What Are Power Supplies?

Power supplies are electrical devices that convert power from a source into the form required by a load, providing controlled voltage and current at levels appropriate for the connected equipment. The conversion may involve changing AC to DC, stepping a voltage up or down, isolating the load from the source, or regulating the output against changes in input voltage and load current. Power supplies range from milliwatt-class units powering microcontrollers to megawatt installations supplying industrial electrochemical processes, and their design determines the efficiency, noise, and reliability of virtually every electronic system.

The field draws on circuit theory, magnetics, semiconductor device physics, control systems theory, and thermal engineering. The principal distinction in design philosophy is between linear regulation, which dissipates excess voltage as heat in a pass transistor, and switching regulation, which rapidly switches an energy storage element to transfer power in discrete packets at high efficiency. A third category, the pulsed power supply, accumulates energy over a relatively long interval and releases it in an extremely brief, high-power pulse for specialized scientific and industrial applications.

Linear and Switching Architectures

A linear power supply steps down the input AC voltage with a mains-frequency transformer, rectifies and filters the output, and then regulates the DC level using a series-pass transistor operated in its linear region. The transistor continuously dissipates the voltage difference between the rectified input and the regulated output, making efficiency dependent on that voltage margin. Linear supplies offer low output noise, typically below a few millivolts of ripple, and are preferred for audio equipment, laboratory instruments, and RF circuits where electromagnetic interference from switching transients would degrade performance.

A switched-mode power supply (SMPS) rectifies the mains input directly or steps it down through a high-frequency transformer, then uses pulse-width modulation to control one or more power semiconductor switches, transferring energy through an inductor or transformer at frequencies from tens of kilohertz to several megahertz. Because the switch operates in either the fully-on or fully-off state, conduction losses are low and efficiencies routinely exceed 90%. The high operating frequency allows the transformer and filter capacitors to be much smaller and lighter than those of a comparable linear supply. According to Analog Devices application note AN-140, the efficiency advantage of switching over linear regulation is most pronounced when the input-to-output voltage ratio is large.

DC Power Supplies and Uninterruptible Power Systems

DC power supplies, both bench-type laboratory instruments and production units embedded in equipment, provide one or more regulated DC rails to power digital logic, analog signal chains, and electromechanical actuators. Thyristor power supplies, used in electroplating, electrolysis, and large DC motor drives, convert AC mains power to high-current DC at controllable voltage levels using phase-controlled thyristor bridges. Uninterruptible power systems (UPS) add a battery or flywheel energy reserve to the AC-DC conversion chain, providing seamless transfer to stored energy during mains outages. Online double-conversion UPS units rectify the incoming AC to DC and then re-invert it, placing the battery permanently in the power path and eliminating any transfer time. The IEEE standard for UPS performance covers measurement methods, efficiency ratings, and output quality requirements for these systems.

Pulsed Power Systems

Pulsed power systems store energy in capacitor banks over seconds to minutes and discharge it into a load in microseconds to nanoseconds, delivering peak powers far exceeding what could be drawn continuously from the mains. The Marx generator, which charges multiple capacitors in parallel and then connects them in series to stack their voltages, is the classical architecture. Modern impedance-matched Marx generators at Lawrence Livermore National Laboratory have demonstrated outputs of 60 gigawatts in 100-nanosecond pulses while storing six times less energy than equivalent conventional designs.

Applications

Power supplies have applications across virtually every sector that uses electricity, including:

• Computing and server infrastructure, where switch-mode power supplies convert AC mains to the 12 V, 5 V, and 3.3 V rails required by processors and memory
• Medical equipment including MRI machines, diagnostic imaging, and ventilators requiring tightly regulated and electrically isolated supplies
• Industrial electrochemical processes such as electroplating, electrolysis, and aluminum smelting using high-current thyristor power supplies
• Telecommunications base stations and data centers using 48 V DC power distribution architectures with distributed point-of-load converters
• High-energy physics accelerators and plasma research facilities using pulsed power supplies to generate kiloampere, kilovolt pulses