1、THE UC3902 LOAD SHARECONTROLLER AND ITS PERFORMANCEIN DISTRIBUTED POWER SYSTEMSAPPLICATION NOTE U-163Laszlo BaloghUnitrode CorporationTHE UC3902 LOAD SHARE CONTROLLER AND ITS PERFORMANCE IN DISTRIBUTED POWER SYSTEMSby Laszlo BaloghUnitrode CorporationAPPLICATION NOTE U-163UNITRODE CORPORATIONUC3902
2、APPLICATIONSThe current demanded by large electronic systemscontinuously rises due to higher performance andincreased functionality. At the same time, supplyvoltages, especially in digital circuits, are falling topreviously unprecedented low levels. The combination of higher load currents and low su
3、pplyvoltages impose difficult requirements on the powerdistribution and in most cases, forces a higher voltage distribution bus with local voltageconversion.Users of distributed power systems are seekingreliable and economic solutions to supply theirloads. A modular approach, where stand alonepower
4、supplies are utilized for local power conver-sion, is widely accepted in these applications.Thepower rating of the individual converters are usu-ally limited by the available height, board spaceand by the limitation on volumetric heat dissipation. High current loads are supplied by several parallel
5、connected power supplies. In addition to higher currents, the parallel modulescan also offer redundancy, an important factor toachieve reliable, uninterrupted operation andextended life expectancy in these systems.Parallel connected power supplies require a dedicated control mechanism, called load s
6、harecircuitry to assure full utilization of the system.Thepurpose of a load share controller, like UnitrodesUC3902, is to provide for equal distribution of theload current among the parallel connected powersupplies. By equalizing the output currents, uniformthermal stress of the individual modules i
7、s alsoensured which has the utmost importance for longterm reliability of electronic components.For the UC3902 to work properly, the modularpower system has to consist of power supplieswhich possess their own feedback circuits.Furthermore, the stand alone modules have to beequipped with true remote
8、sense capability or withan output voltage adjustment terminal. Each module must have its own load share controller.The operating principle of a load share mechanismis to measure the output current of each individualmodule and to be able to modify the output voltageof the units until all participatin
9、g power suppliesdeliver equal output currents. It is accomplished bythe UC3902 integrated circuits which are connect-ed to the common load share bus and adjust thepositive sense voltage (or the voltage of the outputvoltage adjust pin) of their respective modules toprovide equal load current sharing.
10、UC3902 BLOCK DIAGRAMThe UC3902 is an 8-pin integrated circuit. Its solepurpose is to provide the load sharing function forexisting power supplies.The chip incorporates onlythe building blocks required for load sharing. Tightregulation of the converters output voltage is notanticipated with this circ
11、uit.In fact, the UC3902 per-forms just the opposite task by finely adjusting theregulated output voltage of the converters to matchall output currents evenly.U-163APPLICATION NOTE2Figure 1. UC3902 Block DiagramA unique advantage of the UC3902 is its differentialload share bus. This architecture grea
12、tly enhancesthe noise immunity of the system. Figure 1 showsthe block diagram of the load share controller.The IC consists of a current sense amplifier, ashare bus driver and sense amplifier pair, an erroramplifier, a buffer stage called adjust amplifier, andhousekeeping circuit which provides inter
13、nal biasand the on-chip reference for the circuit.The UC3902 has an inverting current sense amplifierwith the gain of 40.The output of the current senseamplifier is proportional to the output current of thepower supply it is connected to.It provides the inputsignals to the load share bus driver ampl
14、ifier and tothe inverting input of the error amplifier. Becausethe share driver amplifier is configured for unitygain, the voltage on the output of the share driveramplifier also equals the output voltage of the cur-rent sense amplifier. When this voltage is the high-est potential among the parallel
15、 connected loadshare controllers, i.e. the unit delivers the highestoutput current, the module acts as the master unit.The output of the share driver amplifier of the mas-ter also determines the voltage on the share bus. Inthe case, where this voltage is lower then the sharebus voltage, i.e. the mod
16、ule supplies less currentthan any one of the other units in the system, theshare controller will behave as a slave. The outputof the share driver amplifier of the slave controllersis disconnected from the share bus by the diodes inseries with their respective outputs.The share sense amplifier measur
17、es the voltage onthe differential share bus and provides the signalfor the noninverting input of the error amplifier.Similar to the share driver amplifier, this circuit hasa gain of 1 which is fixed internally. Consequently,the output voltage of the share sense amplifieralways corresponds to the hig
