Citterio, M

Kierstead, J A

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ARE SWITCHING POWER SUPPLIES ACCEPTABLE FOR THELIQUID ARGON CALORIMETER FRONT-END ELECTRONICS?*M. Citterio, J. Kierstead (email: citterio@bnl.gov)Brookhaven National Laboratory, Upton, NY 11973-5000AbstractUsing high power density DC-DC converters followedby linear ripple attenuators could satisfy the powerrequirements of the Liquid Argon (LAr) calorimeterfront-end electronics. A solution based on resonantcharging DC-DC converters is discussed in term of noisecharacteristics, radiation and magnetic field tolerance,power efficiency and reliability.1.  INTRODUCTIONThe Liquid Argon (LAr) calorimeter system to beused in the ATLAS experiment has complex readoutarchitecture composed of almost 190,000 high precision,high dynamic and low noise electronic channels. Theglobal layout of the detector is shown in Figure 1. Thecentral cryostat contains the barrel electromagneticcalorimeter (EM Accordion). Each end-cap cryostathouses an electromagnetic calorimeter (EM Accordion),two hadronic end-cap calorimeters and one forwardcalorimeter.Figure 1. Three dimensional view of the LArcalorimeter system and the location of the front-endelectronicsFigure 1 shows that the front-end electronics of theLAr calorimeter system is housed on the detectorcryostats, in 64 “crates” attached to the cold to warmfeedthroughs, in the crack between the barrel and end-cap calorimeters and at the rear of the end-caps.*The U.S. Department of Energy supports this work: ContractNo. DE-AC02-98CH10886.The front-end electronics, which process and preparethe detector signal for the “off detector” data acquisitionsystem, is basically composed of preamplifiers, shapers,pipeline memory, digitisation and digital filtering.Different front-end crates are used for the three-calorimeter types.  The majority of the crates serve theEM calorimeters. A typical crate contains 3584 readoutchannels densely packed in various boards. The powersupply requirements for an EM crate are summarised inTable 1.The large power consumption per crate, about 2.8 kW,combined with the limited space available in the detectorarea for the distribution of multiple low voltage powerlines, jeopardise the use of linear power supplies locatedin the control rooms, i.e. far away from the detector, infavour of a solution which employs compact switchingpower supplies located in proximity of the front-endcrate.Table 1: EM crate power requirementsPin Voltage [V] Current [A]1,6,10 ground2 + 6 1803 + 11 94 + 6 245 + 6 907 + 4 1058 - 4 1509 - 6 122. “ON-DETECTOR” POWER SUPPLIESLinear power supplies, which have been the traditionalpower sources in high energy physics experiments due totheir stability and low output noise characteristics, havebeen generally mounted remotely from the detectorlocation, in areas with no physical space and radiationenvironment constraints. This solution is based on theassumption that low or medium power must bedistributed and/or that enough space is available for thepower cables in the experimental area. Both assumptionsare false for the Atlas LAr calorimeter system. Withremote power supplies, the high power and the lowvoltages needed by the LAr read-out electronics, willrequire the use of massive water-cooled power cableswhich will not fit in the service area available for thedetector. A different power supply system must beenvisioned for the detector.Calorimeters CalorimetersCalorimetersHadronic Tile EM AccordionForward LArCalorimetersHadronic LAr End CapFront-EndElectronicsThe diameter of the power lines could be substantiallyreduced if high voltage lines are run from the controlrooms down to the detector region, where the conversionfrom high to low voltage lines could take place. Thepower supply system could be indeed composed of twoparts: a source of high voltage power to be locatedremotely and a source of low voltage power mounted“on-detector”, i.e. as close as possible to the front-endcrates. In this solution each electronic crate will have adedicated low voltage power supply properlydimensioned to fit into the space available.The high voltage supply can be implemented by usingstandard commercial supplies, while the low voltagepower source needs to be specifically designed for theAtlas LAr requirements. The “on-detector” supply willnot be easily accessible during the experiment andconsequently must be extremely reliable with built-inredundancy and provision for external monitor andcontrols. Moreover the supply should have low outputnoise, operate in magnetic field and be radiation tolerant.High power density and efficiency are also important.Table 2 summarises the power supply specification.Commercially available low noise DC-DC converterscan in principle satisfy most of the above requirements.Few converters are able to operate in magnetic andradiation environments.DC-DC converter modules made by VICORCorporation, Andover, MA 01810, USA, have beenmeasured against specification and the results aresummarised in the following section.Table 1: Power supply specificationInput Voltage 300 VDC, with ripple +/- 10%Output V/I See Table 1Output ripple < 5 mV p-p @ 80% loadEnvironment 20 Gy/year and 1012 n/cm2yearB < 200 gaussRedundancy On each Voltage outputEfficiency > 75% at 80% loadCooling Liquid – T operation  ~ 25 oCDimensions 350 x 360 x 130 mm33.  