Move to 12V bus eliminates need for isolated dc/dc converters

Powering the ICs scattered on and across the multiple pc boards in a system has always presented challenges to power-supply designers. As long as the required voltages were relatively high, it was possible to locate the power conversion in a remote area of the system, where thermal and noise management was easier. However, as the required voltages decreased, power-conversion devices moved closer and closer to the usage points of power.

This migration from multiple output-power supplies, driven by the lower voltage and higher current requirements of the latest microprocessors, has reached a point at which today's systems exclusively use a single 48V front-end power supply with final conversion to the lower voltages done on the pc board. The increasing converter-power densities and the resulting converter-size reductions are major reasons for the rapid adoption of a distributed-power architecture (Table 1).

The next emerging trend, driven by technology-conversion advances and cost considerations, is the shift from a 48V to a 12V distribution bus and the resulting elimination of the onboard isolated dc/dc converter.

Several parameters—including the need for battery backup, the overall system-power level, device efficiency, copper-trace losses, connector consideration, and overall system cost—will drive your decision to use either one of the distribution voltages, and you should examine each new system design for the optimal approach.

In 48V systems, an ac/dc front end or, in the case of battery backup, an ac/dc rectifier to the 48V-distribution-bus voltage converts the input ac. An isolated dc/dc converter further converts the bus voltage on each pc board to an intermediate-bus voltage and distributes it on the board to individual nonisolated POL (point-of-load) converters powering the ICs. Such design is commonly called an IBA (intermediate-bus architecture, Figure 1).

The distribution of a higher 48V-bus voltage at lower currents allows for smaller traces and connectors. Because the output Schottky-rectifier-diode drops are small fractions of the output voltage, 48V front ends can be efficient and cost-effective. Distribution of the higher voltages reduces the need for heavy copper traces, or bus bars, and special high-current connectors, thus saving both system cost and space. However, the pc board still needs a secondary conversion by a dc/dc converter to the lower voltages that the POLs require.

You must compare the penalties of lower overall efficiency—due to the three conversion stages and the cost and pc-board space resulting from the additional 48 to 12V dc/dc converters—with the benefits of power transmission at the higher voltages.

With the widespread adoption of synchronous rectification, the increased efficiency of ac/dc conversion has enabled 12V- output front-end power supplies to be as efficient as higher voltage output converters.

Designs that require no battery backup can effectively use the 12V bus to directly supply 12V to each pc board. Because the ac/dc front ends provide basic system isolation, you can eliminate additional isolation on the board and channel the 12V bus directly to the individual POLs (Figure 2).

The trade-offs of the lower voltage distribution are the requirements for higher distribution currents and the resulting need to use heavier pc-board traces and bus bars.

So, how do you decide which bus level to use? Although there are many factors to consider, system-power level can be a quick starting point in the selection process (Table 2).

When a system requires battery backup, the 48V bus is the obvious choice. At the power levels higher than 3000W, the 48V bus is usually a better choice. However, for systems using less than 3000W, the 12V bus is gaining ground.

As Figure 3 shows, 3000W at 12V would require that 250A be distributed across the system. Distributing such high currents requires serious consideration for proper power-distribution methods. Factors include the number of pc boards in a system, the overall system-bus-distribution schemes, the required pc-board-trace widths, and the need for heavier pc-board bus-bar arrangements.

Mainframe computers have for many years used such power-distribution methods. Distribution of high currents on the pc board itself is a fairly recent development. Higher current distribution on the pc board requires wider traces, thicker copper, and consideration for extra layers (Table 3). Proper airflow on the pc board is also a big factor in reducing the trace widths.

Connections to the pc board are other important considerations with higher currents. New connector designs that can accommodate such high currents are currently available.