Series Voltage Regulator: series pass regulator

The series regulator or series pass regulator is the most widely used form of voltage regulator used in linear power supplies.

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The series voltage regulator or as it is sometimes called the series pass regulator is the most commonly used approach for providing the final voltage regulation in a linear regulated power supply.

The series linear regulator provides a high level of performance, especially when low noise, ripple and transients are required in the regulated output.

There is a good variety of circuits using discrete electronics components that provide linear regulation with a series pass element, and in addition to this, virtually all linear regulator ICs use this approach.

This means that there are many options for series voltage regulators that are open when undertaking the electronic circuit design of a power supply.

Series voltage regulator basics

The series voltage regulator or series pass voltage regulator uses a variable element placed in series with the load. By changing the resistance of the series element, the voltage dropped across it can be varied to ensure that the voltage across the load remains constant.

Block diagram of a series voltage regulator - this basic concept is used in most linear power supplies
Block diagram of a series voltage regulator

The advantage of the series voltage regulator is that the amount of current drawn is effectively that used by the load, although some will be consumed by any circuitry associated with the regulator. Unlike the shunt voltage regulator, the series regulator does not draw the full current even when the load does not require any current. As a result the series voltage regulator is considerably more efficient.

Instead of drawing the current not required by the load to maintain the voltage, it drops the voltage difference between the input voltage and the required stabilised voltage.

To maintain a sufficient level of regulation and rejection of noise and transients that may be on the incoming voltage, series linear voltage regulators need to drop a significant voltage. Many high quality, low noise and ripple voltage regulators need several volts across the series regulator element. This means that significant levels of power many be dissipated in this component, and good heat sink and heat removal capability is required for the series pass regulator device and also the power supply as a whole.

Even though a series regulator is considerably more efficient than a shunt regulator, it is considerably less efficient than a switch mode power supply. The efficiency of a series voltage regulator and any linear power supplies using them will depend on the load, etc, but often efficiency levels of less than 50% are achieved, whereas switch mode power supplies can achieve levels greater than 90%.

Series voltage regulators have relatively low levels of efficiency when compared to a switch mode power supply, but they have the advantages of simplicity and also their output is free of the switching spikes seen on some switch mode supplies, although SMPSs are improving and the performance of many is exceptionally good nowadays.

Simple emitter follower voltage regulator

The electronic circuit design for a simple transistor emitter follower voltage regulator is very straightforward. This circuit is not widely used on its own in a linear power supply, but may be used within other equipment to provide a step down voltage, etc from a higher voltage rail.

Basic series regulator using a zener diode and emitter follower
Basic series regulator using a zener diode and emitter follower

The circuit uses a single pass transistor in the form of an emitter follower configuration, and a single Zener diode or other voltage regulator diode driven by a resistor from the unregulated supply.

This provides a simple form of feedback system to ensure the Zener voltage is maintained at the output, albeit with a voltage reduction equal to the base emitter junction voltage - 0.6 volts for a silicon transistor.

It is a simple matter to design a series pass voltage regulator circuit like this. Knowing the maximum current required by the load, it is possible to calculate the maximum emitter current. This is achieved by dividing the load current, i.e. transistor emitter current by the Β or hfe of the transistor.

The Zener diode will generally need a minimum of around 10mA for a small Zener to keep its regulated voltage. The resistor should then be calculated to provide the base drive current and the minimum Zener current from a knowledge of the unregulated voltage, Zener voltage and the current required. [ (Unregulated voltage - Zener voltage ) / current ]. A small margin should be added to the current to ensure that there is sufficient room for margin when the load, and hence the transistor base is taking the full current.

The power dissipation capacity for the Zener diode should be calculated for the case when the load current, and hence the base current is zero. In this case the Zener diode will need to take the full current passed by the series resistor.

Sometimes a capacitor may be placed across the Zener diode or voltage reference diode to help remove noise and any voltage transients that may occur.

Output sampling

The simple emitter follower series voltage regulator circuit directly compared the output with the voltage reference. In this way the output voltage was equal to that of the reference, neglecting the base emitter voltage drop.

