[Technology Report]
Distributed Power Moves To Intermediate Voltage Bus
An increasing number of supply voltages and rising power demands encourage two-stage dc-dc conversion with point-of-load converters spread across the pc board.
The 48-V power bus is the basis of distributed power architectures found in telecom and many computing applications. In these systems, ac-dc front ends convert line voltages down to 48 V dc for distribution among various pc boards in the system. Isolated dc-dc converters then generate the lower voltages required by logic, mixed-signal, and analog circuits. Distributing 48 V throughout the system, rather than the lower IC-level voltages, improves efficiency. In particular, it lowers I2R losses associated with busing high currents at low voltages.
Using isolated dc-dc converters to generate on-board voltages works well in cases of just one or two voltages. But in recent years, the number of voltage levels needed on boards has risen dramatically as new generations of chips were introduced with lower voltage requirements. In addition to the 5- and 3.3-V supplies, designers have to plan for supplies at different voltages that range down to 1 V.
Isolated converters developed initially for telecom offer different output voltages and power levels packaged in standard brick-style formats, including full, half, quarter, and the new eighth bricks. However, the inclusion of an isolation transformer adds to their cost and size. So, to generate the voltage range needed on-board, designers frequently turn to nonisolated, point-of-load (POL) dc-dc converters (often simply called POLs).
POLs, which are typically buck converters, may either be built by the system designer using ICs and discrete components, or purchased from power-supply merchants as functionally complete power modules. The latter typically specify operation from a 3.3-, 5-, or 12-V input and come packaged in SIP, DIP, or surface-mount modules. For a given voltage and current output, POLs cost much less than brick-style converters and usually take up much less board space. Also, they can be placed close to their loads, cutting down on I2R losses in the board and improving transient response.
For some time, it has been common for system designers to run POLs off the isolated 5- or 3.3-V bus. But with current requirements on the rise, some designers are opting to power POLs off of a higher voltage bus such as 12 V. This 12-V bus then becomes an intermediate voltage bus that powers all the POLs (Fig. 1).
Implementing a distributed power system with an intermediate voltage bus can potentially save pc-board real estate and overall cost. Other factors must also be considered in adopting an intermediate voltage bus. These include the power, efficiency, thermal-management, and noise requirements of the application.
The intermediate voltage bus must provide isolation, but its regulation isn't critical because the POL outputs are regulated. While telecom equipment maintains a need for a wide input-voltage range (35 to 72 V, necessary for battery backup), enterprise equipment like servers, which also run off 48 V, can operate from a much narrower input-voltage range.
Power-supply manufacturers are exploiting these factors to develop bus converters that provide isolation with higher power levels and higher efficiency at lower costs than their counterparts among the standard telecom bricks. At the same time, vendors are developing more powerful POLs to help optimize the implementation of intermediate voltage bus architectures.
Tradeoffs: System cost counts heavily in the decision to add an intermediate voltage bus to a distributed power system. POLs are considerably cheaper than isolated brick-style converters. So changing from a distributed power scheme that relies solely on half-brick, quarter-brick, or eighth-brick dc-dc converters to generate the board-level voltages will reduce costs as the designer pays for isolation just once.
Moreover, the cost savings increase as the number of output voltages rises. The intermediate voltage bus architecture might become cost-effective with only four voltages on board. Yet, converter costs must be weighed against the necessary current levels. As the current per output rises, the number of POLs per output may grow, adding cost that could offset the savings accrued from using fewer isolated converters (Fig. 2).
However, savings accomplished by using an intermediate voltage bus aren't limited merely to the reduced costs of the converters. They also include savings in the overall system cost—for instance, reduced board space. These savings can be significant, particularly when the board in question may have as many as 20 layers. There can also be reductions in the amount of copper or number of metal layers needed to distribute power across a pc board. For certain applications, less copper could mean lower overall cost.
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