Voltage regulator wikipedia

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Voltage regulator

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A popular three pin +12 V DC voltage regulator IC.

A voltage regulator isdesigned to automatically maintain a constantvoltage level. A voltage regulator may be asimple "feed-forward" design or mayinclude negativefeedback controlloops. It may use anelectromechanical mechanism,or electronic components. Depending on the design, it may be usedto regulate one or more AC or DC voltages.

Electronicvoltage regulators are found in devices such ascomputer powersupplies where they stabilize the DC voltagesused by the processor and other elements. Inautomobile alternators andcentralpowerstation generator plants, voltage regulatorscontrol the output of the plant. In an electricpower distribution system, voltage regulatorsmay be installed at a substation or along distribution lines sothat all customers receive steady voltage independent of how muchpower is drawn from the line.

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Measures of regulatorquality[edit]

The outputvoltage can only beheld roughly constant; theregulation is specified by two measurements:

loadregulation is the change in output voltage fora given change in load current (for example: "typically15 mV, maximum 100 mV for loadcurrents between 5 mA and 1.4 A,at some specified temperature and input voltage").
lineregulation or inputregulation is the degree to which outputvoltage changes with input (supply) voltage changes - as a ratio ofoutput to input change (for example "typically13 mV/V"), or the output voltage change over theentire specified input voltage range (for example "plus or minus 2%for input voltages between 90 V and260 V, 50-60 Hz").

Otherimportant parameters are:

Temperaturecoefficient of the output voltage is thechange with temperature (perhaps averaged over a given temperaturerange).
Initialaccuracy of a voltage regulator (or simply"the voltage accuracy") reflects the error in output voltage for afixed regulator without taking into account temperature or agingeffects on output accuracy.
Dropoutvoltage is the minimum difference betweeninput voltage and output voltage for which the regulator can stillsupply the specified current. A lowdrop-out (LDO) regulator is designed to workwell even with an input supply only a volt orso above the output voltage. The input-output differential at whichthe voltage regulator will no longer maintain regulation is thedropout voltage. Further reduction in input voltage will result inreduced output voltage. This value is dependent on load current andjunction temperature.
Absolute maximumratings are defined for regulator components,specifying the continuous and peak output currents that may be used(sometimes internally limited), the maximum input voltage, maximumpower dissipation at a given temperature, etc.
Outputnoise (thermal whitenoise) and output dynamicimpedance may be specified as graphs versusfrequency, whileoutput ripple noise (mains"hum" or switch-mode "hash" noise) may be given as peak-to-peakor RMS voltages,or in terms of their spectra.
Quiescentcurrent in a regulator circuit is the currentdrawn internally, not available to the load, normally measured asthe input current while no load is connected (and hence a source ofinefficiency; some linearregulators are, surprisingly, more efficientat very low current loads than switch-mode designs because ofthis).
Transientresponse is the reaction of a regulator when a(sudden) change of the load current (calledthe load transient) or input voltage(called the line transient) occurs. Someregulators will tend to oscillate or have a slow response timewhich in some cases might lead to undesired results. This value isdifferent from the regulation parameters, as that is the stablesituation definition. The transient response shows the behaviour ofthe regulator on a change. This data is usually provided in thetechnical documentation of a regulator and is also dependent onoutput capacitance.
Mirror-image insertionprotection means that a regulator is designedfor use when a voltage, usually not higher than the maximum inputvoltage of the regulator, is applied to its output pin while itsinput terminal is at a low voltage, volt-free or grounded. Someregulators can continuously withstand this situation; others mightonly manage it for a limited time such as 60 seconds, as usuallyspecified in the datasheet. This situation can occur when a threeterminal regulator is incorrectly mounted for example on a PCB,with the output terminal connected to the unregulated DC input andthe input connected to the load. Mirror-image insertion protectionis also important when a regulator circuit is used in batterycharging circuits, when external power fails or is not turned onand the output terminal remains at battery voltage.

Electronic voltageregulators[edit]

A simplevoltage regulator can be made from a resistor in series witha diode (orseries of diodes). Due to the logarithmic shape of diode V-Icurves, the voltage across the diode changes only slightly due tochanges in current drawn or changes in the input. When precisevoltage control and efficiency are not important, this design maywork fine.

