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Ssp

Bob Mammano and Mark Jordan

IntroductionWhen designing low-voltage power systems to

supply large load cunents, paralleled lower-cunentmodules are often preferred over a single, largepower converter for several reasons. These includethe efficiencies of designing and manufacturingstandard modular converters which can be com-bined in whatever number necessary to meet agiven load requirement; and the enhanced reliabilitygained through redundancy. Additional gains areoften achieved in mechanical packaging consider-ations and in distributed heat removal.

While most modem-day power supplies can beparalleled for higher currents, the load current willnot share equally between modules without someextra effort in the design process. With unequalload sharing, the stress placed on the individualmodules will be unequal, resulting in some unitsoperating with higher temperatures--a recognizedcontributor to reduced reliability .Therefore, thechallenge in paralleling modular supplies is toinsure predictable, uniform current sharing-regard-less of load levels and the number of modules.Another major goal should be to provide enhancedsystem reliability through complete redundancy suchthat the failure of one or more modules could betolerated as long as the total remaining capacity isequal to or greater than the demands of the load.

Over the years, a variety of schemes have beendevised to accomplish load sharing and, as onewould expect, these schemes offer a wide range ofperformance characteristics. A brief description ofsome of these approaches will, perhaps, be useful incomparing their performance capabilities against thedegree of difficulty in their implementation.

Current Limit ParallelingThe connection diagram for this most basic a

proach is shown in Figure 1 where it can be sethat each unit is completely independent except the common load. Each individual module muhave inherent current limiting because in practicthe output voltage of all the modules will never exactly equal. Thus, when several modules aparalleled, the one with the highest output voltawill attempt to supply all the load cuITent, up to tpoint where its current limit is reached. As this ugoes into current limiting, its output voltage wfall to the level of the next highest module, whicthen begins to conduct and supply additional locurrent. When the second module reaches current limit, number three starts conducting, andon.

Of course, there is no current sharing at except for the units which are in current limitingand it could be expected that the dynamic loregulation, particularly as each current limit thresold is passed, would be less than desirable.

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Output Voltage Droop MethodAs the name "droop" would imply, this method

of paralleling is accomplished by designing theindividual power modules with a finite outputresistance so that the output voltage falls slightly asthe load current is increased. The modules wouldthen be connected together as shown in Figure 2. Ifthe individual units are initially adjusted for equal

culTents at some load. the action of the outputresistances will be to maintain sharing over fairlywide changes in load demand. If one module wereto conduct less CUlTent. he reduced voltage dropacross its output resistance would lower the voltageat the sense point. causing it to increase its conduc-tion and correct for the unbalance. While thisapproach has seen considerable usage. it also haspoorer load regulation. and the problem gets worseas the output resistance is increased to increase theaccuracy of sharing. Additional difficulties resultfrom the need for accurate initial balancing and thesusceptibility to reference drift in the individualmodules.Common Point V oltage- Mode Control

Figure 3 shows a paralleling configuration wherethe control is achieved by a separate block contain-ing the load voltage sensing. the system reference.and a gain stage. The individual modules then onlyneed to contain the power stage--the ultimatesimplicity if low system cost is a primary objective.While current sharing will not be perfect ifpower stage variables give different switchingwaveforms for the same duty cycle command. the

fact that most of the gain is in the single controlunit minimizes this problem and reasonable sharingcan be achieved with excellent line and load regula-tion over a wide range of operating conditions.With a single control point, output voltage adjust-ment or margin testing is also easily accomplished.The power stages in this configuration can bedesigned with a blocking diode at the output suchthat if an individual unit fails, the remaining mod-ules will merely increase conduction to make up forthe loss. However, even recognizing the argumenthat the greatest propensity for failure will be withthe high-stress power stages, the fact remains thain this approach, and the one following, a failure inthe control stage will cause the entire power systemto fail.Closed Loop Current Mode Control

Current-mode control means that a power supplyis controlled by feedback from both inductor currenand output voltage. Here, each module's currenlevel is programmed by a voltage control loop andwith this capability, current sharing can be veryexact. The circuit approach for paralleling caneither have a separate controller, as shown above inFigure 3, or the more usual Master / Slave configu-ration as illustrated in Figure 4. In either case, thtransfer function between control input and modulecurrent is well defined and a changing controvoltage will cause a proportionate change in eacmodule's current.The module unit configured as a master wil

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Voltage SenseMaster Su~1y

Current FIB

SlaveSu~ly1

Current FIBiio()

