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.