Feedbackvoltage regulators operate by comparing the actual output voltageto some fixed reference voltage. Any difference is amplified andused to control the regulation element in such a way as to reducethe voltage error. This forms anegativefeedback controlloop; increasing the open-loopgain tends to increase regulation accuracy butreduce stability (avoidance of oscillation, or ringing during stepchanges). There will also be a trade-off between stability and thespeed 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 apoint, to produce a higher output voltage–by dropping less ofthe input voltage (for linear series regulatorsand buck switchingregulators), or to draw input current for longer periods(boost-type switchingregulators); if the output voltage is too high, the regulationelement will normally be commanded to produce a lower voltage.However, many regulators have over-current protection, so that theywill entirely stop sourcing current (or limit the current in someway) if the output current is too high, and some regulators mayalso shut down if the input voltage is outside a given range (seealso: crowbarcircuits).

Electromechanicalregulators[edit]

Circuit design for a simple electromechanical voltageregulator.

A voltage stabilizer using electromechanical relays forswitching.

Graph of voltage output on a time scale.

Inelectromechanical regulators, voltage regulation is easilyaccomplished by coiling the sensing wire to make an electromagnet.The magneticfield produced by the current attracts amoving ferrous core held back under spring tension or gravitationalpull. As voltage increases, so does the current, strengthening themagnetic field produced by the coil and pulling the core towardsthe field. The magnet is physically connected to a mechanical powerswitch, which opens as the magnet moves into the field. As voltagedecreases, so does the current, releasing spring tension or theweight of the core and causing it to retract. This closes theswitch and allows the power to flow once more.

If themechanical regulator design is sensitive to small voltagefluctuations, the motion of the solenoid core can be used to move aselector switch across a range of resistances or transformerwindings to gradually step the output voltage up or down, or torotate the position of a moving-coil AC regulator.

Early automobile generators and alternators hada mechanical voltage regulator using one, two, orthree relays andvarious resistors tostabilize the generator's output at slightly more than 6 or 12 V,independent of the engine's rpm orthe varying load on the vehicle's electrical system. Essentially,the relay(s) employed pulsewidth modulation to regulate the output of thegenerator, controlling the field current reaching the generator (oralternator) and in this way controlling the output voltageproduced.

Theregulators used for DC generators (but not alternators) alsodisconnect the generator when it was not producing electricity,thereby preventing the battery from discharging back into thegenerator and attempting to run it as a motor.The rectifier diodes inan alternator automatically perform this function so that aspecific relay is not required; this appreciably simplified theregulator design.

More moderndesigns now use solidstate technology (transistors)to perform the same function that the relays perform inelectromechanical regulators.

Electromechanical regulators are used for mains voltagestabilisation—see ACvoltage stabilizers below.

Coil-rotation AC voltageregulator[edit]

Basic design principle and circuit diagram for the rotating-coil ACvoltage regulator.

This is anolder type of regulator used in the 1920s that uses the principleof a fixed-position field coil and a second field coil that can berotated on an axis in parallel with the fixed coil, similar toa variocoupler.

When themovable coil is positioned perpendicular to the fixed coil, themagnetic forces acting on the movable coil balance each other outand voltage output is unchanged. Rotating the coil in one directionor the other away from the center position will increase ordecrease voltage in the secondary movable coil.

This typeof regulator can be automated via a servo control mechanism toadvance the movable coil position in order to provide voltageincrease or decrease. A braking mechanism or high ratio gearing isused to hold the rotating coil in place against the powerfulmagnetic forces acting on the moving coil.

AC voltagestabilizers[edit]

Magnetic mains regulator

Electromechanical[edit]

Electromechanical regulators called voltagestabilizers or tap-changers,have also been used to regulate the voltage onAC powerdistribution lines. These regulators operateby using aservomechanism toselect the appropriate tap on an autotransformer withmultiple taps, or by moving the wiper on a continuously variableautotransfomer. If the output voltage is not in the acceptablerange, the servomechanism switches the tap, changing the turnsratio of the transformer, to move the secondary voltage into theacceptable region. The controls provide a deadbandwhereinthe controller will not act, preventing the controller fromconstantly adjusting the voltage ("hunting") as it varies by anacceptably small amount.

Constant-voltagetransformer[edit]

The ferroresonanttransformer, ferroresonantregulator or constant-voltagetransformer is a type of saturatingtransformer used as a voltage regulator. These transformers usea tankcircuit composed of a high-voltage resonantwinding and a capacitor toproduce a nearly constant average output voltage with a varyinginput current or varying load. The circuit has a primary on oneside of a magnet shunt and the tuned circuit coil and secondary onthe other side. The regulation is due to magnetic saturation in thesection around the secondary.