Load

SlaveSu~ly n

Current FIB

Fig 4. -Master/Slave Current Mode Control Fig 5. -Independent Current Sharing

the second is that while each module must haveown voltage sensing and reference circuitry , they must regulate to the same common output voltalevel.Since there will always be differences betwethe individual references--no matter how slight-thtechnique used by most implementations is to apthe current sharing control information to adjust voltage reference in each module. Properly dothis allows each module to achieve a referenvalue which equalizes the individual currents witheach module at exactly the value needed to regulathe load voltage. Since the reference adjustmerange need only be large enough to accommodunit-to-unit tolerances, it is reasonable to include added feature that allows the module to continueregulate, losing only the current-share capabilishould the current control line open.

The means for generating and distributing curresharing information between the modules is notriyial task. The signal should be transmitted withsingle interconnecting line (the Share Bus). It mube insensitive to noise pickup and parasitic ements. The modules must current share when tcontrol signal is present and must continue operate when the Bus is either open or shorteTypically, this involves adding an AdjustmeAmplifier within each module. This amplifierfunction is to compare a signal derived from thmodule's current with that received from the ShaBus, and to adjust the reference as needed to drithis difference to zero. At least two techniques ha

work singlely as an independent supply for loadswithin its rating. However, when higher loadsdictate the need for more paralleled modules, slaveunits may easily be added which will automaticallydeliver a proportionate share of the total load. Theproblem, of course, is that redundancy is achievedonly with the slave units-not with the master.Independent Current SharingWhile current-mode techniques can give eachmodule the ability to control its own current,completely independent sharing typically requiresmore effort. Figure 5 illustrates a more desirableapproach for paralleling where each module can acteither as a stand-alone supply, or as a proportionatemember of a paralleled group. A distinction shouldbe made here between "current-mode control",which uses an internal control loop derived frominductor current, and "current sharing", which addsa current loop based on external module outputcurrent. Current sharing can apply equally to eithervoltage or current-mode designs. In Figure 5, thecurrent sense signal measures the module outputcurrent and its function has no bearing on whetheror not the modules contain an internal current-modecontrol loop.

Independent current sharing presents at least twodifficulties, however, which must be overcome inorder to make a connection as shown in Figure 5practical. The first is the need for a bidirectionalbus for communicating current sharing informationbetween the modules, (more about this later), and

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To Load~ .VoltageFeedback r-*

noise rejection. One potential difficulty isthat anything which loads down the Share:~I Bus-an external short or a single failed

Voltage module, for example-could pull down theAmp ,-,!bus. AveragellVc's entire system. A possible protection againstL. ?urr~ntVc ~ ~ this problem is to limit the range of adjust-Monitor ~/ Share us ment allowed for the reference but, while

this would keep the supplies from collaps-ing, it would still reduce the load voltage tothe value defined by this adjustment limit-alimit which must be lower than the expected

I reference tolerance.Master / Slave Automatic Selection

Properly done, a master/slave system with auto-matic selection would preclude this mode of failureas only the master has control and if it fails, thesystem would merely select a new master. One suchapproach is shown in Figure 7. Here, with theseries resistor replaced by a diode, the reference isallowed to adjust only in an upward direction. TheShare Bus now represents the highest current beingdelivered by any of a paralleled array of powermodules. This is because only the module with thehighest output current can forward-bias its diodeand drive the Share Bus. The differential voltage atthe input to this Adjust Amplifier will force itsoutput low, leaving the reference unchanged. Thisis appropriate because the module initially supply-ing the highest current already has the highestreference voltage.Modules initially supplying lower output currentswould use the Share Bus voltage as an input com-mand for their Adjust Amplifiers. Their reference

. c~ . ~ r- -,. };

.AdjustAmp

Fig 6. -Sharing the A verage Current

election.utomatic A verage Output Current

SharingThis system, which was developed (and patented)y Ken Small at Boschert, is shown in simplifiedform in Figure 6. The module's power stage iscontrolled by the Voltage Amplifier in response toa comparison between the sensed output voltagefeedback and the adjustable voltage reference. Themodule's contribution to load current is sensed bythe Current Monitor and a voltage proportional tooutput current is applied to the Share Bus througha series resistor, RSHARE.With the resistors in eachmodule all summed together at the Share Bus, thevoltage at this node then always represents theaverage of all the output currents of all the modulesconnected together. If the individual currents are notequal, there will be voltage drops-eitherpositive or negative-across the share resis-tors. The Adjust Amplifiers within each v Itagemodule will then force these signals to zero Feedbackby changing the value of the individualreferences for each Voltage Amplifier.With all modules matching up such thateach one delivers a current equal to theaverage of all, excellent sharing can beachieved. Additionally, since the Share Busis not a part of the voltage control loop, itsbandwidth can be low and the Bus can bedecoupled with a large capacitor for good

To Loaj~ ..

