A voltage regulator is designed to automatically maintain a constant voltage level. Voltage Regulator regulator, usually having three legs, converts varying input voltage and produces a constant regulated output voltage. They are available in a variety of outputs.
For example this small device or circuit that regulates the voltage fed to the microprocessor. The power supply of most PCs generates power at 5 volts but most microprocessors require a voltage below 3.5 volts. The voltage regulator's job is to reduce the 5 volt signal to the lower voltage required by the microprocessor. Typically, voltage regulators are surrounded by heat sinks because they generate significant heat.
Every voltage regulator consists of four basic elements( as shown in the below block diagram):
a stable reference voltage.
a voltage sampling element.
a voltage comparator.
a power dissipating control device.
A voltage regulator generates a fixed output voltage of a preset magnitude that remains constant regardless of changes to its input voltage or load conditions. There are two types of voltage regulators: linear and switching.
IC voltage regulators are versatile and relatively inexpensive and are available with features such as a programmable output, current-voltage boosting, internal short-circuit current limiting, thermal shutdown, and floating operation for high voltage applications-Voltage regulators comprise a class of widely employed ICs.
Theory of work
The main structure of voltage regulator is the zener diode, which basically consist of reverse biased silicon P-N junction.
Reverse-bias usually refers to how a diode is used in a circuit. If a diode is reverse-biased, the voltage at the cathode is higher than that at the anode. Therefore, no current will flow until the diode breaks down. Connecting the P-type region to the negative terminal of the battery and the N-type region to the positive terminal corresponds to reverse bias. The connections are illustrated in the following diagram:
http://upload.wikimedia.org/wikipedia/commons/0/01/PN_Junction_in_Reverse_Bias.png
A silicon p-n junction in reverse bias.
Because the p-type material is now connected to the negative terminal of the power supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the depletion zone to increase. Likewise, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore, the depletion region widens, and does so increasingly with increasing reverse-bias voltage. This increases the voltage barrier causing a high resistance to the flow of charge carriers, thus allowing minimal electric current to cross the p-n junction. The increase in resistance of the p-n junction results in the junction behaving as an insulator.
The strength of the depletion zone electric field increases as the reverse-bias voltage increases. Once the electric field intensity increases beyond a critical level, the p-n junction depletion zone breaks down and current begins to flow, usually by either the Zener or the avalanche breakdown processes. Both of these breakdown processes are non-destructive and are reversible, as long as the amount of current flowing does not reach levels that cause the semiconductor material to overheat and cause thermal damage.
This effect is used to one's advantage in Zener diode regulator circuits. Zener diodes have a certain - low - breakdown voltage. A standard value for breakdown voltage is for instance 5.6 V. This means that the voltage at the cathode can never be more than 5.6 V higher than the voltage at the anode, because the diode will break down - and therefore conduct - if the voltage gets any higher. This in effect regulates the voltage over the diode.
The Zener Voltage, VZ, of a zener diode is the breakdown voltage of a zener diode when it is connected in reverse-biased in a circuit.
The reason the breakdown voltage of a zener diode is so important is because once the voltage that falls across a zener diode exceeds its breakdown voltage, the voltage across the zener remains constant, even if the current going through the zener diode continues to increase.
Because the zener diode holds its zener voltage so steady and constant, it has huge application in circuits, most importantly, voltage regulation.
Below is the voltage-current characteristics curve of a zener diode:
Zener Diode Characteristics Curve
it can be noticed from the above curve that the how steady and constant the voltage across the zener diode is after it reaches the breakdown voltage, despite large changes in current.
This makes the zener diode very useful in circuits where steady voltages need to be supplied.
The zener voltage of zener diodes comes in a range of values. You can find them in 3.3V-12V easily in widespread use.
The circuit below has a zener voltage of 5.1V.
Zener Diode Voltage Regulator Circuit
Therefore, the 12-volt power supply drops 5.1V and the voltage across the zener remains constant at this voltage. This 5.1 zener voltage is then placed in parallel to a load device, which it powers. This is a voltage regulator circuit.
The zener voltage of the diode can be any voltage, as is needed to be constant and power a circuit.
Types of voltage regulator
A simple voltage regulator can be made from a resistor in series with a diode (or series of diodes). Due to the logarithmic shape of diode V-I curves, the voltage across the diode changes only slightly due to changes in current drawn. When precise voltage control is not important, this design may work fine.
