Power
Small and mighty – DC/DC converters rise to the challenges of AI computing
By Gary Edmonds, product marketing manager for AI applications, Flex Power Modules O
ne definition of ‘Artificial Intelligence’ (AI) is software that mimics and generates human-like behaviours. Whether this is desirable is a
subject of debate and there will be many who would prefer guaranteed rational and deterministic responses from the machines and automated services that surround us. However, AI is here to stay and, supported by the cloud, it represents a business that is worth around $136B in 2022, projected to rise to nearly $2T by 2030, according to Grand View Research. One worthy function of AI is the minimisation of energy consumption both in the home and in industry, but AI computing itself consumes huge amounts of power, both in the ‘learning’ phase and in routine use. The largest data centres require a feed of more than 100MW as a result and this represents a high cost in dollars and to the environment, through the resulting carbon footprint. We’re considering AI, but of course other increased burdens include crypto currency mining, the IoT and social media/streaming.
A success story has been the relatively slow growth of energy consumption of data centres compared with ballooning data throughput and this has been largely due to continued improvements in energy efficiency of hardware and their power supplies. However, rack power density requirements are said to be 3x higher for AI than traditional data centre functions and further efficiency gains get progressively more difficult to achieve. As a result, system designers are continually revisiting their power delivery architecture to look for ways to improve efficiency in the process of converting utility AC down to the sub-1V DC levels often required for GPUs, CPUs, FPGAs and ASICs.
46 February 2023
Figure 1: Datacentre rack power architecture evolution
The evolution of rack power architecture
There are many considerations when designing a power distribution architecture in a data centre. These include voltage and current levels for safety and loss considerations in distribution and conversion, the need for functional and/or safety isolation, the physical size of power conversion stages, cooling arrangements, where and if tight voltage regulation is needed and of course, cost. All these factors have changed in emphasis over the years and as power levels have increased, the optimum arrangement also changes. When loads were relatively low and end-voltages were 12V or 5V with perhaps -12V for analogue and interface circuits, it was practical to have an AC/DC converter for each rack shelf with multiple DC outputs, Figure 1, (left). Wiring for this arrangement is bulky and expensive with the necessary safety isolation, back-up can only be via a preceding UPS and duplicating the AC/DC function in every shelf is also costly. A logical progression as loads increased was to have a bulk AC/DC converter
Components in Electronics
for a cabinet, perhaps with redundancy, and then to route DC via bus-bars around to each shelf, Figure 1, (right). This was initially at 12V but 48VDC became common to keep current low and ohmic losses 16x less, but still at a ‘safe’ voltage, also allowing easier integration of 48V batteries as backup. Each blade then needed to down-convert 48V to the end voltage and this was most often done with an isolated, regulated intermediate bus converter to 12V, then down to the chip level voltages with non-isolated, fixed-input ‘Point-of-Load’ (PoL) converters.
‘Current’ practice
As power levels have inexorably increased, with 30kW per rack now common for AI applications, power density of conversion becomes more of an issue, so there is an increased emphasis not only on efficiency but also on the ‘watts/in3’ specification for DC/ DCs, along with the conflicting demand of ease of heatsinking. This has led to the approach of using an intermediate bus converter DC/ DC with a fixed conversion ratio and without
isolation, which yields the highest efficiency, smallest size and lowest cost for a given power output. Without isolation, common ground paths have to be managed but this is not a major concern with a limited power distribution range. The following PoL converters must now have a wider input range, as their supply varies in a fixed ratio to the 48V bus and the ratio of the bus converter can be chosen from typical values of 4:1 (48V to 12V), 6:1 (48V to 8V), or 8:1 (48V to 6V) to suit the peak efficiency ‘sweet spot’ of the PoL converter. Lower conversion ratios and higher PoL supply voltages result in lower bus currents and lower losses in PCB connections while PoLs are often more efficient with a lower input voltage from higher bus converter ratios. A further option, pioneered by Flex Power Modules, is their ‘Hybrid Regulated Ratio (HRR) scheme, where the bus converter is unregulated up to a certain input voltage, above which the output is regulated. This reduces the overall input voltage variation to following PoLs allowing more efficient types to be utilized. 48V over-voltage transients
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