Linear Regulator

What is Voltage Regulator – Explained in Details

It is interesting to discuss what is voltage regulator all about. It is a common circuit that is widely used in electronics to provide a constant voltage level. A voltage regulator is an electronic circuit that can maintain a voltage level despite of several factors like increase in the input voltage, tolerances and temperature caused variations. For example, a circuit is expected to have a 12V output, so put a voltage regulator to maintain this 12V output all the times. There are two main categories of a voltage regulator. First is linear and second is switching. Linear is can be series or shunt types. There are several classes of switching regulators, but the simplest forms are buck and boost and the buck-boost that is basically the combination of the two.

When to Use a Voltage Regulator

Any applications that needs a constant voltage level will need a voltage regulator. Constant voltage level simply means the level is the same anytime. In short word, a straight-line voltage level. Some circuits that needs voltage regulator are power supplies, reference circuits, and any voltage sensitive integrated circuits or sub circuits.

Types of Voltage Regulator

Linear Voltage Regulator

A linear voltage regulator is the easiest to deal with. It is very easy to create and do not need much technical knowledge to understand its behavior. It has two variations: series and shunt.

(1) Series Linear Regulator

A series linear regulator is a type wherein it is installed in series to the output or the circuit that uses the regulator or simply the load.

Series linear regulator will maintain a voltage regulation by absorbing the excess power made from the difference of the input and output voltage levels.

Supposing a voltage regulator has a fixed voltage regulation of 5V, and the input voltage level is 8V. In order to maintain the 5V regulation, the internal circuit of the voltage regulator will manage the excess 3V that is the difference between the 8V input and 5V output.

Below is a popular series linear regulator from Texas Instrument. The images are taken from Texas Instruments website. Click below link if you want to visit Texas Instruments website.

http://www.ti.com/document-viewer/LM317-N/datasheet/abstract#SNVS7742093

LM317 is a positive variable output series linear regulator. In the above circuit, the input is 28V and the output is can be adjusted to a minimum level of 1.25V per LM317 datasheet.

Power Dissipation of a Series Linear Voltage Regulator

Setting the output voltage is not only the thing to consider in using a voltage regulator. Power dissipation is the next very important thing to look for. A series linear voltage regulator circuit is easy to make. But the power dissipation must be taken cared with. Power dissipation is the amount of power that the regulator absorbs. This directly related to the heat or how hot the regulator is. What causes this power dissipation? This is the result of the voltage difference between the input and output times the load current.

For example, in above figure, if the output voltage is set to 10V, the difference in voltage is 18V (28V-10V). Supposing the load current is 1A, this means a power dissipation of

Power Dissipation = (Vin – Vout) X Load Current = (28V – 10V) X 1A = 18 Watts

18 watts is a huge power dissipation and result to a hotter case temperature. It needs to be managed like by putting a heatsink to the regulator or a cooling fan and so on.

(2) Shunt Linear Regulator

Shunt linear regulator differs to series linear regulator in a way it is connected to the load or the circuit that needs a regulated voltage.

Another difference of a shunt linear regulator to a series linear regulator is the capacity. In most cases the series type is mostly superior than the shunt type. The shunt type will also need a series resistor to help dissipate the excess power. Without this series resistor, it will not work. The very basic example of a shunt linear regulator is a Zener diode.

Below circuit uses a Zener diode to maintain a regulated voltage to the load. There is a series resistance added as discussed above. If the Zener diode has a breakdown voltage of 12V, that means no voltage higher than 12V will be supplied to the load. In case the Vin level is 20V, the excess voltage will be absorbed by the series resistance and this will result to a power dissipation.

Setting the Rseries for Shunt Regulator

Example: Vzener = 12V, Vin = 20V, Load current = 2A, Rseries = ?

(a) Load current + Shunt current = Current flowing to Rseries

(Where Shunt current is the current needed by the Zener diode to maintain voltage regulation. This is provided by the datasheet. If the datasheet gives minimum and maximum level, simply get the average value and use it.)