-~.VoltageAmp--+

VcCurrentMonitfX ~~ i Highest of all Vc's

--7 Share Bus'-+..:.:!!:!!/-

Vref+()~ Adj~.Amp

Fig 7. Sharing the Maximum CurrentUNITRODE CORPORATION-4

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than CUlTentand voltage sensing, only a single liinterconnects the modules for current sharing aeach controller communicates with its own powstage, either directly or through an isolating meum.

The overall block diagram of this IC is shown Figure 9, however, its capability and application cprobably be more readily visualized by discussithe functions separately.Current Sharing with the UC1907

The UC1907 Load Share Regulator takes aumatic master / slave selection one step further replacing the diode described above with a unidiretional Buffer Amplifier as shown in the simplifiesketch of Figure 10. In theory, this addition coueliminate the sharing error caused by the diodevoltage drop, and deliver perfect current sharing. practice, however, this might also result in a costant hunting as modules with coincidently equreferences could fight for position as master. Tsolution chosen for the UC1907, which is mospecifically illustrated in Figure 11, is to add50mY offset to the Adjust Amplifier (as comparto the 700mY of a diode). This offset acts hysteresis to insure a positive selection of a mastThe amount of output current unbalance can determined by dividing the 50 mY offset by tgain of the current monitor circuit which in t

voltages will be adjusted upwards, increasing theiroutput currents until the voltage at the output of thecurrent monitor equals that on the Share Bus.The forward drop of the master's diode in thissystem represents an error in current matchingbetween modules. The magnitude of the error is theratio of the diode voltage drop to the Share Busvoltage and it represents the maximum deviationbetween the master and any of the slaves. A faulton the Share Bus will now only hold all the AdjustAmplifiers low, leaving all the references un-changed. While forced sharing will be defeated, allthe modules will still operate within their toleranceswith sharing determined by their individual currentlimits.The UC1907 Load Share Regulator

From the above discussion, it would appear thatproviding good load sharing capability could add asignificant degree of complexity to power supplydesign. To ease this task, an integrated circuitdesignated the UC1907 has been developed to com-bine both the voltage regulating and current sharingfunctions into a single device. The UC1907 wouldnormally be built into each power module andapplied as shown in Figure 8 to sense its ownmodule output current and the output voltagedirectly at the load. For maximum accuracy, differ-ential sensing is used for both measurements.Other

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(+)SENSE 1!:!1 ~

VccI , 175V ~ COMP

~ 10V DrivoAmp OPTO.. 9 DRIVE2OK 8 ISET

5OK

Vcc(45VTO35V) 1!::21

IK li~.~~VREF1.750V0250V

-=-tr~ -

(-)SENSE 0--. G,,",nciPONER RTN 5 Amp

ARTIACIALGND 0 ;

VREF 0 ,

C/SOUT

rent, only the onein the modulewith the highestsensed currentwill be active,serving as a lowimpedance driverADJ UT for the Current

5()nV Share Bus. All~t=-@ ADJ(-) others will beinactive with eachexhibiting a lOk.Qload impedanceto ground at theBus. While themaster modulehas the onlyactive BufferAmplifier, its Ad-just Amplifier isheld inactive at zero. The Adjust Amplifiers in the

slave modules are all active, using the Share Busvoltage to increase their references to the appro-priate value.The Adjust Amplifier is a transconductancecircuit, allowing its bandwidth to be limited, andnoise decoupled from the reference adjust loop,with a single capacitor to ground at its output. Thevalue of the capacitor can be determined from

Cl=~ 21tfWhere = desiredbandwidthand ~ = 3 mSiemensorthe UCI907.As an added feature, since the master-and onlythe master--will have its AdjustAmplifier driven into low-side

saturation, this can be detected andused to activate a flag output iden-tifying which module is acting asmaster. This open-collector outputhas adequate current capabilities foran LED or lamp indicator, as wellas serving as a logic output.

A t Amp

-

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.'V

.~ ~

Vcc ~ ,if ~ 16Mastor ,In(jcate : -

c: =~3 5~V, -.I I.