However it is possible to improve the performance of the voltage regulator by sampling a proportion of the output voltage and comparing it to the reference. A differential amplifier like an operational amplifier can be used for this function. If this is done, then the output voltage becomes greater than the reference voltage as the negative feedback in the circuit fights to keep the two compared voltages the same.

If for example the reference voltage is 5 volts and the sampling or potential divider provides 50% of the output voltage, then the output voltage will be maintained at 10 volts.

 Series pass voltage regulator sampled output - a differential amplifier compares the reference and sampled output
Series pass voltage regulator with sampled output

The potential division or sampling can be made variable, and in this way, the output voltage can be adjusted to the required value. Normally this method is only used for small adjustments as the minimum output level obtained by this method is an output equal to the reference voltage.

It should be remembered that using a potential divider has the effect of reducing the feedback loop gain. This has the effect of reducing the loop gain and thereby reducing the regulation performance. Normally there is sufficient loop gain for this not to be a major problem except when only a very small proportion of the output is sampled.

Care should also be taken not to increase the voltage of the output to a point where the regulator does not have sufficient drop across it to regulate the output voltage sufficiently.

Series pass regulator with feedback

In order to provide improved levels of performance over that provided by a simple emitter follower, it is possible to add a more sophisticated feedback network into the voltage regulator circuit. This is achieved by sampling the output, comparing it to a reference and then using some form of differential amplifier to feedback the difference to correct the errors.

It is possible to use a simple two transistor circuit for a series pass regulator with voltage sensing and feedback. Although it is quite straightforward to use an operational amplifier, which will provide higher levels of feedback, and hence better regulation, this two transistor circuit illustrates the principles well.

Simple two transistor series pass regulator circuit
Simple two transistor series pass regulator circuit

In this circuit TR1 forms the series pass transistor. The second transistor, TR2 acts as the differential amplifier, feeding the error voltage between the reference diode and the sensed output voltage which is a proportion of the output voltage as set by the potentiometer. The resistor, R1 provides the current for the collector of TR2 and the voltage reference diode ZD1.

Voltage reference

Any linear voltage regulator can only be as good as the voltage reference that is used as the basis of the comparison within the system. While a battery could in theory be used, this is not satisfactory for most applications. Instead Zener diode based references are almost universally used.

Integrated circuit regulators and references use sophisticated on-chip combinations of transistors and resistors to obtain temperature compensated and precise voltage reference sources.

The voltage reference must be driven from the unregulated supply. It cannot be taken from the regulated output as there are start-up issues. At start-up there is no output and therefore the reference output will be zero and this will be maintained until the reference starts-up.

Reference source for series pass voltage regulator
Simplified reference source for series pass voltage regulator

Often the output from the reference source is fed via a potential divider. Not only does this reduce the output voltage which is normally very useful, but it also allows a capacitor to be added to the output to help remove any ripple or noise that may be present. The reduced voltage is also useful because the minimum voltage output is governed by the reference voltage.

Low drop out series voltage regulators

One of the considerations of any regulator is the voltage that must be placed across the series pass element. Often for linear regulators a significant drop across the series pass element is required to achieve the best regulation and noise rejection. For example a linear regulator with an output of 12 volts may be designed to have an input voltage of 18 volts or more.

With any linear regulator there is a minimum voltage that is required across the series element before the regulator "drops out." This drop out voltage may be seen in many linear regulator integrated circuits.

In some circuits, having a low drop out regulator is important. If the input voltage available is not particularly high, having a low drop out linear regulator can be important. It will need to regulate well, despite having a limited voltage across it.

While the circuits shown here are simple transistor circuits, the same principles are used in larger circuits and also within integrated circuits. The same series pass regulator concepts as well as the reference diode circuits, sampling and other areas all use the same elements.

The concepts used here are used within virtually a linear regulated power supplies which can offer very good levels of performance. The linear regulated power supplies are larger and heavier than switch mode power supplies, however they have a name for low noise and good regulation on the output, free from the spikes that some switch mode power supplies have.

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