Let us assume a shunt current of 0.01A. So,

2A + 0.01A = Current flowing to Rseries = 2.01A

(b) Rseries = (Vin – Vshunt) / Current flowing to Rseries

= (20V – 12V) / 2.01A = 3.98 ohms

(Where; Vshunt is the voltage regulation of the Zener diode)

 Select a resistor value that is nearest to the computed value and recheck the calculation

(c) Supposing a resistance of 3.9 ohms is selected, the new current flowing to the Rseries would be

Current flowing to Rseries = (20V – 12V) / 3.9 ohms = 2.051 A

The load current is fixed at 2A, so the new Zener current is 0.051A. Check the datasheet if this does not exceed the limit.

(d) Compute the Rseries Power Dissipation

The series resistor is the one absorbing the huge power, so it is critical to size it accordingly. The power dissipation on the Rseries is

Pdiss = (Current flowing to Rseries) X (Current flowing to Rseries) X Rseries = 2.051A X 2.051A X 3.9 ohms = 16.4 Watts

Be sure to select the resistor that can carry this power dissipation.

If you want to know how a linear regulator provides regulation and what’s inside the chip, read my article How Linear Regulator Provides Output Regulation .

Switching Voltage Regulator

Both series linear regulator and shunt linear regulator discussed above are only practical for small power applications. Though the series linear regulator may offer higher power capability than the shunt linear regulator but still this is very low power if we talk about power supply. In power supply, switching voltage regulator is used.

A switching voltage regulator is the one uses a power electronic switch that continuously operate between on and off states and charging and discharging a tank circuit. There is then a control loop mechanism that in charge of the system regulation to attain a stable output. Unlike linear voltage regulator that are simple to design, switching voltage regulator is more complicated. However, it can offer high power operations due to its very small power losses compared to the linear versions.

Commonly used switching voltage regulators on PCB assemblies are buck (step down), boost, the combination of the two that is a buck-boost, flyback, sepic and so on.

If you want to know further on switching power supplies and topologies, read my articles Switching Power Supply Operation Principle and Design and Switch Mode Power Supply Explained with Common Topologies.

If you want to venture how switching power supply do the voltage regulation, read my article How Switch Mode Power Supply Regulates its Output.

I have also an article entitled ACDC Linear Power Supply Simulation in LTSpice Step by Step Guide. This will give you guidance on how to simulate a simple ACDC linear power supply in LTspice. LTSpice is a freeware from Linear Technology. If you want to know how to begin with LTSpice simulation, read my article LTSpice Circuit Simulation Tutorials for Beginners.

For more tutorials in LTSpice, choose from the list the topic you want.

Buck Switching Converters

In power electronics, buck voltage regulator is called also as buck switching converter. It is a step-down switch mode regulator that has an output that is lower than its input. For instance, the input is 12V and the output is 5V. Below is the power section circuit of a buck converter.

Buck Converter Schematic

A buck converter is a duty cycle operated switching converter. For complete explanation about duty cycle, read the article Buck Converter Duty Cycle Derivation.

Do you want to learn the details of buck converter design? Read my article Buck Converter Design Tutorial.

Or maybe you are interested to know how to size or select the inductor of a buck converter, read the article Sizing the Inductor of Buck Converter and Setting its Operation.

If you want to design a buck converter but don’t have enough knowledge yet, don’t worry. I have a ready made template that will do the design for you. Go to Buck Converter Design Template Mathcad.

Boost Switching Converters

If a buck converter is called a step-down switching regulator, the boost converter on the other hand called a step-up switching regulator. Its output is higher than its input. For instance, an output of 20V from an input of 10V. Below is a common schematic of a boost converter.

A boost converter is also a duty cycle controlled switching converter. If you want to know more about the duty cycle, read the article How to Calculate the Duty Cycle of Boost Converter.

In dealing with a boost converter, one of the very important component is the inductor. The inductor will control the ripple current. To know more about this, read How to Calculate Boost Converter Ripple Current.

Switching converters has smaller power losses compared to the linear voltage regulator. In order to give you idea, read Power Losses in Boost Converter.

Do you want to design a boost converter? No problem as there is already a ready made template for it. Go to Boost Converter Design Template Mathcad.

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