Low Voltage Power Supply Project (2001 - 2008)

Introduction

This project presents custom made Low Voltage Power Supply (LVPS) system developed for the ATLAS – TileCal detector of the LHC (The Large Hadron Collider) particle accelerator at CERN, Geneva. The power is distributed by two stage system, the first stage from 3x230V~ into +200Vdc was designed, the second stage was made of dc/dc converters from +200Vdc into 8 low voltage levels for the front-end detector electronics of +3.3V, +/-5V, +/-15V, respectively. It is designed to be manufactured by using only COTS (Commercial of The Shelf) components, it is qualified to be radiation tolerant up to 40krad, and can operate in external DC magnetic field above 0.020 Tesla (200 Gauss). The LVPS design described has been developed and produced at CERN, Geneva, CH, for the ATLAS TileCal detector during the years 2001 – 2008.

Paper.pdf,  doc,  ppt  presented on TWEPP07 workshop, Prague, September 2007

The Tile Calorimeter front-end electronics [1] of the ATLAS detector is powered by 256 custom made low voltage power supplies (LVPS) called LVBOX. Each LVBOX contains eight 150W DC/DC single-output modules transforming 200VDC input into various independent low voltage outputs (+3.3V, +/-5V, +/-15V). A local control and communication board using ELMB [2] permits to monitor behavioral parameters (two temperatures on each dc/dc module, IN/OUT LVBOX cooling temperature, Iin, Iout, Vin, Vout, sense lines reading) and trim Vout of each DC/DC module (Brick) using CAN Bus communication. The power supply is water cooled, is capable to survive a total integrated radiation dose of 40krad, and can work in external magnetic field higher than 0.02Tesla[3]. The LVPS is now manufactured in 303 production units. The 200VDC input voltage for these LVBOXes are delivered from 24 bulk power supplies HPS1 [4] located in USA15 control room. Sixty-four auxiliary control and power supply boards (AUX Board) in the same control room are required to give power for the LVBOX monitoring and control circuits. Four LVPS Interlock Boards are capable to switch off all LVPS and HPS1 supplies in case of water cooling system leak or general switch off of the Tile detector.

LVBOX

The power supply, called LVBOX, is mounted in the vicinity of Tile Calorimeter detector modules (drawers) inside so called FINGERS [1]. Each LVBOX contains eight independent DC/DC converters - bricks (3.3VDIG, +5VMB, -5VMB, +5VDIG, +15VMB, +5VHV, +15VHV, -15VHV), so called ELMB Motherboard, ELMB plug-in board [2], 200VDC distribution Fuse-Board, internal cable set, and chassis and water cooled heatsink. The ELMB Motherboard and ELMB module are connected to DC/DC converters and permit remote PC monitoring of their behavioral parameters (temperatures, Iin, Iout, Vin, Vout, DIG and MB side sense line voltages) and set trimming of Vout. Start-up sequence of LVBOX is carried out in 3 groups of bricks[5]. The HV or DIG side brick group shutdowns are triggered by most critical power lines: -5VMB, +15VMB, and -15VHV. The LVBOX is exposed to radiation and magnetic field in the ATLAS cavern. The Tile LVPS system is computer managed by the Detector Control System (DCS) software through a CAN Bus line that communicates with the local ELMB of each power supply. The LVBOX of each Tile detector module has to deliver average power of 300W. Production of final 256 units started at CERN in January 2007.

DC/DC SWITCHING POWER SUPPLY MODULE - "BRICK"