Feedback voltage regulators operate by comparing the actual output voltage to some fixed reference voltage. Any difference is amplified and used to control the regulation element in such a way as to reduce the voltage error. This forms a negative feedback control loop; increasing the open-loop gain tends to increase regulation accuracy but reduce stability (avoidance of oscillation, or ringing during step changes). There will also be a trade-off between stability and the speed of the response to changes. If the output voltage is too low (perhaps due to input voltage reducing or load current increasing), the regulation element is commanded, up to a point, to produce a higher output voltage-by dropping less of the input voltage (for linear series regulators and buck switching regulators), or to draw input current for longer periods (boost-type switching regulators); if the output voltage is too high, the regulation element will normally be commanded to produce a lower voltage. However, many regulators have over-current protection, so that they will entirely stop sourcing current (or limit the current in some way) if the output current is too high, and some regulators may also shut down if the input voltage is outside a given range.
There are many types of voltage regulators: linear, switching, SCR type, and hybrid.
A linear regulator employs an active (BJT or MOSFET) pass device (series or shunt) controlled by a high gain differential amplifier. It compares the output voltage with a precise reference voltage and adjusts the pass device to maintain a constant output voltage.
A switching regulator converts the dc input voltage to a switched voltage applied to a power MOSFET or BJT switch. The filtered power switch output voltage is fed back to a circuit that controls the power switch on and off times so that the output voltage remains constant regardless of input voltage or load current changes.
SCR regulators: Regulators powered from AC power circuits can use silicon controlled rectifiers (SCRs) as the series device. Whenever the output voltage is below the desired value, the SCR is triggered, allowing electricity to flow into the load until the AC mains voltage passes through zero (ending the half cycle). SCR regulators have the advantages of being both very efficient and very simple, but because they cannot terminate an on-going half cycle of conduction, they are not capable of very accurate voltage regulation in response to rapidly-changing loads. An alternative is the SCR shunt regulator which uses the regulator output as a trigger, both series and shunt designs are noisy, but powerful, as the device has a low on resistance.
Combination (hybrid) regulators
Many power supplies use more than one regulating method in series. For example, the output from a switching regulator can be further regulated by a linear regulator. The switching regulator accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage slightly above the ultimately desired output. That is followed by a linear regulator that generates exactly the desired voltage and eliminates nearly all the noise generated by the switching regulator. Other designs may use an SCR regulator as the "pre-regulator", followed by another type of regulator. An efficient way of creating a variable-voltage, accurate output power supply is to combine a multi-tapped transformer with an adjustable linear post-regulator.
Except for the, switching regulators, all other types of regulators are linear regulators. The impedance of the active element of the linear regulator may be continuously varied to provide a desired current to the load, on the other hand, in a switching regulator a switch is turned on and off at a rate such that the regulator provides the desired average current in periodic pulses to the load. The switching regulators are more efficient than the linear regulators. This is because there is negligible power dissipation in switching elements in either the on or off state. Nevertheless, in switching regulators the power dissipation is substantial during the switching intervals (on to off or off to on). Also, most of the loads (devices) cannot accept the average current in periodic pulses. Therefore, most practical voltage regulators are of the linear type.
Voltage Regulator IC
Regulator IC units contain the circuitry for reference source, comparator amplifier, control device, and overload protection all in a single IC. Although the internal construction of IC is somewhat different from that of discrete voltage regulator circuits, the external operation is almost the same.
A power supply can be built using a transformer connected to the ac supply line to transform the ac voltage to a desired level, then rectifying the ac voltage, filtering with a capacitor and RC filter, if desired, and finally regulating the dc voltage employing and IC regulator. The regulators can be selected for operation with load currents ranging from hundreds of milli amperes to tens of amperes, corresponding to power ratings from milliwatts to tens of watts.
Voltage Regulator Block Diagram
Voltage Regulator Block Diagram
Voltage regulators, especially the switching type, are employed as control circuits in pulse width modulation, push-pull bridges, and series type switch mode supplies. Almost all power supplies make use of some type of voltage regulator IC because they are simple to use, reliable, cheaper in cost, and, above all, available in a variety of voltage and current ratings.