Theferroresonant approach is attractive due to its lack of activecomponents, relying on the square loop saturation characteristicsof the tank circuit to absorb variations in average input voltage.Saturating transformers provide a simple rugged method to stabilizean AC power supply.

Olderdesigns of ferroresonant transformers had an output withhigh harmonic content,leading to a distorted output waveform. Modern devices are used toconstruct a perfect sinewave. The ferroresonant action is a flux limiterrather than a voltage regulator, but with a fixed supply frequencyit can maintain an almost constant average output voltage even asthe input voltage varies widely.

Theferroresonant transformers, which are also known as ConstantVoltage Transformers (CVTs) or ferros, are also good surgesuppressors, as they provide high isolation and inherentshort-circuit protection.

Aferroresonant transformer can operate with an input voltage range±40% or more of the nominal voltage.

Outputpower factor remains in the range of 0.96 or higher from half tofull load.

Because itregenerates an output voltage waveform, output distortion, which istypically less than 4%, is independent of any input voltagedistortion, including notching.

Efficiencyat full load is typically in the range of 89% to 93%. However, atlow loads, efficiency can drop below 60%. The current-limitingcapability also becomes a handicap when a CVT is used in anapplication with moderate to high inrushcurrent like motors, transformers or magnets.In this case, the CVT has to be sized to accommodate the peakcurrent, thus forcing it to run at low loads and poorefficiency.

Minimummaintenance is required, as transformers and capacitors can be veryreliable. Some units have included redundant capacitors to allowseveral capacitors to fail between inspections without anynoticeable effect on the device's performance.

Outputvoltage varies about 1.2% for every 1% change in supply frequency.For example, a 2 Hz change in generator frequency, which is verylarge, results in an output voltage change of only 4%, which haslittle effect for most loads.

It accepts100% single-phase switch-mode power supply loading without anyrequirement for derating, including all neutral components.

Inputcurrent distortion remains less than 8% THD evenwhen supplying nonlinear loads with more than 100% current THD.

Drawbacksof CVTs are their larger size, audible humming sound, and the highheat generation caused by saturation. Also, the regulation is notas good as solid state devices, so these units are obsolete formost purposes.

DC voltagestabilizers[edit]

Many simpleDC power supplies regulate the voltage usinga shunt regulator such asa Zenerdiode, avalanchebreakdown diode, or voltageregulator tube. Each of these devices begins conducting at aspecified voltage and will conduct as much current as required tohold its terminal voltage to that specified voltage. The powersupply is designed to only supply a maximum amount of current thatis within the safe operating capability of the shunt regulatingdevice (commonly, by using a series resistor).

If thestabilizer must provide more power, the shunt regulator output isonly used to provide the standard voltage reference for theelectronic device, known as the voltage stabilizer. The voltagestabilizer is the electronic device, able to deliver much largercurrents on demand.

Activeregulators[edit]

Activeregulators employ at least one active (amplifying) component suchas a transistor or operational amplifier. Shunt regulators areoften (but not always) passive and simple, but always inefficientbecause they (essentially) dump the excess current not needed bythe load. When more power must be supplied, more sophisticatedcircuits are used. In general, these active regulators can bedivided into several classes:

Linear series regulators
Switching regulators
SCR regulators

Linearregulators[edit]

Main article: Linearregulator

Linearregulators are based on devices that operate in their linear region(in contrast, a switching regulator is based on a device forced toact as an on/off switch). In the past, one ormore vacuumtubes were commonly used as the variableresistance. Modern designs use one ormore transistors instead,perhaps within an IntegratedCircuit. Linear designs have the advantage of very "clean"output with little noise introduced into their DC output, but aremost often much less efficient and unable to step-up or invert theinput voltage like switched supplies. All linear regulators requirea higher input than the output. If the input voltage approaches thedesired output voltage, the regulator will "drop out". The input tooutput voltage differential at which this occurs is known as theregulator's drop-out voltage.

Entirelinear regulators are available as integratedcircuits. These chips come in either fixed or adjustablevoltage types.

