Voltage Regulator Circuits

- an overview of the basics of the linear and switching voltage regulator circuits used in electronics power supplies.


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Voltage regulators are widely used in electronics power supply circuits. They provide very high degrees of regulation and low levels of ripple, although their levels of efficiency are much lower than another popular form of regulator called the switch mode regulator. However linear regulators are still used in large quantities because of their relative simplicity and high levels of performance.

It is possible to make voltage regulator circuits from both discrete components as well as being able to use IC regulators. The IC regulators enable very high levels of performance to be achieved, often using comparatively few components, but often for many projects it is possible to use a few available components to make a perfectly adequate voltage regulator circuit.

Basic concept behind voltage regulator circuits

Although there are many different voltage regulator circuits and integrated circuit regulators, the basic concepts for these circuits fall into one of two basic categories:

  • Series regulator circuit
  • Parallel or shunt regulator circuit.

All voltage regulator circuits fall into one of these categories, although of the two, the most common type seen for full voltage regulator circuits is the series regulator.

In addition to voltage regulators being categorised as series and shunt regulators, it is also possible to categorise them into two other categories depending upon the mode of operation:

  • Linear voltage regulators.
  • Switching voltage regulators.

Both linear and switching regulator circuits are widely used. Each type has its own advantages and disadvantages and therefore a choice of the type of regulator needs to be made dependent upon the application envisaged.

Series voltage regulator circuit

The series voltage regulator circuits operate by using a series control element such as a bipolar transistor or a field effect transistor. The basis of operation of the circuit hinges around controlling the conductance of this series element by a control voltage. If the voltage of the output is tending to rise, then this will be detected and the control voltage will be adjusted to reduce the conductance of the series element which will cause the voltage across the series element to rise. As the series element and the load form a potential divider circuit, any voltage increase across the series control element will cause the voltage across the load to fall.

Similarly if the voltage across the load is tending to fall too low, then this will be detected, the control voltage for the series element will then cause the conductance of the series element to rise and the voltage across the load will be maintained.

This is a typical form of negative feedback system. The control voltage must have a reference against which the output can be compared. This is often provided by a voltage reference circuit based around a zener diode. The output voltage from the regulator is taken, often through a potential divider, and compared with the reference voltage, and the error voltage is fed back as the control voltage to alter the conductance of the series control, element.

It is possible to vary the output voltage by changing the amount by which the output is divided down. By placing a variable resistor in the potential divider, the voltage that is compared with the reference voltage can be changed. This in turn will change the output voltage from the voltage regulator circuit.

Shunt voltage regulator circuit

As the name implies, a shunt voltage regulator operates in parallel with the load, rather than being in series with it. Using a form of constant current device, which could be as simple as a resistor, it operates in parallel with the load, shunting, or absorbing the current so that the voltage across the load remains the same.

The simplest forms of shunt regulators utilise constant voltage devices such as zener diodes. These circuits use a series resistor to provide a current limiting action, and zener diode is placed between the resistor and ground in parallel with the load. As the zener diode maintains a constant voltage, and variations in current by the load will not cause any (significant) voltage variations because the diode will maintain a constant voltage, taking up any current variations. Naturally there are other, more sophisticated forms of shunt regulator, but the zener diode version is the simplest and most straightforward.

Linear voltage regulator

A linear voltage regulator circuit is one in which the conductance of the series regulator element varies in a linear fashion to ensure that the required voltage is maintained at the output. In this way the output voltage is maintained as exactly as possible and the cleanest output is obtained.

Although a linear voltage regulator circuit offers very high levels of performance in terms of noise, ripple and regulation, this type of circuit is not efficient. The series regulator element requires a significant voltage to be dropped across it for it to be able to maintain the high levels of noise and ripple rejection required. The series regulator element must be able to dissipate significant levels of power dependent upon the output required. This means that these power supplies can be large and heavy.

Switching voltage regulator

Unlike linear regulators where the series element varies in a linear fashion, series element in switching regulators only have two states - on and off. The regulator works by charging up a large capacitor on the output. As the voltage falls as the charge is used powering the load, the series regulator switches on. Once it ahs reached the required voltage it switches off again. By having a sufficiently large reservoir capacitor on the output the switching spikes are removed in the main.

The advantage of switching regulators is their very much greater level of efficiency they are able to offer. The series element dissipates very little power as it is either on or off. As a result these power supplies are not only very efficient, but they can also be made very much smaller. The problem is that there are always some switching spikes present on the output, and the level of overall noise on the output is greater than those offered by linear regulators. However they are quite adequate for many applications and as a result they are very widely used.

Summary

Linear voltage regulators are very widely used within electronics circuits. With circuits operating at high speeds and requiring the supply rails to be accurately maintained, voltage regulator circuits are used to provide the supplies for most circuits.

By Ian Poole



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