~40KHa: IILt I~ ~

i.

I !I ., I

L+! ~CIII.Cllront-i!I--oo ShareI BusII.

LJ10KAIT1'Load :

Qxrent:Sense: ,

UC3907 ,,' !

Fig 11. -Current Share Components within the UC1907Adjusting the reference: Before discussing thereference, there are some aspects of the UCl907which need to be clarified with respect to ground-ing. Practical power systems must address the issueof undefined resistance in series with both the feedand return lines between the supply and the load.For accurate load voltage control, high-impedanceKelvin sense inputs are an important part of thevoltage control loop. In the UCl907, these areprovided by the Voltage and Ground Amplifierswhich can measure the load voltage differentially atthe point of use. To allow the Ground Amplifierheadroom so that it can provide effective systemground sensing, an arbitrary low-impedance refer-ence level is established within the UCl907 whichis exactly 250 m V above the load ground. This iscalled an Artificial Ground and it serves as a returnpoint for all the currents associated with the opera-

I!:!](4!Son.. ~Vccrof R.,ge200to210V

rREF1 ~~ I r;;;;:;--~J~'1.75V-~~~

I

tion of the UC1907. To accommdate undefmed voltage drops in tsystem power return line, tUCI907 supply return is connectclosest to the supply, at the monegative end of the return linThese three different "grounreferences within the UCI907 ashown in Figure 12 and differenated as follows:

Negative Sense (Symbol: * )This is the high-impedance pintended for remote sensing of tload or system ground, bypassiany voltage drops which migappear in the power return line. It is the input

the Ground Amplifier and should be considered the "true" ground. Unless otherwise stated, all voage measurements will be referenced to this poin

Artificial Ground (Symbol: V) -This islow-impedance reference point which is exactly 25millivolts more positive than the (-) Sense erminaThis offset allows the Ground Amplifier to diveall the control bias and operating cun-ents awfrom the high impedance at the (-) Sense input. also serves as a point to return all external resistoand compensating capacitors associated with thoperation of the UC1907.

Power Return (Symbol: nI7 ) -This should the most negative voltage available and can ranfrom zero to -5V below the (-) Sense terminal. should be connected as close to the power source

possible so that voltage dropacross the return line and the curent sensing resistance lie betweethis terminal and the ( -) Senpoint.

With this understanding, we caF now look at Figure 12 and recog-rom~~ont nize that while the reference vol

age is 1.75V with respect to artif;o.r:'. cial ground, it is actually 2.00Bu. with respect to the load. The adjusment range for cuITent sharing mube enough to encompass all thtolerances associated with settin

2~V~~

"'f-FGround GroundSen.. Reun(TrueGround) (M(t Neg)

~--rr- 14~-~-R2

'175KGND(+25OmV)

Fig 12. -Reference Adjustment in the UC1907

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the output voltage. In the UC1907, reference accu-racy is %1.5% (30mV) and the adjustment range isset at 5% (loomV). The adjustment is accomplishedby adding current to R1 to create a voltage dropwhich is defined by the output of the Adjust Ampli-fier and resistor R2. Q3 clamps the Adjust Amplifi-er to limit the voltage across R2 to 1.75 V andtherefore, with R1 = 1k, the maximum adjustvoltage is limited to 100mV.Voltage Control with the UCl907

Figure 13 shows the elements within the UC1907associated with closing an overall feedback loop toregulate the power supply output voltage. In addi-tion to the adjustable Reference and the GroundAmplifier discussed above, this circuit contains aVoltage Amplifier intended to serve as a high-gainerror amplifier, and a fixed-gain Drive Amplifierincluded to ease interfacing with the power stage.With most of the gain in the Voltage Amplifier, thisis the place to provide the frequency compensation

necessary to stabilize the loop. The Drive Amplifiercan be used as either a f1Xed-gain voltage buffer ora voltage-to-current transconductance stage with thecurrent established externally by RSET.This config-uration is particularly appropriate for isolated powersupplies where an optocoupler will be used totransmit the feedback control across an isolationboundary .The polarity of this control loop is suchthat increasing the sense voltage increases theoptocoupler current, a requirement for starting anisolated power supply.Figure 13 also shows the usual method forpowering the UC1907, as long as the supply'soutput voltage is five volts or above. Note thawhile the voltage sense ines are normally connect-ed as close to the load as possible, the controller'ssupply is taken from close to the power stage suchthat line drops, the current sensing resistor, andblocking diode (if present) all tend to raise the chipsupply voltage, rather than reduce it.