The key component of the LVBOX is a small size (80 x 80 x 30mm), radiation and magnetic field tolerant, single output DC/DC dual transistor synchronous forward converter based on LT1681 chip[6]. Full custom design using only COTS (Commercial Off The Shelf) components and the universal printed circuit board design enabled to select three brick versions of various outputs (+3.3V, +5V, or 15V) by means of only several components change. Brick input voltage is +200VDC due to the radiation degradation. The maximum output power from the brick is 150W. The DC/DC converter design enables high efficiency of energy conversion: 72% for 3.3V @10A Iout, 89% for 5V version @12A, and 82 % for 15V version @6A, respectively[3]. Switcher uses the current mode control and has fixed switching frequency of 300kHz. The brick frequency can be synchronized with other DC/DC modules inside LVBOX. In order to start, the brick needs a remote auxiliary Start-Up voltage of +15V coming from the AUX Board in USA15 control room. Important brick features are active Over current and Overvoltage protection circuitry[5]. Innovative technology of brick cooling (especially of the switching FET power transistors, power inductors and high frequency transformer) was applied by means of custom made Al2O3 ceramic spacers. 2048 bricks plus spares are needed for LVBOX production. Brick has integrated remote control and measurement circuitry for both input (Iin/Vin) and output (Iout/Vout) parameters. Moreover, two temperature sensors are also present to control brick cooling. The Vout voltage can be trimmed in range of 15%. This feature enables a remote correction of Vout voltage during radiation degradation of output voltage of LVPS. The trimming, sensing and remote control of the brick are carried out by ELMB module [2]. Extensive irradiation test campaigns of COTS components and of the brick prototype were performed at PSI, Villigen, CH (protons 2002, 2003, 2004), at CERN (neutron P2B 2004, TCC2 SPS tunnel 2002, 2003 campaigns), Saclay (Co60 gamma test 2003) and at INT Portugal NIEL tests in 2003[3]. Experimental results demonstrated that bricks and ELMB motherboards could work together without problems up to 20krad TID, and survive up to 2.15E+12 n/cm^2 neutron fluency. Bricks radiation tolerance is even higher reaching 35-38krads. Bricks are also designed to be operational in external DC magnetic field without damage. Magnetic field decreases efficiency caused by DC magnetization of the HF transformer ferrite core. A decrease of the DC/DC converter's efficiency measured in a dc magnet with external B field at 200 Gauss (TileCal nominal level) was max -1.5%, and at 500 Gauss was -3%. Bricks were tested up to 1000 Gauss (0.1 Tesla) without functionality failure.

200VDC BULK POWER SUPPLIES

The input for the Tile power system is 200V DC voltage that is converted from three phase 3x230V~ mains. One HPS1 unit contains three 1.7kW power channels delivering 200VDC/8.5A [4], and the whole PS can then power 12 LVBOX finger supplies. In order to fully power 256 LVBOXes in the ATLAS PIT, we need 64 channels, so that is 22 HPS1 power supplies. For the maintenance requirements and duration of approx 10 years in operation 24 HPS1 units were finally installed in the USA15 control room (including  8x 200Vdc channels ready spares). The HPS1 power supplies are not radiation tolerant. They were developed in cooperation with TESLA Company, Prague, CZ.

AUX BOARD

The Auxiliary (AUX) Board is 6U sized PCB board and generates all auxiliary signals and power voltages for one cluster of 4 LVBOXes. The board consists of 3 x 4 isolated power supplies for the ELMB motherboard (trimmed between 6.8 – 14.5V), for ELMB chip (6.8 – 14.5V) and Startup signal (15 – 25V) for LVPS bricks. It also contains current sources for LVBOX HV/DIG side remote on/off current loop controls and an interlock circuitry. The board is controlled via ELMB module hosting custom made version of ELMB software. So all AUX boards are creating another CAN bus network and theirs control is embedded into the overall remote control system. The distance between 64 AUX Boards or 22 HPS1 supplies in USA15 room and 256 LVBOXes in the ATLAS cavern varies from 60 to 100 meters depending on the position of particular power supply.

DSS-LVPS INTERLOCK BOARD

The LVPS interlock board is receiving the ATLAS-DSS (Detector Safety System) cooling interlock signals. In case of interrupted water cooling to the Tile Calorimeter, one LVPS Interlock Board will disable function of 64 LVBOXes, will switch-off 16x 200Vdc channels of HPS1 units, and will disable 64 channels of interconnected High Voltage Power Supply crates (HVS1, 850 - 930Vdc/20mA for photomultipliers). In total, four DSS-LVPS Interlock PCB boards were needed to stop/control  256 finger LVBOXes, they are installed in the USA15 control room together with 16 HVS1 units, 64 AUX boards, and 24 HPS1 power supplies.

REFERENCES

[1] “Design of the front-end analog electronics for the ATLAS tile calorimeter”, K. Anderson et al., Nucl. Instr. and Meth. A, Vol. 551 (2005), pp. 469–476.

[2] “The Embedded Local Monitor Board (ELMB) in the LHC Front-end I/O Control System”, B. Hallgren, 7th Workshop on Electronics for LHC Experiments, LECC2001.

[3] Production Readiness Review of LVPS design, LVPS Meeting, CERN/ATLAS/TileCal, 10 Dec 2004.

[4] ”High power supply HPS1 Operational Manual”, TESLA Hloubetin, Prague, CZ, technical manual, 2005.

[5] “Specifications for the Modification of the ATLAS TileCal Bricks for Low Voltage Power Supplies”, B.Palan, L. Price, G. Drake ,CERN ATLAS Draft report, Dec 29, 2006.

[6] “Radiation and magnetic field tolerant 150W DC/DC converter brick”, CERN-ATLAS/TileCal internal technical datasheet, 2006