Switchingregulators[edit]

Main article: Switched-modepower supply

Switchingregulators rapidly switch a series device on and off.The dutycycle of the switch sets howmuch charge istransferred to the load. This is controlled by a similar feedbackmechanism as in a linear regulator. Because the series element iseither fully conducting, or switched off, it dissipates almost nopower; this is what gives the switching design its efficiency.Switching regulators are also able to generate output voltageswhich are higher than the input, or of opposite polarity —something not possible with a linear design.

Like linearregulators, nearly-complete switching regulators are also availableas integrated circuits. Unlike linear regulators, these usuallyrequire one external component: an inductor thatacts as the energy storage element. (Large-valued inductors tend tobe physically large relative to almost all other kinds ofcomponentry, so they are rarely fabricated within integratedcircuits and IC regulators — with some exceptions.^[1]^[2])

Comparing linear vs.switching regulators[edit]

The twotypes of regulators have their different advantages:

Linear regulators are best whenlow output noise (and low RFI radiatednoise) is required
Linear regulators are best when afast response to input and output disturbances is required.
At low levels of power, linearregulators are cheaper and occupy less printedcircuit board space.
Switching regulators are bestwhen power efficiency is critical (such asin portablecomputers), except linearregulators are more efficient in a small number of cases (such as a5V microprocessor often in "sleep" mode fed from a 6Vbattery, if the complexityof the switching circuit and the junction capacitance chargingcurrent means a high quiescent current in the switchingregulator).
Switching regulators are requiredwhen the only power supply is a DC voltage, and a higher outputvoltage is required.
At high levels of power (above afew watts), switching regulators are cheaper (for example, the costof removing heat generated is less).

SCRregulators[edit]

Regulatorspowered from AC power circuits can use siliconcontrolled rectifiers (SCRs) as the seriesdevice. Whenever the output voltage is below the desired value, theSCR is triggered, allowing electricity to flow into the load untilthe AC mains voltage passes through zero (ending the half cycle).SCR regulators have the advantages of being both very efficient andvery simple, but because they can not terminate an on-going halfcycle of conduction, they are not capable of very accurate voltageregulation in response to rapidly-changing loads. An alternative isthe SCR shunt regulator which uses the regulator output as atrigger, both series and shunt designs are noisy, but powerful, asthe device has a low on resistance.

Combination (hybrid)regulators[edit]

Many powersupplies use more than one regulating method in series. Forexample, the output from a switching regulator can be furtherregulated by a linear regulator. The switching regulator accepts awide range of input voltages and efficiently generates a (somewhatnoisy) voltage slightly above the ultimately desired output. Thatis followed by a linear regulator that generates exactly thedesired voltage and eliminates nearly allthe noise generatedby the switching regulator. Other designs may use an SCR regulatoras the "pre-regulator", followed by another type of regulator. Anefficient way of creating a variable-voltage, accurate output powersupply is to combine a multi-tapped transformer with an adjustablelinear post-regulator.

Examplelinear regulators[edit]

Transistorregulator[edit]

In thesimplest case emitterfollower is used, the base of the regulatingtransistor is directly connected to the voltage reference:

Voltage stabiliser transistor, IEC symbols.svg

Thestabilizer uses the power source, having voltage U_in that may vary overtime. It delivers the relatively constant voltage U_out. The output load R_L can also vary overtime. For such a device to work properly, the input voltage must belarger than the output voltage and Voltage drop must not exceed thelimits of the transistor used.

The outputvoltage of the stabilizer is equal to U_Z - U_BE where U_BE is about0.7 V and depends on the load current. If theoutput voltage drops below that limit, this increases the voltagedifference between the base and emitter (U_be), opening the transistor anddelivering more current. Delivering more current through the sameoutput resistor R_L increasesthe voltage again. When the output voltage increases, due to thereduced base-emitter voltage the BJT will conduct less and theregulation will be maintained.

R_v valuelimits the maximum output current I_out, therefor has a upper limits.R_v (as arough estimate) should not be larger than (U_In - U_z) / I_B, where I_B is maximumI_out dividestransistor's current amplification ratio.

Regulator with anoperational amplifier[edit]

Thestability of the output voltage can be significantly increased byusing an operationalamplifier:

In thiscase, the operational amplifier drives the transistor with morecurrent if the voltage at its inverting input drops below theoutput of the voltage reference at the non-inverting input. Usingthe voltagedivider (R1, R2 and R3) allows choice of thearbitrary output voltage between U_z and U_in.