CONVERTER CHARACTERISTICS3.1  Zero-Current-SwitchingVICOR switching power supply modules make use ofa patented resonant charging mechanism, called zero-current-switching (ZCS)[1]. Current flows through theMOSFET switch as half sine wave loops. The current isvirtually zero both when the switch is closed and when itis opened. This result in a much less high frequencyelectromagnetic interference (EMI) than withconventional, fixed frequency switching power supplies.Any residual differential high frequency interference iseasily removed by passive filtering.VICOR converters can operate at frequencies inexcess of 1MHz, with efficiencies greater than 80% andpower densities of up to 7.3 W/cm3. Two different seriesof VICOR DC-DC converter modules, with nominalinput voltage of 300V, have been tested: the VI-200series, or 1st generation modules, and the VI-300 seriesalso called “2nd generation”. Both converter familiesshare some common features such as availability ofvarious standard programmable output voltages, stableregulation, current limit, internal overvoltage andovertemperature protection. All modules are parallelablewith N+M fault tolerance and current sharing, and arephased-array-control compatible.The second-generation modules contain a completeredesign of the control, magnetic, switching andpackaging elements. The modules have also one-thirdthe number of parts of the previous series [2].The tested modules have output voltages from 5 to 12VDC, similar to the ones of interest for the LAr powersupply.  For the VI-200 series the maxi size moduleswere tested, for the VI-300 series the maxi and mini sizemodules were tested.3.2  Noise CharacteristicsThe virtually loss-less energy transfer from input tooutput achieved with the zero-current-switchingtechnology greatly reduces the conducted and theradiated noise. The typical peak-to-peak (p-p) ripplenoise of the VICOR converters for the output voltages ofinterest is ~ 100 mV on a 20 MHz bandwidth.A large fraction of this noise concentrates above10 MHz [3]. If a filter, the RAM module made byVICOR or an equivalent one, is added to the output ofthe converter, the noise reduces to less than 3 mV p-p.However questions remain to be solved in term ofcoherent noise contribution to the front-end electronics(FEB) noise due to the power supply. The issues to beproved are:- Can the common mode high frequency switchingsignal be diverted from the input cables andpreamplifiers on the FEB?- Can the differential modulation at the switchingfrequency be reduced to a level where its noisecontribution is negligible?Measurements made using a VI-200 switching powersupply on the most sensitive FEB supply, have shownthat the coherent noise is ~ 56 µV rms compared to arandom noise of 2.1 mV. The dominant coherent noise isa low frequency, almost sinusoidal ripple.The additional low-voltage dropout regulators,foreseen for the FEB on-board filters, will virtuallyeliminate the already small residual ripple. No increasein coherent noise was measured by replacing linearpower supplies with VICOR VI-200 DC-DC convertersmounted in a MEGAPAC unit and followed by outputripple attenuators. A 0.5% noise degradation wasmeasured with the ripple attenuators filter bypassed.Similar results should be expected for the VI-300converter family.3.3  Magnetic Field SensitivityUnits from the VI-200 and VI-300 series wereoperated at Brookhaven National Laboratory in auniform magnetic field adjustable from zero to 103gauss.The parameters under measurements were the outputvoltage and the switching frequency of the device undertest. The temperature of operation of the module waskept constant at ~ 50oC during the test through properair-cooling. The results were identical for both familiesof converters and are illustrated in Figure 2 and 3 for aVI-300 module with 5V output.Figure 2. Magnetic field sensitivity with a fieldperpendicolar to the converter module width.Figure 3. Magnetic field sensitivity with a fieldparallel to the converter module