High-Voltage DC-DC ConversionSponsored by: NATIONAL SEMICONDUCTOR

The delivery of dc voltages and currents to ICs in modern communications systems is turning traditional power-supply design into a new art form, driven by the need for greater bandwidth, faster data rates, tighter security, upgradability, larger numbers of customers, and a broad spectrum of user features. Next-generation power systems based on dc-dc converters must operate over wide input voltage ranges, sometimes from 30 to 100 V. Simultaneously, they’re generating a number of low-level dc output voltages to supply high-performance communication system ASICs, DSPs, and microprocessors designed in deep-submicron CMOS processes.

In communications and network server applications, this means converting a 48-V input voltage into not only legacy 5- and 3.3-V supply rails, but also into new lower voltages that range from below 1 to 2.5 V at load currents of 10 to 35 A. Moreover, the power systems must hold tight tolerances and generate minimum noise to preserve signal integrity. These increased demands take place in an environment where space constraints and thermal management are major considerations.

To meet these multiple demands, power-system architectures are moving from earlier centralized delivery of lower voltages and currents to the present distributed approach. Instead of a single supply to produce all of the necessary voltage levels, power is distributed along a network of secondary and tertiary buses to dc-dc converters that step down the voltage levels to suit the requirements of individual circuits or subsystems.

At each level, you can design or buy a dc-dc converter that delivers the necessary voltages and currents to supply a number of ICs, ASICs, mixed-signal devices, or complete pc boards. Each dc-dc converter will have a specific topology that depends on many factors of the circuitry it powers and the system in which it operates, such as efficiency, noise levels, physical factors (height, weight, size), number of output voltages required, power consumption, and heat removal. This tutorial will discuss specific tradeoffs and suggest topologies best suited to meet these various system power design objectives.

Distributed Power: A Top-Level View In a distributed power architecture,a front-end power supply converts ac power to dc and distributes a dc voltage via first-level buses (usually —48 V in communication systems) to dc-dc intermediate bus converters (IBCs).The IBC ’s purpose is to first provide isolation ,as well as reduce the ac-dc front-end distributed dc voltage to a lower voltage level.This should occur before sending it to a final set of non-isolated dc-dc (buck) converters via second-level distribution buses.These so-called dc-dc point-of-load (POL) converters deliver the required voltages and currents to the system.

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