18、hest output currentlevel, delivered by the master unit in the system.A transconductance amplifier is utilized for the erroramplifier function in the UC3902. If a feedback network were connected between the output and theinverting input of the error amplifier, the output currentrepresentation would b
19、e inaccurate. The transcon-ductance amplifier allows the feedback network tobe connected from the amplifier output to the groundwhich preserves the authenticity of the current signalat the inverting input of the error amplifier. Also notethat this type of amplifier requires a voltage differencebetwe
20、en its inverting and noninverting inputs.Furthermore, the transconductance amplifier hashigh input and output impedances. Instead of theusual low impedance voltage source output ofoperational amplifiers, this circuit has a high imped-ance current source output. Accordingly, the gain isdefined as tra
21、nsconductance (A/V) but it can beconverted back to the general V/V gain term byUDG-95133U-163APPLICATION NOTEmultiplying the transconductance of the amplifier(Gm) with the impedance of the compensation network (XCOMP).The steady state output voltage of the erroramplifier is the function of the volta
22、ge differencebetween the outputs of the current sense and theshare sense amplifiers. When a particular controller works as the master in the system thisvoltage difference is zero. To guarantee the correctstate of the error amplifier, a 50mV offset is insert-ed in series with the inverting input. Thi
23、s artificialoffset will ensure zero volts at the output of theerror amplifier when the unit is the master module.All slave controllers will develop a non zero error voltage at the output of the transconductanceamplifier which is proportional to the differencebetween the output current of the respect
24、ive powersupply and the output current value of the mastermodule represented on the share bus.The output voltage of the error amplifier is used toadjust the output voltage of the power converter tobalance the load current among the parallel connected modules. This is done by the adjust amplifier and
25、 its companion NPN transistor. Theadjust amplifier provides signal conditioning for theerror signal and its output drives a NPN transistorwhich is configured as a programmable currentsource.A resistor from its emitter to ground and theerror voltage defines the current, IADJ, which flowsthrough a res
26、istor connected between the ADJ pinof the load-share controller and the positive outputterminal. The IADJcurrent causes a voltage dropacross the resistor which requires the power supply3Figure 2. Typical Applicationto increase its output voltage. The resulting higherpower supply output voltage incre
27、ases the outputcurrent of that particular module until the outputcurrent levels equal out among the units. At thatpoint load sharing has been established.UC3902 LOAD SHARE CONTROLLER DESIGNThe UC3902 load share controller requires only afew external components. Before the values ofthese components c
28、an be calculated, the DC/DCconverter module parameters must be reviewedand some application specific limits have to bedetermined. There are three parameters whichmust be known; VO,NOMis the nominal output voltage of the converter, IO,MAXis the maximumcurrent delivered by the unit and VO,MAXis themax
29、imum adjustment range of the output voltage.The output voltage adjustment can be accomplishedby using either the positive sense terminal or theadjust pin of the DC/DC converters and it is usuallylimited by the design of the modules. Depending onthe output current level, the allocated voltage dropand
30、 the allowed power dissipation, the currentsense resistor can be selected. Since the UC3902is operational from 2.7V to 20V, in most cases theIC is powered directly from the output voltage of thesystem. If that is not possible because of an inappropriate value of the output voltage, the supply voltag
31、e, VCC, has to be determined as well.SETTING UP THE DC OPERATING POINTFigure 2 shows a typical application of the UC3902load share controller. This circuit must be repeatedUDG-962454U-163APPLICATION NOTEfor each power supply sharing the current for acommon load in the system. The design processstart
32、s with calculating the component values forsetting up the basic operating conditions for the IC.In order to precisely share the load current amongparallel connected modules, the output current ofeach individual power supply has to be measured.A current sense resistor, RSENSEis placed in thenegative
33、return path of the units.Choosing RSENSEis based on two factors; the maximum power dissipation and the maximum voltage drop acrossthe sense resistor. While the power dissipation isonly limited by practical considerations like efficiencyand component ratings, the maximum voltage drophas to comply wit
