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Key Considerations for Linear Power Supply Design

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Learn what a linear power supply is and its applications

  • Design tips and requirements for creating a barebones linear power supply design

  • Voltage regulator component function, structure, and operating elements

A simple linear power supply with: transformer, rectifier, smoothing capacitor, and voltage regulator IC

A simple linear power supply with a transformer, rectifier, smoothing capacitor, and voltage regulator IC

Linear power supplies are power supply units that function without any switching or digital components. Linear power supplies (PSUs) are capable of generating multiple output voltages — both very high or very low voltages. They’re relatively larger and heavier and require larger heat sinks. They feature low noise, low ripple, immunity from mains noise, relatively fewer components, and can be easily repaired and designed. 

Let’s explore exactly what goes into linear power supply design. 

Linear Power Supply Features

Linear power supplies generally convert a mains AC voltage to a lower DC voltage for various electronics. AC mains voltage is typically 120V@60Hz or 220V@50Hz, which is then converted to the desired DC-level voltage through the linear power supply.

Linear PSUs can provide fixed arbitrary DC voltages or a variety of voltages for larger circuits (+5, +12, -12 V). They can also create bipolar voltages, such as ±15V used for an op-amp, or dual tracking supplies that are voltage-locked to each other. Common uses of linear power supplies are in communications systems, amplifiers, laboratory test equipment, various control circuitry, and high-power industrial equipment.

Examples of linear power supply output voltages include:

  • +5V logic for microprocessor circuits

  • 0-30V used on a bench test supply

  • ±15V for op-amp circuits

  • +12V LED lighting and other general electronics

  • +14.5V for battery charging

Linear Power Supply Design Considerations

The following are common considerations and requirements for linear PSU design:

  • Range of inputs including voltage, current, and frequency

  • Conversion rate or the turns ratio on the transformer

  • Required common mode rejection ratio (CMRR) for noise rejection between the output voltage and ground

  • Isolation between the source and power supply

  • The difference between peak and average values of the output voltage, known as ripple voltage 

  • Output power requirements

  • Specific load and line regulation requirements

  • Additional features such as fold-back current limiting 

  • Low-dropout features 

Linear Power Supply Design Architecture

Components required for a basic linear power supply architecture

Components required for a basic linear power supply architecture

The main components necessary for linear power supply design include:

  • A connection to the mains power through a switch, fuse, or pilot lamp. Ensure good grounding is used and insulate any exposed wiring with sleeving to protect the supply.

  • A transformer with turns and structure based on the required output voltage. This is used for isolating the rest of the downstream circuitry from being mains-referenced. The transformer might have multiple taps to allow for different mains input voltages and secondary taps for the desired output voltage. Some transformers may also have a copper foil shield between primary and secondary taps to reduce capacitive coupling to high-frequency mains noise. 

  • A rectifier that converts AC signal to DC. Generally, full-wave bridge rectifiers are most suitable. However, ensure that your choice of diodes is well-calculated. Utilizing cheap or small diodes may result in failure from too much current or over-voltage spikes. Selecting diodes with high peak inverse voltage may help in addition to using longer leads for better heat dissipation. Small capacitors may also be placed in parallel for faster recovery.

  • A smoothing capacitor is used to reduce the amount of AC ripple that rides on the rectifier-output quasi-DC voltage. Capacitors with low equivalent series resistance (ESR), such as tantalum-based caps, may handle ripple better.

  • A voltage regulator is used for stepping down and providing a constant DC voltage to the load. It can be a series, shunt, simple, or complex regulator. This component is complex and discussed further in the section below. 

Voltage Regulator Design

Voltage regulators can come in all shapes in sizes, and ultimately serve in taking a DC voltage and stepping it down. Voltage regulators use feedback to keep the output voltage constant, independent of changes in the input voltage or load. If the load begins to draw a greater amount of current, the linear regulator will detect this and adjust to provide greater power. It accomplishes this through a feedback mechanism. A fraction of the output voltage is sampled and compared to a fixed reference. The difference is amplified and used to control the regulation element (usually a transistor or configuration of multiple transistors) to reduce the voltage error.

For example, in the case that the load pulls more current and the output is too low, this difference is sampled, amplified, and then used to control the regulation transistor to allow more current through, thus raising the voltage to the desired value.

In the case of fold-back current limiting, low-dropout features, line and load regulation requirements, or other power-supply-specific requirements, your voltage regulator design is where this will be implemented. Linear power supply designs are inefficient (when compared to switching PSUs); any extra power is dissipated as heat through the control element. For this reason, having an appropriately sized heat sink attached to your voltage regulator, and specifically, the control transistor, is critical.

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