Linear Power Supply Basics Tutorial

- summary or tutorial about the basics of linear power supplies, their design, operation and advantages and disadvantages.

Linear power supplies are widely used because of the advantages they offer in terms of overall performance.

Linear power supplies are often used in exacting situations where the regulation and removal of noise is paramount.

While linear power supplies may not be as efficient as other types of power supply technology, they offer the best performance and are therefore used in many applications where noise is of great importance. Often audio amplifiers and many other items of electronic equipment use linear power supplies to obtain the best performance.

Linear power supply basics

Linear power supplies gain their name from the fact that they use linear, i.e. non-switching techniques to regulate the voltage output from the power supply. The term linear power supply implies that the power supply is regulated to provide the correct voltage at the output. Sometimes the sensing of the voltage may be accomplished at the output terminals, or on some occasions it may be achieved directly at the load.

In terms of the overall make-up of a linear power supply, it can be split into several blocks as detailed below.

Block diagram showing the architecture of a linear power supply with blocks for transformer, rectifier, smoothing adn the regulator
Block diagram of linear power supply architecture

The main elements of the linear power supply are:

  • Input transformer:   As many power supplies take their source power from an AC mains input, it is common for linear power supplies to have a step down or occasionally a step up transformer. This also serves to isolate the power supply from the mains input for safety.
  • Rectifier:   As the input from an AC supply is alternating, this needs to be converted to a DC format. Various forms of rectifier circuit are available.

    Note on Diode Rectifiers:

    Diode rectifiers are an essential element within many circuits including power supplies and RF detectors. They convert the AC signal into DC. Various forms are available from single diode half wave rectifiers to multiple diode full-wave rectifiers. The type chose will depend upon the application and its requirements.

    Click for more information on Diode Rectifiers

    Even for DC powered regulators, a rectifier may be placed at the input to guard against inverse connection of the supply.
  • Smoothing:   Once rectified from an AC signal, the DC needs to be smoothed to remove the varying voltage level. Large reservoir capacitors are used for this.

    Note on PSU Smoothing Circuits:

    In order to remove the undulations on the voltage produced after a diode circuit has rectified an AC or RF signal, it is necessary to smooth the voltage. This is generally accomplished using a capacitor reservoir capacitor.

    Click for more information on PSU Smoothing Circuits

  • Linear regulator:   Once a smoothed supply is available, this can then be applied to the linear regulator. This will provide a properly regulated output.

Linear power supply regulators

There are two main types of linear power supply:

  • Series regulator:   This is the most widely used format for a linear power supply. As the name implies a series element is placed in the circuit, and its resistance varied via the control electronics to ensure that the correct output voltage is generated for the current taken. Read more about the Series regulator
  • Shunt regulator:   The shunt regulator is less widely used as the main element within a voltage regulator. For this form of linear power supply, a variable element is placed across the load. There is a source resistor placed in series with the input, and the shunt regulator is varied to ensure that the voltage across the load remains constant. Read more about the Shunt regulator

Both of these types linear regulator used in power supplies has its uses and can be used in different situations.

Linear power supply advantages / disadvantages

The use of any technology is often a careful balance of several advantages and disadvantages. This is true for linear power supplies which offer some distinct advantages, but also have their drawbacks.

Linear PSU advantages

  • Low noise:   The use of the linear technology without any switching element means that noise is kept to a minimum and the annoying spikes found in switching power supplies are now found.
  • Established technology:   Linear power supplies have been in widespread use for many years and their technology is well established and understood.

Linear PSU disadvantages

  • Efficiency:   In view of the fact that a linear power supply uses linear technology, it is not particularly efficient. Efficiencies of around 50% are not uncommon, and under some conditions they may offer much lower levels.
  • Size:   The use of linear technology means that the size of a linear power supply tends to be larger than other forms of power supply.
  • Heat dissipation:   The use of a series or parallel (less common) regulating element means that significant amounts of heat are dissipated and this needs to be removed.

Despite the disadvantages, linear power supply technology is still widely used, although it is more widely used where low noise and good regulation re needed. One typical application is for audio amplifiers.

By Ian Poole

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