Series Voltage Regulator (Series Pass Regulator)

- overview and theory of the series regulator or series pass regulator used in many linear power supply circuits.

The series voltage regulator format or as it is sometimes called the series pass regulator is the most commonly used format for providing the final voltage regulation in a linear voltage regulator circuit.

As the name suggests, the series voltage regulator or series pass voltage regulator operates by using a variable element in series with the load.

In this way a series voltage regulator provides an effective form of voltage regulation within a linear 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.

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 regulator, the series regulator does not draw the full current even when the load does not require any current. As a result the series regulator is considerably more efficient.

Concept of the series voltage regulator / series pass regulator
Concept of the series voltage regulator / series pass regulator

Simple emitter follower voltage regulator

One of the simplest implementations of this concept is to use a single pass transistor in the form of an emitter follower configuration, and a single Zener diode drive 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.

Simple emitter follower series pass regulator
Simple emitter follower series pass regulator

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.

Series pass regulator with feedback

In order to provide improved levels of performance it is possible to add a more sophisticated feedback network into the regulator circuit.

Using feedback within a voltage regulator enables the output to be sampled, and compared with a stable reference voltage. The error is then used to correct the output voltage. In this way, a far higher level of performance can be obtained in terms of the required output voltage as well as ripple and spikes.

 Series pass voltage regulator with feedback
Series pass voltage regulator with feedback

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
Simple two transistor series pass regulator

In this circuit TR1 forms the series pass transistor. The second transistor, TR2 acts as the comparator, 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 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.

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 sample a proportion of the output voltage and compare this to the reference. 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
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.

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. In addition to the drop across the regulator itself, there must be sufficient voltage to run the drive circuitry. In some circuits, a low drop out regulator is important - i.e. where the level of voltage drop available across the series regulator element is limited. This minimum drop out voltage can be important and is often a specified parameter in many integrated regulator chips.

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.

By Ian Poole

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