width.The performance of the converter depends from therelative orientation between the magnetic field and themodule under test. Figure 2 illustrates that, when themagnetic field is orthogonal to the width of the module,the converter operates with minor variation in outputvoltage and switching frequency, up to 520 gauss. Athigher field the unit shuts off and does not restart untilthe magnetic field is decreased below the criticalintensity. However the worst condition happens whenthe applied field is parallel to the width of the module.As shown in Figure 3, the converter operates correctlyup to approximately 180 gauss, when it suddenly stopsfunctioning. The failure is irreversible. A failure analysisperformed by VICOR engineers found that one of thereset transistors of the unit was damaged by an excessivecurrent flowing in the transistor during the main switch’soff time, the increase in current was very likely causedby the saturation of the main transformer core. Therelative orientation between the magnetic flux confinedin the transformer core and the external magnetic fieldcould also explain the observed not-isotrope behaviourof the converter. The magnetic field lines interfere withthe operation of the transformers inside the VICORmodule mostly when they are parallel to the transformercores and if all cores are oriented in the same direction,such as in this case.3.4  Radiation Tolerance to NeutronsConverters from both VICOR families have beenirradiated with 1 MeV neutrons at the University ofMassachusetts, Lowell, USA. The parameters measuredduring the test were again the output voltage and theswitching frequency of the converter. Through properair-cooling, the module temperature was kept constant at~ 50oC during the test. The results were very differentfor the two families of converters.The VI-200 devices showed degradation almostimmediately and failed in approximately 3 hours, at atotal fluence of ~ 3x1011 n/cm2. A second unit wassubsequently irradiated at ten time lower fluence toexclude a fluence-dependent behaviour. This second unitalso failed at the same total fluence. The damage was inboth cases irreversible.Failure analysis made by VICOR on the irradiated unitfound that only the opto-coupler circuit in the convertershad failed. Replacing the damaged opto-coupler with anew one successfully repaired both units. The standardotpo-couplers used in the VI-200 units are commercialcircuits, with no special precautions taken to make themrad-tolerant.Neutron irradiation results were very different for the“2nd generation” VI-300 converters. As illustrated inFigure 4, for a VI-300 module with 5V output, thisfamily of devices were able to withstand up to~ 1.3x1013 n/cm2 without noticeable degradations. The05101520250123450 100 200 300 400 500 600Vicor  2nd gen. M odule5  V olt/300 W att(Iload ~ 5  A , T  ~ 50 C )Switching frequency [kHz]Vout [Volt]Switching frequency [kHz]Vout [Volt]Magnetic Field [Gauss]05010015020002468100 100 200 300 400 500 600Vicor 2nd gen. M odule5 Volt / 300 W att(Iload~ 20 A, T ~ 50 C)Switching frequency [kHz]Vout [Volt]Switching frequency [kHz]Vout [Volt]Magnetic Field [Gauss]control units in these “new” modules are made of bipolarintegrated circuits without an opto-coupler circuit [4].The measurements also indicate the presence ofannealing effects related to the fluence. The irradiatedconverters showed a partial recovery after few days fromthe end of the test. However, no load current effectswere measured.Figure 4. Radiation tolerance to neutrons, for a VI-300converter with 5 V output voltage.3.5  Radiation Tolerance to Ionising RadiationUnits from the VI-200 and VI-300 series wereirradiated at Brookhaven National Laboratory with a60Co source. Figure 5 illustrates the typical response ofthe two families of DC-DC converters as a function ofdose.VICOR 1st gen. units can withstand 103 Gy (100 krad)with only a small decrease in output voltage (~ 1.5 %).Switching frequency and efficiency remain constant.Figure 5. Radiation tolerance to ionizing radiation fora VI-200 and a VI-300 converter as a function of dose.At higher doses the units are less efficient, theswitching frequency and the output voltage decreasemore and more. At 1.1x103 Gy (110 krad) bothcharacteristics reduce to ~ 50 % of their original value.No recovery was observed after the conclusion of thetest.Second generation converters stop functioning at~ 200-250 Gy (20-25 krad). The failure happens rapidlywith no annealing.The different behaviour of the two families is stillunexplained. VICOR has indicated that the 2nd gen unitsuse a “proprietary” integrated power MOSFET (IPD),which differ from the one used in the 1st gen