Kierstead, J A

Takai, H

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Switching Power Supply Technology for ATLAS Lar CalorimeterJ. Kierstead and H. TakaiBrookhaven National Laboratorykierstead@bnl.gov, takai@bnl.govAbstract  We present an update on the development of  a3kW DC-DC converter for the ATLAS Liquid ArgonCalorimeters. The power supply will be located close to theCalorimeter front end crate in a region where magnetic fieldand nuclear radiation are present. In order to keep the cost lowa solution based on components of the shelf is sought. Recenttest results on single event effects on commercial converters ispresented.I. INTRODUCTIONEach liquid argon calorimeter front end crate will requireseven different voltage lines and a total of 3 kW of power. Atearlier stages of the design it was considered unacceptable tobring the power to the crates , located in the gap between thebarrel and endcaps, from large distances. The powerdissipated in the cables by ohmic losses would require the useof water cooled cables. In addition the use of large cablebundles would make the movement of the endcaps a verydifficult task. Therefore the only acceptable solutionconsidered was  the use of DC-DC converters near to the frontend crates. By bringing DC lines between 200 to 400V alldesired voltages can be generated locally and would requirethe routing of small gauge cables to the interior of thedetector.1In this particular application the DC-DC converter will beinstalled in regions where magnetic field and nuclear radiationare present. Converters  in this type of environment are onlymanufactured by the space industry at unreasonable  cost.Therefore it was felt that we should investigate the  use ofcomponents of the shelf (COTS) that could operate adequatelyin  our environment.I. FIGURES AND TABLESThe environment where the power supplies will be located issummarized in table 1. The magnetic field value assumes a 20mm iron shielding around the power supply and is  a result ofa simmulation performed by  et. al. The radiation field isequaly a result of simulations and  includes the  safety factorsrecomended  by the ATLAS experiment.                                                          1 Copies of the transparencies of the presentation given duringthe workshop can be found at the following web address:http://www.usatlas.bnl.gov/~takai/leb2000.Table 1: Radiation and Magnetic Field in the power supply region.Doses are for 10 years of operation at full luminosity. Safety factorsas suggested in ref 1 are included. Values are from ref 2,3.Quantity Total in 10 yearsTotal Ionizing Dose 5 kRadNeutrons 1MeV equiv. 3.0x1012 n/cm2Hadrons > 10 MeV 1x1011 h/cm2Magnetic Field 20 GaussThe presence of magnetic field causes is the first concern asfar as operating DC-DC converters. High values of magneticfield will saturate the transformer cores. The radiation  on theother hand causes damages in the internal semiconductordevices used in the feedback loops and switching transistors.Figure 1: Particle Flux in the location of the power supplies2104 105 106 107 108 109 1010Particle Energy ( eV)10−810−610−410−2100102E. dN/dE (kHz/cm2 )Particle Flux in the Power Supply Region(Preliminary − March 2000)γnpipeHadrons with E>10 MeV are responsible for single eventeffects in semiconductors. In DC-DC converters the mainproblem concerns the operation of power MOSFETs thatcould undergo a single event burn-out or gate rupture. Othersemiconductors in the device can also suffer from upsets orlatch-ups.I. INDUSTRIAL PARTNERSHIPThe design of the power supply is made together with anindustiral partner, an integrator, which specializes in theoperation of DC-DC converters in environments similar toours.  To keep the cost within reasonable range we havechosen to use components of the shelf instead of morespecialized radiation qualified parts. There are twoimplications with this choice. First it implies that we areforced to involve other industrial outfits, suppliers,  other thanthe integrator itself. The second implication is that anextensive testing program has to be launched.The first task undertaken between us and the integrator was toagree on the design of the  power supply to be built. This hasbeen summarized in a short specificiation document written byus. The second task is to establish a clear path for thequalification of commercial parts which is also done togetherwith the integrator.At this stage, one needs to involve the suppliers at the level ofunderstanding which is much deeper than what is availablethrough standard documentation. For example, it is interestingto learn what their parts qualification procedure is. Thisusually requires the signing of a non-disclosure agreementwhich is not only time consuming but could be confusing attimes because the depth of the non-disclosure is not alwaysclear. However the acquired information may haveconsequences in the estimating the most appropriate safetyfactors for the overall radiation doses.The integrator chosen is Modular Devices4, a company thathas experience in providing power supplies forcommunication