Understanding Alternating Current (AC) in Power Supply Design
To design a power supply using alternating current (AC), the input current must first be determined. Most grid power sources are AC, which typically exhibits a sinusoidal waveform.
Figure 1: AC waveform and key parameters
When working with AC, the following parameters must be considered:
Peak Voltage/Current: The maximum amplitude of the waveform.
Frequency: The number of waveform cycles per second. The time for one complete cycle is called the period.
Average Voltage/Current: The mean value of all voltage points over one cycle. In a pure AC waveform without DC offset,this value is zero because the positive and negative halves cancel each other.
RMS (Root Mean Square) Voltage/Current: The root mean square of instantaneous voltage values over one cycle. For a pure AC sine wave, this can be calculated using Formula.
RMS is equivalent to the DC power required to produce the same heating effect. This definition, though complex, provides the "effective value" of AC and is widely used in electrical engineering, often denoted as VAC.
Phase: The angular difference between two waveforms. A full AC cycle spans 360°, with peaks at 90° (positive) and 270° (negative), and zero crossings at 180° and 360°.
If two waveforms are plotted together and one reaches its positive peak while the other reaches its negative peak (e.g., 90° vs. 270°), they have a 180° phase difference and are termed out of phase. A 0° phase difference indicates waveforms are in phase.
AC in Power Transmission
AC is the primary method for transmitting electricity from power plants to end users. This is because AC allows efficient voltage transformation during transmission.
Power plants generate electricity at ~40,000 V (40 kV), which is then stepped up to 150–800kV for long-distance transmission to minimize resistive losses.
Near the destination, the voltage is reduced to 4–35kV, and finally to 120V or 240V for residential use.
DC cannot easily achieve such voltage transformations, as linear transformers (which rely on voltage fluctuations) only work with AC.
AC/DC Linear Power Supplies
An AC/DC linear power supply follows these steps:
Figure 2: Functional block diagram
Step-down transformer: Reduces AC input voltage.
Rectification: Converts AC to pulsating DC.
Filtering: Smoothens the DC waveform.
Advantages: Simple design, matured over decades with improved efficiency and power range.
Limitations:
Size: Low-frequency operation (e.g., 50 Hz) requires large, heavy transformers to transfer power between primary and secondary coils.
Voltage regulation: Linear regulators dissipate excess energy as heat. At high power, this demands massive heat sinks, making thermal management impractical.
AC/DC Switching-Mode Power Supplies
To address limitations of linear designs, switching-mode power supplies were developed :
Input rectification/filtering: AC is directly converted to DC.
Chopper circuit: Converts DC into high-frequency pulses.
Step-down transformer: Uses a compact, high-frequency transformer.
Output rectification/filtering: Produces stable DC.
Advantages:
Compact size: High-frequency operation allows smaller transformers.
High efficiency: Minimal energy loss compared to linear designs.
Thermal management: Reduced heat dissipation eliminates bulky heat sinks.
Challenges:
Noise: High-frequency switching introduces electromagnetic interference (EMI), requiring advanced filtering.
Complexity: Control circuits are more sophisticated but manageable with modern integrated components.
Summary
AC/DC power supplies are ubiquitous, converting AC to stable DC for electronic devices. Key points:
AC vs. DC transmission: AC dominates grids due to efficient voltage transformation.
Single-phase vs. three-phase systems:
Single-phase: Simpler, sufficient for households.
Three-phase: Delivers stable, high power for industrial applications.
Design evolution:
Linear supplies: Limited by size and efficiency but simple.
Switching-mode supplies: Compact, efficient, and dominant in modern applications.
