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Chapter 3

Chapter 3. Common Diode Applications Basic Power Supply Circuits. Power Supply. A group of circuits used to convert ac to dc. Rectifier – Converts ac to pulsating dc. Filter – Reduces variations in the rectifier output. Voltage regulator – Maintains a constant dc output voltage.

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Chapter 3

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  1. Chapter 3 Common Diode Applications Basic Power Supply Circuits

  2. Power Supply • A group of circuits used to convert ac to dc. • Rectifier– Converts ac to pulsating dc. • Filter – Reduces variations in the rectifier output. • Voltage regulator – Maintains a constant dc output voltage.

  3. Transformers • Transformers are described in terms of the relationship between secondary voltage (VS) and primary voltage (VP). • Step-up– VS is greater than VP. • Step-down – VS is less than VP. • Isolation – VS is approximately equal to VP.

  4. Transformer Voltage and Current Ratios

  5. Half-wave Rectifiers • Half-wave rectifier – A diode placed in series between a transformer (or ac line input) and its load.

  6. Positive Half-wave Rectifiers This circuit converts an ac input to a series of positive pulses.

  7. Average Load Voltage and Current • Average voltage (Vave) – The dc equivalent of a voltage waveform. • Average current (Iave) – The dc equivalent of a current waveform. For the output from a half-wave rectifier:

  8. Negative Half-wave Rectifiers This circuit converts an ac input to a series of negative pulses.

  9. Peak Inverse Voltage (PIV) Peak inverse voltage (PIV) – The maximum diode reverse bias produced by a given circuit. For the diode in a half-wave rectifier:

  10. Full-wave Rectifier

  11. Full-wave Rectifier Operation • Diodes conduct during alternate half cycles of the input signal. • VL(pk) is approximately half the value of VS(pk). • The circuit produces two positive half-cycles for each input cycle.

  12. Average Load Voltage and Current • Average voltage (Vave) – The dc equivalent of a voltage waveform. • Average current (Iave) – The dc equivalent of a current waveform. For the output from a full-wave rectifier:

  13. Peak Inverse Voltage (PIV) • Peak inverse voltage (PIV) – The maximum diode reverse bias produced by a given circuit. For the diode in a full-wave rectifier:

  14. Negative Full-wave Rectifiers • The negative full-wave rectifier converts an ac input to a series of negative pulses.

  15. Full-Wave Bridge Rectifiers • The most commonly used because: • It does not require the use of a center-tapped transformer. • It can be coupled directly to the ac power line. • It produces a higher dc output than a comparable full-wave center-tapped rectifier.

  16. Bridge Rectifier Operation • Conduction alternates between two diode pairs.

  17. Calculating load voltage and current relationships

  18. Working with Rectifiers • Rectifiers are high current circuits, so IFRB can have a significant impact on diode forward voltage (VF ). • Cooling is often used to minimize the effects of high power rectifier reverse current. Components may be cooled using a fan and/or a heat sink. • High transformer tolerances can introduce significant discrepancies between calculated and measured current and voltage values.

  19. Integrated Rectifiers • Integrated Rectifier – A rectifier circuit that is etched on a single piece of silicon (housed in a single case).IC rectifiers are: • Cheaper to produce • Easier to troubleshoot • The diodes in an IC rectifier operate at the same temperature, so they have equal values of forward voltage and leakage current. • IC rectifier cases are designed to be attached easily to a heat sink.

  20. Filters • A filter reduces the voltage and current variations in the output from a rectifier.

  21. Ripple Voltage (Vr) • Ripple voltage (Vr) – The variation in the filter output voltage.

  22. The Basic Capacitive Filter • The capacitor: • charges through the rectifier diode. • discharges through the load.

  23. Surge Current • The high initial current through a power supply. • Surge current lasts only long enough for the filter capacitor to charge.

  24. Limiting Surge Current Surge current can be limited by: • Inserting a current limiting resistor between the filter capacitor and the rectifier. • Using a low-value filter capacitor (which shortens the duration of the surge). • Using an inductive filter.

  25. Filter output voltages

  26. Filtering and Half-wave Rectifier PIV

  27. LC Filters

  28. Voltage Regulators • Voltage regulator – The final circuit in the dc power supply.

  29. Zener Voltage Regulators

  30. Regulator Response to Load Variations

  31. Zener Reduction of Vr

  32. A Basic DC Power Supply

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