Mauro Citterio on behalf of the Atlas Liquid Argon Collaboration
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The power supplies for the LAr Atlas calorimeter Front End Electronics
must be engineered to reliably operate in a harsh environment in which 
space is a premium. The location of the power supplies in proximity of the 
front-end electronics crates requires compact power supplies with high 
power density. A typical electronics crate will need approximately 
2.5 - 2.7 kW of power distributed on six different voltages ranging from
+10V to - 5V. The estimated volume for one power supply, comprehensive
of cooling, is approximately 12 liters. The use of remote low-voltage 
linear power supplies is not feasible due to the unacceptable large 
amount of cables, which need to be brought to each individual crate.
Dc-dc converters which make use of a resonant charging mechanism
have all the necessary characteristics to be successfully used in the design
of the ATLAS power supplies. The resonant charging mechanism generates
very much less high frequency electromagnetic interference than conventional,
fixed frequency switching power supplies. The common mode high frequency
switching signals and the differential modulation at the switching frequency
can be reduced to a level where their noise contribution is negligible.
The ripple noise is less than 5 mV P-P when custom designed ripple-attenuator
output filters are in place. The dc-dc converters were tested for their magnetic 
field susceptibility and they were found to withstand magnetic field in excess 
of 200 Gauss. The converters have been also proved to tolerate 100 Krad of 
ionizing radiation and 10^13 n/cm^2 of neutron fluence. Each individual module
can provide up to 600 Watts and the modules are parallelable (to obtain more 
power) with N+M fault tolerance and current sharing, and are phased-array-
control compatible. Other specifications include a no-load to full-load
regulation of 0.1%, a programmable output of 10% to 100%, conversion 
efficiencies of up to 92% depending on the voltage combination and power
level chosen, and an input-to-output isolation voltage of 3000 V rms.

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The power requirements of the LAr calorimeter Front End Electronics, 
could be satisfied by using high power density dc-dc converters 
followed by linear ripple attenuators. A solution based on resonant 
charging dc-dc converters will be discussed in terms of noise 
characteristics, radiation and magnetic field tolerance, power 
efficiency and reliability.

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