EEAC-105-Learning-Material-1

Overview of DC Power Supply Units (PSU)

• Definition and Purpose: A DC Power Supply Unit (PSU) derives power from the AC mains (line) supply to provide energy for electronic circuits. It is a critical component; failures in the supply affect all other connected circuits.

• Primary Tasks of a PSU:

  • Voltage Level Adjustment: It changes (usually reduces) the supply level to a value appropriate for the load circuit.

  • AC to DC Conversion: It produces a DC supply from a pure AC wave using rectifier diodes.

  • AC Suppression: It prevents alternating current from appearing at the output.

  • Regulation: It ensures the output voltage stays constant regardless of fluctuations in the input AC supply voltage, the load current drawn, or temperature changes.

• System Reliability: Modern PSUs are highly reliable but remain susceptible to failure because they handle higher voltages and currents than the rest of the system. They often incorporate feedback systems for stability and automatic safety circuits to prevent over-voltage or over-current conditions.

The Four Main Stages of a Power Supply

• Stage 1: Transformer: Provides electrical isolation and steps the voltage up or down.

• Stage 2: Rectifier: Converts the AC signal into a pulsating DC signal.

• Stage 3: Filter (Smoothing): Reduces the AC ripple to provide a smoother DC level.

• Stage 4: Regulator: Stabilizes the output voltage against load and line variations.

Transformer and Rectification Details

• The Transformer Stage:

  • The primary winding connects to the mains supply.

  • The secondary winding is electromagnetically coupled but electrically isolated from the primary.

  • The transformer must be sized correctly to supply the required current. If too small, it will fail to maintain output voltage, and losses will increase dramatically at full load.

  • Safety features often include thermal fuses and strict electrical isolation.

• Half-Wave Rectification:

  • Utilizes a single silicon diode to block one half-cycle of the AC input.

  • Average DC Output Formula: $V_{pk} \times 0.318$

  • Diode Voltage Drop: The output is reduced by approximately $0.6\,V$ due to the forward voltage drop of the silicon rectifier. This is significant in low-voltage supplies.

  • Characteristics: Cheap, but inefficient. It leaves large gaps between pulses (50Hz or 60Hz), making ripple removal difficult.

• Full-Wave Rectification (Center-Tapped):

  • Uses a transformer with a center-tapped secondary producing two anti-phase outputs and two diodes.

  • Diodes conduct on alternate half-cycles, resulting in two current pulses per cycle.

  • Frequency: The output frequency is twice the input frequency.

  • Average DC Output Formula: $V_{pk} \times 0.637$

  • Characteristics: More efficient than half-wave; easier to filter due to higher frequency. Requires a more expensive center-tapped transformer.

• Bridge Rectifier:

  • Uses four diodes in a bridge arrangement to provide full-wave rectification without a center-tapped transformer.

  • Advantages: Diodes effectively operate in series pairs, requiring only half the reverse breakdown voltage capability compared to other designs.

  • Operation: Opposite pairs of diodes conduct on each half-cycle, maintaining a constant polarity across the load.

Filter Circuits and AC Ripple

• Reservoir Capacitor:

  • A large electrolytic capacitor (hundreds or thousands of $\mu F$) acts as temporary storage.

  • It charges to the peak value ($V_{pk}$) when the rectifier conducts and discharges into the load when the rectifier is non-conducting.

  • Adding this capacitor increases the DC level from $0.637\,V_{pk}$ (full wave) or $0.317\,V_{pk}$ (half-wave) to nearly the peak value of the input.

• AC Ripple:

  • Ripple is the remaining peak-to-peak AC amplitude in the DC output. Typically, it should be no more than 10% of the DC output voltage.

  • Charge Formula: $Q = I \times t$

  • Design Trade-off: Larger capacitors reduce ripple but require shorter, much higher current pulses from the transformer and diodes, risking damage if the components are not rated for such peaks.

• Low Pass Filters (LPF):

  • Used after the reservoir capacitor to remove remaining ripple.

  • RC Filter: Uses a resistor ($R$) and a capacitor ($C$).