/.SupplyPowerOutputStage

I LOADAs

CurrentSense R3Voltage LoopCompensationowerReturn Vcc

1- ---G ]- "11II

Vre 1III,,", I

[II~ II

--i

Or"

InternalBias

T75V)~~

1.0V

firR4

~ :~ Sense~I

Voltage Am"p" l :I(1:)-+-Oto100mv Ifrom Share Loop II.REF 171

j 250mV

1~~

GrOUn~

2OK

I REF IL.lZYJ

~ -solation .for Primary : D .-

Side Control I ArlV8pI8I 5OKIIII UC3907 AnilicialGND

Fig 13. -Voltage Control Loop Elements in the UC1907

~4j---vsetL,

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Application IssuesSystem start-up: Since the current share loop willnormally have a bandwidth limited to somethingless than 500Hz, there are dynamic factors toconsider in designing a multiple-module powersystem. The addition of CI to the Adjust Amplifierwill limit its slew rate to the value defmed by itsmaximum output current of 200uA. Since thevoltage feedback loop will probably be significantlyfaster, the start-up characteristics shown in Figure14 will be typical. These curves assume fourparalleled modules with nominal references lOrnVapart, ranging from 1.98V on module #4 to 2.OIVfor module #I. It is further assumed that the load islight enough that current limiting does not comeinto play. Under these conditions, at TO the loadvoltage and current rise with the speed of the

voltage control loop until the module with thighest reference regulates the output at TI. At thtime, the other three modules with lower referencare contributing nothing and the full load currentcoming from module # I. During this time, tAdjust Amplifiers in modules #2, 3, and 4 aresponding but with a lesser slew rate. At 1'2 tinternal threshold of the reference adjust circuitry reached and the references on the lower-outpmodules begin to rise. At T3, module #2 matchmodule #1 and it picks up half the load currendropping the module #1 current to 50%. At T4 aT5 the other modules have completed their adjument and sharing is complete at T5 with each oconducting 25%.The point of all this is to show that at turn-oone module may initially attempt to supply tentire load current and, if the load were heaver, into current limiting. This would no problem unless the modules a

equipped with over-current shudown which might prevent tsystem from starting as differeunits alternately start up and shdown. Several solutions are posble: A faster current adjust loo

25% (although stabilization may difficult), a soft-start in the powstage to slow the initial voltagramp-up to less than the AdjuAmplifier slew rate, or a delay the over-current shutdown functio

SO2SVVout

OVVouVRL

LoadCurrents '50%---33%r#4

OA2010V

Voltage 2000VA~ (+) 1990VUC3907 1980V

ov2.7V

AdjustAmpOutUC3907

MaxOutnut=2.7V## #4 = 1 53V

#3 = 135V= 118V

Adjust Threshold = 1.0V#I = 07V

ovTO TI 12 13 14

TimeT5

Fig 14. -Start-up Timing, 4-Module System (without soft-start orcurrent limiting).Load Sharing with Paralleled Power Supplies 2-

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~jillP.-,

Tran~nTIer

ILoadlCo

D2~ -~R1 ~, .~ ;;nent3 A"1'

R2C2.Rcs

CunentTransfcxmer

the UC1907 was designed

ng resistor, in highapplications these resistive

are less desirable. In switch-feasible to current sense ahead of

output rectifiers, using a cUITentsignal with minimal power Fig 15. -Current Sense Transformer and Averaging Circuit

Figure 15 shows a way Eliminates High Power Current Shuntng this with a for-

voltage waveform across Rcs. Assuming that

ill follow the rising inductor current during then-time of the power switches. During the off-time,2 discharges slowly through R2 to approximatethe down-slope of inductor current. In this manner,the average voltage on C2 can be made equivalentto the average load current through the inductor.An Off-Line Load Share Application

A possible application for the UC1907 in an off-line, isolated flyback design is shown in Figure 16.Here all the control intelligence is provided by theUC1907 on the secondaryside of the power trans-former. An optocouplerpasses he control informa-tion over to the primaryside pulse-width modula-tor. This modulator couldbe implemented with anyof a variety of approaches,including either voltage orcurrent mode control. Notethat in this application, theReference, Voltage Ampli-fier, and Opto-Driverwould be needed underany circumstances and theUC1907 merely addscUITent sharing.

0-I~tLO..0- ~ca -""-~.i " :;;

x y -C--.~

~ I~~v"'

r:ri~ :v: .