satelites. We have also chosen three differentsources  for the most critial component: the DC-DC converterbricks. However due to manpower limitations we areconcentrating our efforts in understanding the performace ofthe Vicor5 bricks. The other choices are seen as backupsolutions with the implicit  understanding that if the firstchoice fails delays in the program will occur.I. SINGLE EVENT EFFECTSThe behaviour of the Vicor power bricks as far as magneticfield, integrated dose, and 1 MeV equivalent neutron damagehas been reported during the last Workshop.We have carried out initial single event effect tests. The singleevent effects which are relevant to power supplies are twotypes: destructive and transient. The destructive upsets mayhappen for all types of DC-DC converters which uses Vinlarger than 100V. The transient type of upsets could happen toDC-DC converters that uses integrated circuits in the controlsof the switching circuit.   To carry out the tests we feltimportant to also add all the representative anciliary circuitsthat will be used for the controls of the power supply.Therefore a special board was manufactured by ModularDevices for the test.The irradiation was carried out at the Harvard CyclotronFacility. The cyclotron delivers 150 MeV protons at rates of1x108 protons/cm2/s. We use protons instead of neutrons whichas far as SEU probabilities are concerned they are equivalent6.The rate is chosen so that we can irradiate samples with thesame number of particles expected in 10 years of operation.The supplies were mounted on stages and a laser alignmentsystem to locate the center of the beam spot which is 30 mmin diameter was used. The exact positioning of the powersupply was checked by placing a polaroid film behind thesupply.The DC-DC converters irradiated were loaded to 5% of theirfull capacity with the rational that SEGR/SEB is most likely tohappen while the supply is not conducting. We have exposed acombination of Mini and Maxi converters.I. SUMMARY OF RESULTSThe following conclusions were drawn as a result of theirradiation tests.a. The failure cross section at Vin=300V is 0.5x10-10 cm2. Thisresults are based on three destructed power supplies byirradiating their power MOSFETs. The destruction mode,SEGR,  was reported by Vicor which have send us theirFailure Analysis Report. This is one aspect where involvingthe suppliers is very important.b. At Vin=200V no failure was observed. We determine thatthe failure cross section is < 1x10-12 cm2. This is a expectedbehaviour for power MOSFETs. By derating the input voltagethe probability of burnout/gate rupture is greatly decreased6.c. None of the logic in the power supply seems to suffer fromsingle event effects to the level of 1x10-12cm2.d. None of the logic on the test board seems to suffer fromsingle event effects to the same level.I. CONCLUSIONSAt this point of the  development we conclude that the powersupply meets the requirements as far as magnetic field, totalintegrated dose, and 1 MeV equivalent neutron damage. As faras single event effects are concerned we  conclude that at 5%of total load the failure cross section is smaller than1x10-12cm2. The exact failure cross sections as function of theVin  are not known at this point. Because the total number ofconverters is nearly 1000, a cross section of 1x10-12 wouldimply in a loss of approximately 100  converters in a 10 yearsoperation. Even with a N+1 redundancy it is very likely thatwe would loose few power supplies because of the singleevent effects.  Note that this failure rate needs to becompounded with other more traditional failure modes.  As far as the operating point is concerned, it is clear that weneed to operate the power supply at reduced voltage. We  arein the process of designing a more complete test with de-ratedinput voltage, Vin=200V. The results of the tests will permit usto decide if we continue with the present supplier, Vicor, orwe would need to exercise a second option that we have: useradiation hardned converters. Of course the cost incrementwill be substantial.The involvement of commercial partners in the power supplyhas been very fruitful. Both the integration house and thesupplier of DC-DC converters has provided us withinformation that allow us the understanding of how differentparts are being assembled. One of the most important aspectswhen dealing with COTS is to acquire enough information asfar as how suppliers do component selection. This allows forlower risks as far as radiation is concerned.I. REFERENCES1. M. Dentan,  ATLAS Policy on Radiation TolerantElectronics. Draft Version 03/04/00, april 2000.2. M. Shupe, University of Arizona, private communication.3. Vorojtsov, Dubrovin and Nessi, private communicationagust 1999.4. Modular Devices Inc, Brookhaven R&D Plaza, One RonedRoad, Shirley NY 11967.http//www.modev.com5. Vicor Corporation. 25 Frontage Road, Andover MA01810-5413. http://www.vicr.com6. D.L. Oberg et al., IEEE Trans on Nucl Science43(1996)2913

Switching power supply technology for ATLAS Lar calorimeter

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