Column: Electric Vehicles
20kW and higher range, server-rack PDN designs continued to evolve. Although sticking to 12V legacy systems is simpler, the introduction of AI into data centres, with processors up to 1000A steady-state and peak currents approaching 2000A, are making 12V-based PDNs near impossible for these applications. AI is all about performance and 12V PDNs limit performance and competitiveness. In a move to address the many challenges of high-power racks,
the OCP consortium is changing its rack design to accommodate a 48V PDN; see Figure 2.
Rapid adoption of 48V Moving distribution from 12V to 48V reduces the input current requirement by a factor of 4 (P = V • I) and cuts losses by 16x (power loss = I2
Figure 2: To achieve the optimal point-of-load power system: 1. A regulator delivers top effi ciency when Vin
= Vout R losses ;
2. Effi ciency is maximised when high-current delivery is closest to the point of load, minimising I2
R); see Figure 3. As automotive, 5G and industrial
applications move to 48V, the ecosystem of 48V power converters is rapidly expanding. T ere are many options for engineers to leverage both 12V and 48V systems today – although it’s worth noting that not all 48V converter topologies and architectures are the same. T ere’s greater performance diff erentiation in the 48V converter market, which must be carefully considered for each design. With high performance and power effi ciency at the top of
the list of requirements for high-power racks and data centres, several companies are moving to convert 3-phase AC to 48V for distribution to the blades. Alternatively, high-voltage DC (at 380V, derived from a rectifi ed 3-phase feed) distribution within the rack can be used, and several high-performance computing (HPC) companies are using HVDC PDNs for racks up to 100kW. And as PDNs that supply the blades are changing to 48V, a change in power conversion on the blade is required, too. T is is now a very interesting area of change, as many alternatives in architecture, topology and packaging of the DC-DC converters and regulators are being introduced.
Power architectures in HPC Although new to the data centre server application, 48V is widely used in communications, including routers and network switches, primarily because 48V lead-acid rechargeable battery systems already power telecoms equipment. T e common architecture used there is the Intermediate Bus Architecture (IBA), which uses an isolated unregulated bus converter to change the 48V to +12V, then feeds it to a bank of multiphase buck regulators to handle the 12V conversion and regulation for the PoL. Some of the cloud computing and HPC companies copied this architecture for their 48V systems, but as power increased and voltage at the PoL decreased to below 1V, alternative architectures and topologies were needed.
Changing game rules T e latest state-of-the-art AI processors have steady-state currents of almost 1000A, with peak currents reaching 2000A. With typical PDN resistance from the output of a conventional multiphase buck regulator to the processor of 200-400µΩ, this would result in
Figure 3: By overcoming the obstacles imposed on high-power processors by high-current delivery through the “last inch”, Vicor technology does not only improves performance and simplifi es motherboard design, but it enables processors to achieve previously unattainable performance levels that are necessary to fulfi l the promise of high-performance applications such as AI
power losses of 200-400W steady-state (P = I2 R), which is too high
for any system to handle. PDN losses are now the dominant factor in the effi ciency and performance calculation of DC-DC regulator design. Since this is a PoL problem and higher voltage cannot be considered (PoL voltages are being reduced to maintain Moore’s Law), the only solution available to engineers is to reduce the PDN resistance, by placing the regulator as close to the processor as possible. In the case of a multiphase buck regulator, typically 16-24 phases are needed to support the high AI processor current. T is is not a high-current-density solution and doesn’t solve the PDN power loss problem, however.
Factorised power architecture An alternative to IBA is Vicor’s Factorized Power Architecture (FPA), which consists of a pre-regulation stage (PRM) followed by a voltage-transformation stage (VTM). T is proprietary architecture enables the optimisation of the performance of each stage, where the PRM performs a non-isolated (48V is SELV, or Safety Extra Low Voltage) regulation function, where its 48V input is tightly regulated to provide a 48V output and leaves the
www.electronicsworld.com April 2023 15
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