BuW 15 cu"..,1UC3907 : : SI.. Bu.1

, r1v' \;~ E,,

,,,

x--W---y --ff'--- i3>-

UC1907 Load Sharing with Off-Line, Isolated Supplyig 16.UNITRODE CORPORATION-10

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Vo~

~,,-.tTTTT"I .~ I-.-J~

-v I01

Q ~~;-ls.Wlyl

, [ ~ fi-Vcc... 2OK

IIIII

21 ,rTI! .

III

T ~M.-~--J Var~,II Ref

,RI

Fig 17. -UC1907 Output Amplified by Q1 Provides Reset Current for MagAmp Regulator

Load Sharing with Mag Amp ControlThe use of a saturable reactor, or mag amp, is a

popular approach for power control where thepower source is an AC square-wave or pulse-widthmodulated supply. Controlling a mag amp is ac-complished by varying its reset cun-ent which, asshown in Figure 17, is done by driving QI with theoutput from the Drive Amplifier. Here again, theUC1907 provides all the regulating circuitry whileadding current sharing. Note that in this application,the control is self-powered, using the module'sregulated output as a source of power for bothcontrol bias and the mag amp reset cun-ent. Toprovide short-circuit current limiting for a magamp, an auxiliary power source would be required.

Load Sharing with Paralleled Power Supplies 2-11

'"T"m-

: ~II "X ~ ---~ ~ '15l---2OX B~ l.:;I-~rrenly 2 l.!C~~QI-: "Share Bus

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Fig 18. -Non-lsolated DCIDC Converter with Load Sharing

Non-lsolated DC-to-DC ConverterApplicationsWith a non-isolated source providing power formultiple modules, a11 riving the same load, currentsensing must be done on the high side as shown inFigure 18. The reason for this is that with a com-mon source--and a common load-all the sense esis-tors in the return path would effectively be inparallel, defeating their ability to monitor individualmodule currents. The only limitation which theUCl907 presents to this configuration is that thecurrent amplifier has a maximum common-moderange ofVcc-2V which must be accommodated byeither raising the supply voltage to the controller-as shown in Figure 18-or level-shifting the currentsense voltage.

Since an optocoupler is not required, the feed-back signal from the Voltage Amplifier in theUC1907 is taken from the lset pin of the DriveAmplifier. With two inversions between the voltagesense input and the ISET pin, another reversal isrequired and is provided by the error amplifierwithin the UC1524A. This amplifier is set for unityor low gain inversion to be compatible with the 0.3to 4.0V range of the ISET terminal.

In this application, the same sense resistor usedfor current sharing is also shown providing informa-tion for current limiting of the PWM controller. TheCurrent Amplifier provides both a fixed gain oftwenty and level shifting to a ground reference.

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A Linear Regulator ExampleFigure 19 shows the use of the UCl907 as aload sharing controller for a linear regulator. The

only added components are a Darlington powerstage, plus a transistor which uses the CurrentAmplifier output to provide current limiting inaddition to its primary function of load sharing. Asthis is another non-isolated example, current sensingmust be done again in the positive power line andcommon-mode range considerations must be made.

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Power (.)Sense (.) RI40na1N2222

R2r

350i

.,~Load~

StandardPowerSupply

ACIN

I \---,ense ( -)Power (-): f'i

I -.

~I x yI01- 112'" ,

'EJJ+-Vcc ~~~ Chip ~""~19~ JV~

~50K'V'-"'-

VarRef

&IIII1141

I :i3>- ~I $---icurrenlUC3907 I: Share usI '

Fig 20. -Load Sharing Added to Existing Power Modules

x [!Jy~

Load Sharing to Existing PowerWhile the UC1907 is most efficiently utilized by

it into the initial design of a powerexisting modules as long as provisions for

have been provided. In this applica-as the voltage loop is already closed

the supply. However, current is sensed andAdjust Amplifier is used to drive added transis-providing a variable voltage drop across Rl.

accomplishes the same function as varying thevoltage. Thus, the UCl907 can be used as

a built-in load sharing feature.

References:[I] Walter J. Hirshberg, "Current Sharing of

Paralleled Power Supplies," PESC Proceed-ings, October 1985

[2] Kenneth T. Small, "Single Wire Current ShareParalleling," U.S. Patent No. 4,717,833

[3] Mark Jordan, "Load Share IC SimplifiesParallel Power Supply Design," PCIM Pro-ceedings, September 1991.

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