converters[4]. This MOSFET is an unpacked device bondeddirectly to the converter substrate. The VI-200 familyuses a MOSFET device packaged in a TO-220 case. Nolayout compatibility exists between the two converterfamilies. The different layout prevents from swappingthe MOSFET transistors between families.4. CHOICE OF THE CONVERTER4.1  Second generation unit advantagesThe VI-300 converter family has some advantagesrespect to the VI-200 family.The 2nd-gen converters have higher power per unitaryvolume, have an improved transformer design whichshould reduce common mode noise [2], have on-third thenumber of part of their predecessors and meet theexperimental magnetic field and radiation environmentconstraints. However some of the specification is barelymet. The extremely high power-density of these unitsleaves some space to introduce some “shielding” in thefull size power supply.4.2  Reliability IssuesThe main argument in term of reliability is the drasticreduction in number of components used in the VI-300family (from ~ 200 components to ~ 35 components).The improved quality screening of the components,the improved thermal management between thetransformer and the baseplate, the reduced thermalimpedance from the IPD junction to the baseplate haveimproved the unit mean-time-between-failure (MTBF)up to 2.5 million hours at 25 oC.Moreover the built-in protection and the currentsharing capability allow N+M fault tolerancearchitecture.The units will be anyway “components out of theshelf”, a quality assurance procedure need to be definedto avoid properties variation from batch to batch. Thequality assurance should be mostly performed at full sizeprototype instead that at single element level.More measurements are needed to study single eventeffects (SEE).0408012016020002468100 5 1012 1 1013 1.5 1013 2 1013Vicor 2nd gen. M odule5 Volt / 300 W att(Iload ~ 20 amp, T ~ 50 C)Switching frequency [kHz]Vout [Volt]Switching frequency [kHz]Vout [Volt]F luence [n/cm2]05010015020002468101214160 20 40 60 80 100 120VICO R M odules1st gen: 12  Volt / 200 W att (I ~ 5  A ,  T ~ 50  C)2nd gen: 5 Volt / 300 W att (I~  20  A ,  T ~ 50  C)Switching frequency [kHz]Vout [Volt]Dose [krad]Vout 1st gen.fsw 1st gen.V out 2nd gen.fsw  2nd gen.4.3  Toward a prototypeA “six-voltages six-currents” power supply for a fullfront-end crate could be made by using 24 VICOR 2nd-gen modules in a configuration which will guarantee100 % redundancy. Twelve converters will be active allthe time while 12 units will be on stand-by and ready tostart in case of a failure. The maximum deliverablecurrent will exceed by 10 % the maximum nominal cratecurrent.Each voltage will have an input section to regulate the300V DC input and an output section to achieve theoutput ripple specification. All the components used inthis IN/OUT circuit will be evaluated for radiationtolerance. The converters will be mounted on threecooling plates and each cooling plate will dissipate thesame power.The limited magnetic field and ionising radiationtolerances of the tested modules will require additional“shielding” in the full size prototype.The location of these “on-detector” power units mustbe carefully estimated to minimise environmentalspecification.The supply control and monitoring electronics will becompatible with the slow-monitor system accepted bythe LAr collaboration.5.  CONCLUSIONSThe commercial VICOR 2nd gen DC-DC converters,family VI-300, meet all the power supply specificationfor the LAr calorimeter system. However VICORprovides only the bare DC-DC converters. The modulesneed to be properly “packaged”; IN/OUT and controlcircuits must still be designed. An industrial counterpartinterested in this application has been identified and afull size supply has been proposed.Preliminary coherent noise measurements indicate thata power supply based on these DC-DC converters shouldbe indistinguishable form a linear power supply,provided proper output filtering.6. ACKNOWLEDGMENTSThe authors wish to thank F. De Giso and K. Nardoneof VICOR Corporation for the fruitful discussions andthe help provided. F. Densing, D. De Simone and G.Kegel for the assistance during the tests of theconverters.7.  REFERENCES1. VICOR, Application Manual 5th edition.2. P. Mannion, “Second-Generation DC-DCConverters, Up Power Density And LowerCost”, Electronic Design, September 2, 1997, p.40.3. R. L. Chase and others, “Can Vicor switchingpower supplies be used with the ATLAS EMCalfront-end boards?”, BNL internal report, 1996.4. K. Nardone, private communication.

Are switching power supplies acceptable for the Liquid Argon calorimeter front-end electronics?

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