Power distribution
Abstract
Compared to their LHC counterparts, the HL-LHC detectors require significantly increased channel
count and rate capability in order to meet physics needs. Low power design techniques and deepsubmicron technologies (130 or 65 nm CMOS) will enable upgraded performance to be delivered at
power levels comparable to the present detectors, however as the improved efficiency of such
technologies comes largely as a result of lower power supply voltages the current drawn by the
front-end electronics is actually is increased. In the case of individual or parallel powering, the losses
in off-detector cabling quickly become unacceptable. It is clear that new power distribution
strategies are required.
Two alternate powering schemes are being studied for HL-LHC detectors: DC-DC Conversion and
Serial Powering. Radiation hard DC-DC converters with air-core inductors have been designed to
meet the needs of Phase 1 upgrades, and the circuit blocks needed to operate a chain of serially
connected detector modules have been prototyped. Both schemes have been shown to perform
well in small-scale system tests. In addition, switched capacitor DC-DC converters have been
demonstrated to be a viable, high efficiency alternative to on-chip linear regulators. The status of
this work shall be shown and the merits and drawbacks of each scheme outlined. The directions of
future development will be described, as required to deliver reliable, low-mass powering solutions
for Phase 2 upgrades on an appropriate timescale.
Summary
The inner detectors of the LHC experiments use classical independent and parallel low voltage
power distribution. Such schemes are very inefficient, with typically more than half of the delivered
power being lost in cables and voltage regulators.
Compared to their LHC counterparts, the HL-LHC detectors require significantly increased channel
count and rate capability in order to meet physics needs. Low power design techniques and deepsubmicron technologies (130 or 65 nm CMOS) will enable upgraded performance to be delivered at
power levels comparable to the present detectors, however as the improved efficiency of such
technologies comes largely as a result of lower power supply voltages the current drawn by the
front-end electronics is actually is increased. In the case of individual or parallel powering, the losses
in off-detector cabling quickly become unacceptable.
New power distribution schemes are therefore needed: the power must be transported to the
detectors at higher voltages and be converted locally to the voltage levels needed by the front-end
chips. This can be achieved by means of DC-DC Conversion or Serial Powering. With either scheme
large-scale system tests are needed in order to verify that performance is maintained (especially
noise levels).
In serial powering, a chain of front-end modules is fed serially with a constant current a little higher
than the peak current needed by a single module. On each module, a local shunt regulator
dynamically regulates its impedance to generate the appropriate local supply voltages for the frontend electronics. Protection circuitry may be added to maintain the integrity of the chain in the event
that a single link may fail. Several experimental front-end ASICs with built-in shunt regulators have
been successfully designed and small-scale system tests have shown encouraging results.
Commercial DC-DC Buck converters typically use ferrite-cored inductors: such inductors will saturate
in the large magnetic fields present in the HL-LHC tracking detectors, rendering them inoperable.
Similarly the extremely high radiation levels in the experimental caverns would cause commercial
devices to fail quickly. Only customised converters with air-cored inductors and radiation hardened
ASICs can meet our requirements.
Point of Load DC-DC Buck converters tailored to the needs of Phase 1 upgrades will become
available in large quantities next year. The culmination of an R&D programme which started in 2009,
the converters use a custom ASIC (FEAST) in conjunction with a custom oval air-cored inductor. In
parallel with this development, small-scale system tests have shown that detector systems may be
engineered to use DC-DC converters without any significant change to the noise levels observed in
the front-end electronics, provided that careful EMI shielding is used.
It is important to note that further work is needed in order to deliver converters to meet the needs
of phase two upgrades: two candidate technologies are being explored with a view to meeting the
required radiation tolerance and, notably for tracking detectors, revised converter designs with
reduced material are foreseen. More exotic designs are of course possible, for example the use of
GaN to increase the range of possible input voltages, but if such developments are necessary to
meet experimental requirements, costs will rise accordingly.
DC-DC conversion with switched capacitors is an alternative to the more common inductor based
circuits which is of particular interest as a high efficiency alternative to on-chip linear regulation. As
such it may be used as part of either a more general DC-DC or Serially Powered scheme. Whilst the
technique has been successfully demonstrated within the community, we would require access to
ASIC technologies with integrated capacitors to be able to make optimal use of this technique.
Further reductions in cable mass and cost may be achieved by connecting multiple sensor bias nodes
to a common supply. To reduce the risk to the group, a radiation qualified high voltage transistor
may be used as a switch to isolate any faulty units. The search for suitable COTS devices has just
begun but, subject to a successful conclusion, may be of benefit to the larger community.
Commercial “bulk” supplies will be needed to feed the on-detector power distribution system. In
many cases these will have to be located in the experimental caverns in order to reduce the length
of the power cables, hence they must also operate in magnetic fields and have a sufficient radiation
tolerance. This will require a combined R&D effort between our community and industrial suppliers.
This effort took several years for the current LHC experiments and should not be underestimated.
The requirements are very similar for all HL-LHC experiments, hence a coherent and continuous R&D
programme is possible and indeed preferred. Whilst DC-DC Buck converters are optimal for many
systems, Serial Powering requires less space. SP may therefore be a more efficient choice for high
power systems where there is insufficient space to place DC-DC converters directly at the point of
load, such as Phase 2 Pixel barrels. It is therefore important that support continues for both
schemes, to ensure that suitable parts are available in time to build the Phase 2 Upgrades.