For instance, the LM 309 is a fixed positive regulator with an output of + 5 V, a maximum load current of 1 A, a load regulation of 15 mV, a source regulation of 4 mV, and a ripple rejection of 75 db. For the adjustable regulators, LR and SR are given in percentage rather than millivolts. The table also includes the drop-out voltage, or the minimum permissible difference between the input and output voltages. For example an LM 309 has a drop-out voltage of 2 V. It implies that the input voltage must be at least 2 V greater than output voltage i.e. input voltage must be at least 7 V, because its output voltage is 5 V.New devices can supply load current from 100 mA to more than 5 A. Available in plastic or metal packages, these three-terminal voltage regulators have become extremely popular because they are inexpensive and easy to use. Aside from a couple of bypass capacitors, the new three-terminal IC voltage regulators do not need any external component.
The integrated three-terminal voltage regulators typically incorporate many of the functions.
The error amplifier is used to maintain a constant voltage through a negative feedback. The internal voltage reference is tightly controlled during the fabrication of IC. So, the nominal output voltage of most of the three-terminal voltage regulators has tolerances that range from ± 6 % to better than ±2%. The series-pass element is driven by the output of the error amplifier. IF acts as an automatically controlled variable resistor. This resistance varies as required for maintaining the output voltage constant. The series-pass element is typically a BJT that is rated to pass the maximum load current.
The basic connection of a three-terminal voltage regulator IC to a load is shown in figure. The fixed voltage regulator has an unregulated dc input voltage Vin, applied to one input terminal, a regulated output dc voltage, Vout from a second terminal, with the third terminal connected to ground.
The most common part numbers start with the numbers 78 or 79 and finish with two digits indicating the output voltage. The number 78 represents positive voltage and 79 negative one. The 78XX series of voltage regulators are designed for positive input. And the 79XX series is designed for negative input.
Examples:
5V DC Regulator Name: LM7805 or MC7805
-5V DC Regulator Name: LM7905 or MC7905
6V DC Regulator Name: LM7806 or MC7806
-9V DC Regulator Name: LM7909 or MC7909
The LM78XX series typically has the ability to drive current up to 1A. For application requirements up to 150mA, 78LXX can be used. As mentioned before, the component has three legs: Input leg which can hold up to 36VDC Common leg (GND) and an output leg with the regulator's voltage. For maximum voltage regulation, adding a capacitor in parallel between the common leg and the output is usually recommended. Typically a 0.1MF capacitor is used. This eliminates any high frequency AC voltage that could otherwise combine with the output voltage. See below circuit diagram which represents a typical use of a voltage regulator.
http://www.eidusa.com/KITS/7805_SCH_001.jpg
As a general rule the input voltage should be limited to 2 to 3 volts above the output voltage. The LM78XX series can handle up to 36 volts input, be advised that the power difference between the input and output appears as heat. If the input voltage is unnecessarily high, the regulator will overheat. Unless sufficient heat dissipation is provided through heat sinking, the regulator will shut down.
Disadvantages of LM7XXX:
The input voltage must always be higher than the output voltage by some minimum amount (typically 2 volts). This can make these devices unsuitable for powering some devices from certain types of power sources (for example, powering a circuit that requires 5 volts using 6-volt batteries will not work using a 7805).
As they are based on a linear regulator design, the input current required is always the same as the output current. As the input voltage must always be higher than the output voltage, this means that the total power (voltage multiplied by current) going into the 78xx will be more than the output power provided. The extra input power is dissipated as heat. This means both that for some applications an adequate heat sink must be provided, and also that a (often substantial) portion of the input power is wasted during the process, rendering them less efficient than some other types of power supplies. When the input voltage is significantly higher than the regulated output voltage (for example, powering a 7805 using a 24 volt power source), this inefficiency can be a significant issue.
Even in larger packages, 78xx integrated circuits cannot supply as much power as many designs which use discrete components, and are generally inappropriate for applications requiring more than a few amperes of current.
Application Of Voltage Regulator IC
Dual Power Supply using LM 320 and LM 340
Simple Dual Power Supply
Simple Dual Power Supply
Many discrete and ICs need bipolar (dual or ± V) supplies. This can be easily accomplished with two three-terminal regulators, as illustrated in figure. Opposite-phase ac is provided by the transformer's secondary and a grounded center tap. The single full-wave bridge converts these into positive and negative dc voltages (with respect to the grounded center tap). Filtering (with respect to ground) is provided by capacitors C1 and C2.
The LM 340 provides regulation of the positive voltage, while the LM 320 regulates the negative voltage. It is very important to mention here that LM 320 has a different pin configuration than the LM 340. The case of the LM 320 is not ground. So care is to be taken while mounting the negative regulator.
