Superconducting Technology Assessment - nitrd

6.5 POWER DISTRIBUTION AND CABLES
Superconducting processor chips are expected to dissipate very little power. The cryocooler heat load for a
petaflops system will be dominated by heat conduction through input/output (I/O) and power lines running
between low and room-temperature environments. To reduce heat load, it would be desirable to make the lines
as small as possible in a cross section. However, requirements for large DC power supply currents and low-loss,
high bandwidth I/O signaling both translate into a need for a large metal cross section in the cabling for low
signal losses and low Joule heating. Therefore, each I/O design must be customized to find the right balance
between thermal and electrical properties.
SCE circuits for supercomputing applications are based on RSFQ circuits that are DC powered. Due to the low
voltage (mV level), the total current to be supplied is in the range of few Amperes for small-scale systems and can
be easily kilo-Amperes for large-scale systems. Serial distribution of DC current to small blocks of logic has been
demonstrated, and this will need to be accomplished on a larger scale in order to produce a system with thousands
of chips. However, the overhead of current-supply reduction techniques on-chip can be expected to drive the
demand for current supply into the cryostat as high as can be reasonably supported by cabling.
In addition, because petaflops systems will have very high I/O rates, Radio Frequency (RF) cabling, which can support
high line counts serving thousands of processors with high signal integrity, is needed. Reliable cable-attach
techniques for thousands of connections also require cost efficient assembly procedures with high yield.
6.5.1 POWER DISTRIBUTION AND CABLES – STATUS
Supplying DC current to all of the SCE chips in parallel would result in a total current of many kiloAmps. Several
methods may be used to reduce this total current and heat load. One technique is to supply DC current to the SCE
chips in series, rather than in parallel (known as current recycling). This technique of providing DC current to RSFQ
has been demonstrated, but there is real estate and junction count overhead associated with this method. This overhead
will drive system design to find a high DC current “comfort zone” for the power supply.
Another solution is to use switching power supplies. High voltages/low currents can be brought near SCE circuits
and conversion to low voltages/high currents, all at DC, can occur at the point of use. However, this method
employs high power field-effect transistor switches, which themselves can dissipate significant power.
If sufficient cooling power is available at the intermediate cryogenic temperature (such as 77 K), then high temperature
superconductor (HTS) cables may be used to bring in DC current from 77 K to the 4 K environment. High temperature
stranded DC cabling is a well known product, but there has been little demonstration of flexible ribbon cabling in
HTS—the most desirable implementation for a large system such as a petaflops computer—where the component
count and number of connections makes reliability, modularity, and assembly very difficult with individual cable.
For serial I/O in systems with low I/O counts, coaxial cables can be used for both high-speed and medium-speed
lines. These cables are made in sections, with a middle section having a short length of stainless steel having a high thermal
resistance, and with a bottom section of non-magnetic Copper for penetration into the chip housing.
For systems with hundreds or thousands of I/O lines, coaxial cables are not practical. To meet this challenge,
flexible ribbon cables have been developed 8 (Figure 6-7). These cables consist of two or three layers of copper
metallization separated by dielectric films, typically polyimide. With three copper layers, the outer two layers serve
as ground planes and the inner layer forms the signal lines, creating a stripline configuration. Stripline cables
provide excellent shielding for the signal lines. Successful high-reliability and low cost cable-attach procedures for
signals up to 3 GHz have been demonstrated, and this technique should allow operation up to 20GHz after minor
modifications. Present flex cable line density is sufficient for the MCM-to-MCM connections, but is an order of
magnitude lower than required for the external I/O.
8
“Cryogenic Packaging for Multi-GHz Electronics” T. Tighe et al, IEEE Tran. Applied Superconductivity, Vol 9 (2), pp3173-3176, 1999.
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