34、h internal signal level limits of theintegrated circuit. Most important is to prevent theoutput of the current sense amplifier from saturation.The highest voltage of the amplifier output, VCSAOis a function of VCC. It equals either 10V or VCC- 1.5V, whichever is lower. Consequently,VSENSE, MAX=where
35、 VSENSE,MAXis the maximum voltage dropacross the current sense resistor, VCSAOis themaximum voltage of the current sense amplifieroutput and ACSAis 40, the gain of the currentsense amplifier.Once VSENSE,MAXis determined, the currentsense resistor value can be calculated by the following formula:RSEN
36、SE=IADJ,MAX, the maximum current of the adjustamplifier is 10mA as specified in the datasheet. Itis a good practice to keep the highest current ofthe adjust amplifier in the 5mA to 10mA range aslower values might increase the noise sensitivity ofthe system. This current is determined by the highest
37、possible voltage on the ADJR pin which is2.6V and by the resistor, RG, between the ADJRpin and ground. Thus the resistor value can becalculated as:RG=RADJis the impedance inserted in the positivesense line of the power supply. Its value is thefunction of the maximum adjustment range of the2.6VIADJ,M
38、AXVSENSE,MAXIO,MAXVCSAOACSAoutput voltage, VO,MAXand the previously select-ed IADJ,MAXcurrent value. Since the voltage dropacross the current sense resistor reduces therange available for output voltage adjustment, thatfactor has to be accounted for as shown in theequation for RADJbelow:RADJ=This re
39、lationship points out an important designconstraint for the system. The sum of the voltagedrops across the wiring impedances and the voltagedrop across the current sense resistor must be significantly lower than VO,MAXor the load sharemechanism has no headroom to adjust the outputvoltage of the modu
40、les to achieve proper distributionof the load current.CLOSING THE SHARE LOOPBalanced distribution of the load currents amongthe parallel connected units is accomplished by anadditional control loop provided only for the loadshare function. Similarly to other control loopsinside the power supplies, t
41、he current share loop isbased on negative feedback. Such control loopsmust obey certain stability criteria in order to workproperly. Although this Application Note does notcover the theoretical background of loop stability,some of the most critical conditions for successfullyclosing the load share l
42、oop will be pointed out.Since the load share loop is added to existingpower supplies, interaction between the existingvoltage control loop and the share loop must beavoided. For that reason the crossover frequenciesof the two loops must be well separated.The voltage loop crossover frequency, 0,Vis g
43、reat-ly dictated by the required transient response of theconverter. In order to maintain the stability of thevoltage control loop, the share loop has to be set upnot to cause any excess phase shift at 0,V.This isusually achieved by placing the crossover frequen-cy of the share loop (0,S) at least o
44、ne, but prefer-ably two decades lower than the crossover fre-quency of the voltage loop. Also a zero should beplaced in the transfer function at 0,S.This way, theeffect of the share loop is minimized at thecrossover frequency of the voltage loop.Closing the load share loop at low crossover frequency
45、 is acceptable, if the purpose of load sharing is clarified. The primary concern for loadsharing is to extend the life expectancy of the system and to increase reliability. It is achieved inthe system by paralleling several modules andVO,MAX IO,MAX RSENSEIADJ,MAXU-163APPLICATION NOTE5assuring that t
46、heir thermal stresses are balancedduring the life of the product. Another advantage insuch systems is the possibility for redundancy.These goals can be completely fulfilled by slow acting corrective measures which allow 0,Sto besignificantly lower than 0,V.The first step is to determine the transfer
47、 functionsof the individual building blocks for the frequencyrange from 0.1Hz to approximately 1kHz.The unitygain crossover frequency of the load share controlloop is expected to be within these frequency limits. If off-the-shelf power supplies are beingused, then a network analyzer should be used t
48、omeasure the transfer function. This is also a goodpractice to confirm actual versus calculated performance as well.The valid transfer functions for the range of ourinterest are: APWR(s): the transfer function of the powersupply, measured by injecting an AC signalbetween VSENSE+and VOUT+terminals. A
49、VoVis: this gain term describes the relationship between the output voltage andthe voltage across the current sense resistor.It varies with the load impedance according to AVoVis = ACSA: the gain of the current sense amplifier.This term is a constant and equals 40. ASHA: both amplifiers driving and sensing thesignal on the differential share bus are configured for unity gain. (ASHA= 1) AEA(s): the gain of the error amplifier defined as: AEA(s) = Gm XCOMP(s) where Gm is the transconductance of the error amplifier andXCOMP(s) is the impedance of the compensationcomponent
