply•Battery Charger Circuits

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Ripple factor