Linear Power Supply: linear regulated supply

Linear Power Supply Circuits Primer & Tutorial Includes:
Linear power supply     Shunt regulator     Series regulator     Current limiter     78** series regulators & circuits     LM317 voltage regulator & circuits     LDO, low dropout regulators    

See also: Power supply electronics overview     Switch mode power supply     Capacitor smoothing     Over-voltage protection     PSU specs     Digital Power     Power management bus: PMbus     Uninterruptible power supply    

Linear power supplies are widely used because of the advantages they offer in terms of overall performance, and also the technology is very well established because it has been available for very many years.

While linear power supplies may not be as efficient as switch mode power supplies, they offer the best performance and are therefore used in many applications where noise is of great importance.

While the performance of linear power supplies is very good, they are larger and more costly than their switch mode power supply relations, and the performance of the SMPS units is improving all the time.

One major area where linear power supplies are almost always used is for audio visual applications, hi-fi amplifiers and the like. Here the noise and switching spikes from switch mode power supplies can cause issues - that said SMPSs are improving in performance all the time, but linear supplies tend to be used most of the time.

Linear power supply basics

Linear regulated 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.

The voltage is sensed and this signal is fed back, normally into some form of differential amplifier where it is compared with a reference voltage, and resulting signal is used to ensure the output remains on the required voltage.

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. Remote sensing is used where there may be ohmic losses between the power supply and the load. Often laboratory bench supplies have this capability.

Different linear power supplies will have different circuits and incorporate different circuit blocks if additional capabilities are required, but they will always include the basic blocks as well as some optional additional ones.

Power supply input transformer

As many regulated 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.

The transformer is typically a relatively large electronic component, especially if it is used in a higher power linear regulated power supply. The transformer can add significant weight to the power supply, and can also be quite costly, especially for the higher power ones.

Dependent upon the rectifier approach adopted, the transformer may be a single secondary, or it may be centre tapped. Also additional windings may be present if further voltages are required.

For vintage radios and other vintage electronic electronics, multiple secondary windings were commonplace. Normally the main secondary winding was centre tapped to enable full wave rectification with a double diode valve or tube rectifier, and further secondary windings were required for the valve or tube heaters - often 5 volts for the rectifier and then 6.3v for the valves / tubes themselves.

One of the advantages of using a transformer, is that it is possible to have several windings for the different voltages required, should this be needed.

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.

The simplest form of rectifier that could be used in a power supply is a single diode, providing half wave rectification. This approach is not normally used because it is more difficult to satisfactorily smooth the output.

Normally full wave rectification, using both halves of the cycle is used. This provides a waveform that can be more easily smoothed.

There are two main approaches to providing half wave rectification. One is to use a centre tapped transformer and two diodes. The other is to use a single winding on the power supply transformer and to use a bridge rectifier with four diodes. As diodes are very cheap, and the cost of providing a centre tapped transformer is more, the most common approach these days is to use a bridge rectifier.

Even for DC powered regulators, a rectifier may be placed at the input to guard against inverse connection of the supply.

Typically the diodes used these days are semiconductor PN junction diodes as there is a good variety of low cost diodes capable of carrying high current levels and at high reverse voltages.

In some instances it may be possible to use Schottky diodes as these provide a much lower forward voltage drop, but silicon versions of these diodes normally have a low reverse breakdown voltage and a higher leakage, so it is necessary to be aware of this.

The newer silicon carbide Schottky diodes, SiC diodes can have very much higher breakdown voltages and lower leakage levels should this be needed, although the costs are higher.

Power 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.

The smoothing element of the circuit uses a large capacitor. This charges up as the incoming waveform from the rectifier rises to its peak. As the voltage of the rectified waveform falls away, once the voltage is below that of the capacitor, the capacitor starts to supply charge, holding the voltage up, until the next rising waveform from the rectifier.

The smoothing is not perfect, and there will always be some residual ripple, but it enables the huge variations in voltage to be removed.

Linear power supply regulators

Most power supplies these days provide a regulated output. With modern electronics it is quite easy and not too costly to include a linear voltage regulator. This provides a constant voltage output regardless of the load - within the specified limits.

With many electronic components and electronic devices, etc requiring accurately maintained supplies, a regulated power supply is a necessity.

There are two main types of linear power supply:

• Shunt regulator:   The shunt regulator is less widely used as the main element within a linear 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.

  The power supply is designed for a given current, and wit the load applied, the shunt regulator absorbs any current not required by the load so that the output voltage is maintained.

  Read more about . . . . Shunt regulator.
  
  
• Series regulator:   This is the most widely used format for a linear voltage regulator. 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.

  

  In this block diagram, a reference voltage is used to drive the series pass element which may be a bipolar transistor or a FET. The reference may just be a voltage taken from a reference voltage source, e.g. an electronic component such as a Zener diode.

  The more usual approach is to sample the output voltage and fed this into a differential amplifier to compare the output with a reference, and then used this to drive the final pass element circuitry.

  Read more about . . . . Series regulator.
  
  

Both of these types of linear regulator are used in power supplies, and although the series regulator is more widely used, there are instances where the shunt regulator is also used.

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.

Despite the disadvantages, linear regulated power supply technology is still widely used, although it is more widely used where low noise and good regulation are needed. One typical application is for audio amplifiers where the linear supply is able to provide optimum performance for powering all the stages of the amplifier.

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