CHAPTER 19 (2): FILTER CIRCUIT

Copyright

• Copyright © 2010 Christopher Teoh, Tan HJ & Wong WY Singapore Polytechnic. All rights reserved.

Chapter Overview

• Chapter 19: Diode Applications (Part 2)

Objectives of Chapter 19, Part 2

• Understand the function of a power supply filter circuit.

• Explain how a capacitor filter works.

• Understand ripple voltage and ripple factor.

Power Supply Filters

• The purpose of a power supply filter is to eliminate (or reduce) fluctuations in the output voltage of a rectifier.

• The filter circuit produces a (near) constant-level DC voltage necessary for electronic circuits.

• Electronic circuits require a steady source of DC voltage and current for proper operation, leading to the need for filtering.

• The fluctuations in the filter output are referred to as ripple.

Capacitor Filter Operation

• During the positive first-quarter cycle of the input:

  • The diode is forward-biased.

  • Capacitor charges to within 0.7V of the input peak.

• When input voltage drops below its peak:

  • The diode becomes reverse-biased.

  • The capacitor retains its charge and discharges through load resistance at a rate defined by the RLC time constant (usually long).

  • A larger time constant results in less discharge of the capacitor.

Charging and Discharging of Capacitor

• In the next cycle, the diode becomes forward-biased when the input voltage exceeds the capacitor voltage by approximately 0.7V again.

• The capacitor charges at the beginning of each cycle and discharges slowly through the load after the positive peak.

• The variation in capacitor voltage due to charging and discharging is referred to as ripple voltage.

• Ripple is undesirable; a smaller ripple indicates better filtering.

Ripple Voltage

• Larger ripple means less effective filtering.

• Smaller ripple indicates more effective filtering.

Ripple Frequency

• For a given input frequency, the output frequency of a full-wave rectifier is twice that of a half-wave rectifier.

Ripple Output of Full-Wave vs. Half-Wave Rectifier

• Filtered full-wave rectified voltage possesses a smaller ripple compared to half-wave voltage, assuming the same load resistance and capacitor values.

Ripple Factor

• The ripple factor (r) indicates the effectiveness of the filter defined as:

  • r = Vr(rms) / Vdc

  • Where Vr(rms) is the RMS of the ripple voltage and Vdc is the average output voltage.

Derivation of Ripple Voltage

• The ripple voltage Vr(rms) can be derived by approximating the ripple waveform to a triangular shape, introducing a factor of √3.

• The formula becomes: r = (Vr(p-p) - Vp(rect)) / (2√3)

• A lower ripple factor signifies better filter performance.

• To decrease the ripple factor, increase the filter capacitor value or load resistance.

Surge Current in the Capacitor Filter

• Before the switch is closed, the filter capacitor is uncharged.

• When the switch is closed, the uncharged capacitor behaves like a short circuit, causing an initial surge of current through the forward-biased diodes D1 and D2.

Surge Limiting

• A surge-limiting resistor may be used to protect the diodes.

• The resistor's value should be small compared to the load resistance (RL).

• Diodes must have a maximum forward surge current rating (IFSM) to withstand momentary current surges.

Summary of Capacitor Filters

• A capacitor filter can provide a DC output voltage slightly lower than the input peak voltage.

• The charging and discharging of the capacitor produce ripple voltage; less ripple means better filtering.

• Lower ripple factor translates to a more effective filter performance.