The diodes provide protection, but ensure that they are not reversed. Diodes Dl and D2 ensure that transients on the regulator outputs do not drive the outputs to a potential above their inputs and cause damage to the regulators. Also, the two regulators may not turn on simultaneously. If this occurs, the output of the slower regulator may be driven toward the potential of the faster one. Diodes D3 and D4 prevent these reverse polarities on start up.
Tracking Dual regulator
Tracking Dual regulator
Many applications require several different voltage power supplies. One solution is to build several independent regulators. However, very often it is important that all of these supply voltages track. That is, if one of the supply voltages goes up 2 %, it is best if all of the supply voltages go up by the same amount. It can be accomplished by adding an op-amp to the adjustable three-terminal voltage regulator.
The circuit shown in figure provides current limiting and thermal shutdown for the negative as well as the positive output voltage. The positive regulated voltage is produced with a LM 317 adjustable positive regulator IC, as it was in figure.
The regulated positive voltage is always 1.2 V more positive than the voltage across R3 (on the adjust pin). This regulated output voltage is used as the input of an inverting amplifier. The op-amp A is the input stage of this amplifier, and the LM 337 negative voltage regulator is the power output stage. Consequently, the negative regulated output voltage V" is an amplified and inverted version of the positive regulated voltage V+. Current limiting and thermal shutdown are independently provided by the LM 317 (for the positive output) and the LM 337 (for the negative output). The op-amp assures that the negative output voltage tracks the positive output by driving the LM 337's adjust pin to + 1.2 V above the required negative output. Since the LM 337 is inside the op-amp's negative feedback loop, this + 1.2 V offset appears between the op-amp and the regulator, not at the regulator output.
Measures of regulator quality
The output voltage can only be held roughly constant; the regulation is specified by two measurements:
load regulation is the change in output voltage for a given change in load current (for example: "typically 15 mV, maximum 100 mV for load currents between 5 mA and 1.4 A, at some specified temperature and input voltage").
line regulation or input regulation is the degree to which output voltage changes with input (supply) voltage changes - as a ratio of output to input change (for example "typically 13 mV/V"), or the output voltage change over the entire specified input voltage range (for example "plus or minus 2% for input voltages between 90 V and 260 V, 50-60 Hz").
Temperature coefficient of the output voltage is the change with temperature (perhaps averaged over a given temperature range).
Initial accuracy of a voltage regulator (or simply "the voltage accuracy") reflects the error in output voltage for a fixed regulator without taking into account temperature or aging effects on output accuracy.
Dropout voltage is the minimum difference between input voltage and output voltage for which the regulator can still supply the specified current. A low drop-out (LDO) regulator is designed to work well even with an input supply only a volt or so above the output voltage. The input-output differential at which the voltage regulator will no longer maintain regulation is the dropout voltage. Further reduction in input voltage will result in reduced output voltage. This value is dependent on load current and junction temperature.
Absolute maximum ratings are defined for regulator components, specifying the continuous and peak output currents that may be used (sometimes internally limited), the maximum input voltage, maximum power dissipation at a given temperature, etc.
Output noise (thermal white noise) and output dynamic impedance may be specified as graphs versus frequency, while output ripple noise (mains "hum" or switch-mode "hash" noise) may be given as peak-to-peak or RMS voltages, or in terms of their spectra.
Quiescent current in a regulator circuit is the current drawn internally, not available to the load, normally measured as the input current while no load is connected (and hence a source of inefficiency; some linear regulators are, surprisingly, more efficient at very low current loads than switch-mode designs because of this).
Transient response is the reaction of a regulator when a (sudden) change of the load current (called the load transient) or input voltage (called the line transient) occurs. Some regulators will tend to oscillate or have a slow response time which in some cases might lead to undesired results. This value is different from the regulation parameters, as that is the stable situation definition. The transient response shows the behaviour of the regulator on a change. This data is usually provided in the technical documentation of a regulator and is also dependent on output capacitance.
Mirror-image insertion protection means that a regulator is designed for use when a voltage, usually not higher than the maximum input voltage of the regulator, is applied to its output pin while its input terminal is at a low voltage, volt-free or grounded. Some regulators can continuously withstand this situation; others might only manage it for a limited time such as 60 seconds, as usually specified in the datasheet. This situation can occur when a three terminal regulator is incorrectly mounted for example on a PCB, with the output terminal connected to the unregulated DC input and the input connected to the load. Mirror-image insertion protection is also important when a regulator circuit is used in battery charging circuits, when external power fails or is not turned on and the output terminal remains at battery voltage.
