Shunt Voltage Regulator: shunt regulator

Shunt voltage regulators are used in many areas - they are not the most efficient voltage regulators but they are often very convenient.

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The shunt regulator or shunt voltage regulator is a form of voltage regulator where the regulating element shunts the current to ground.

The shunt regulator operates by maintaining a constant voltage across its terminals and it takes up the surplus current to maintain the voltage across the load.

One of the most common examples of the shunt regulator is the simple Zener diode circuit where the Zener diode acts as the shunt element.

As such the shunt voltage regulator is an essential element within linear power supply technology.

Shunt voltage regulator basics

The basic operation of a shunt voltage regulator can be seen from the diagram. Essentially the load is operated with a resistor in series with the voltage source and the shunt regulator then in parallel with the load.

In order to keep the voltage across the load constant, a level of current must be drawn through the series resistor to maintain the required voltage across the load. The load will take some and the remaining current is drawn by the shunt voltage regulator.

The circuit is designed so that at maximum load current the shunt regulator draws virtually no current and at minimum load current, the shunt voltage regulator passes the full current.

As a result, it can be seen that shunt regulators are inefficient because maximum current is drawn from the source regardless of the load current, i.e. even when there is no load current.

Zener diode shunt regulator

One of the most common and simple forms of shunt regulator is the simple Zener diode regulator circuit shown below.

The operation of this shunt regulator circuit is very straightforward. Once over its small minimum current, the Zener diode maintains an almost constant voltage across its terminals.

In this circuit, the series resistor drops the voltage from the source to the Zener diode and load. As the Zener diode maintains its voltage, any variations in load current do not affect the voltage across the Zener diode.

The Zener diode takes up the current variations required to ensure the correct drop across the series resistor occurs to ensure the required output voltage.

As the Zener or avalanche effect of the diode ensues that an almsot constant voltage is maintained across the diode, it "shunts" sufficient current to maintain the voltage across its terminals and hence the load.

Simple Zener diode shunt regulator
Zener diode shunt regulator circuit

In this shunt voltage regulator circuit, the Zener diode must be capable to dissipating the power from the maximum amount of current it is likely to handle. This is most likely to be a little more than the maximum current supplied to the load as the Zener diode will need to pass all the current when load current is zero.

Thus the total maximum current that will be passed by the diode is the load current plus an allowance for current to maintain the reference voltage when the load is taking its maximum current.

It should also be noted that for the shunt regulator circuit, the series resistance is comprised of the series resistor value, plus any source resistance. In most cases the value of the series resistor will dominate and the source resistance can be ignored, but this may not always be the case.

Zener diode shunt regulator circuit design example

In this example a Zener diode / voltage reference diode circuit design is used to supply a regulated 5.1 Volt rail consuming 2 mA, from an input voltage supply of 12 volts.

The following easy steps can be used to calculate the resistor required:

1) Calculate the difference in voltage across the series resistor

12 volts - 5.1 volts = 6.9 volts

2) Determine the resistor current. Choose this to be 15 mA. This will allow sufficient margin above the minimum Zener diode current for some variation in the load current.

3) Check the Zener diode power dissipation. At a current of 15 mA and a voltage across the power dissipation is:

15 m A     5.1 V = 76.5 m W

This is nicely within the maximum limit for the diode

4) Determine the current through the series resistor. This is 15 mA for the Zener diode plus 2 mA for the load, i.e. 17 mA.

5) Determine the value of the series resistor. Using Ohms law this can be calculated from the voltage drop across it and the total current through it:

6.9 V 17 m A = 0.405 k Ω

The nearest value is 390 ohms

6) Determine the wattage of the series resistor. This can be determined using the value for the current through the resistor and the voltage across it calculated earlier:

V   I = 6.9 V   x   17 m A = 117 m W

The resistor needs to be able to dissipate this level of heat. A quarter watt resistor should be adequate for this.

This simple Zener diode circuit is widely used as an easy method of providing a voltage reference.

Efficiency of Zener diode shunt regulator

The efficiency of a shunt regulator like the Zener diode or reference diode circuit above is very low.

Firstly the series resistor needs to drop the voltage between the incoming power rail voltage to that of the Zener or reference diode. This needs to be calculated for the expected current throught he resistor.

Secondly the current supplied must be sufficient to keep the Zener diode turned on when the load is taking its maximum current - in other words the current drawn must be the current fort he reference diode and that for the load.

If the load is taking no current, this needs to be absorbed by the voltage reference or Zener diode in order that the output voltage remains constant.

In other words the circuit always draws its maximu current and this means that it is very inefficient.

Shunt regulator with feedback loop

The basic shunt voltage regulator above does not have any feedback, i.e. it runs in an open loop manner.

As imagined, the performance of this form of shunt regulator is sufficient for many applications, but much higher levels of performance can be achieved by providing feedback based on the output voltage of the shunt voltage regulator and feeding this back into the system to ensure that the required output voltage is accurately maintained.

Shunt regulator with feedback
Block diagram of shunt voltage regulator with feedback

Using a shunt voltage regulator with feedback as shown above, the output voltage is sensed and the voltage compared to a reference. The level of the shunt current is then altered to return the output voltage to the required level.

Shunt voltage regulators are not particularly effective or efficient in high current situations,. The simple Zener diode voltage regulator, when used as a low current voltage reference is widely used, and its inefficiency can be tolerated in view of the low current. Often shunt regulators are used as voltage reference sources from which much higher current series regulators are driven.

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