Intermediate Bus Architecture (IBA) versus Factorized Power Architecture (FPA)

August 20, 2012

We’ve posted a number of articles over the last few months about the different power architectures available, and the advantages and disadvantages of using each. In this post we continue on this theme, looking at the difference between Intermediate Bus Architecture (IBA) and the latest architecture Factorized Power Architecture (FPA).

The essential features of an Intermediate Bus Architecture (IBA) are that it distributes semiregulated 42-50V voltage (therefore taking advantage of reduced distribution losses); then, by means of an isolated, nonregulated Bus Converter, this voltage is approximately reduced by a factor of 4 (or 5), while the voltage reference is changed from positive to negative (intermediate bus). This intermediate 9-12V bus enables non-isolated Point-Of-Load converters (niPOLs) to provide final step down and regulation function at the same time.

The Factorized Power Architecture (FPA) represents a further evolution onwards from IBA: the high efficiency, fixed ratio, isolated but not regulated converter is used as Point-Of-Load device, providing isolation and current multiplication directly from 48V; upstream, a non-isolated device provides voltage regulation by dynamically adjusting the 48V “Factorized Bus” slightly above or slightly below 48V. This avoids the need for the intermediate bus altogether, therefore further increases distribution efficiency.

A state-of-the-art configuration for an IBA (for a 48V telecom system) is shown in Figure 1. Each stage accomplishes two main functions, as listed in the diagram. An aspect to note about this structure is that the first stage (AC-DC Power Entry Module, PEM) is a high power, highly optimized commodity device, which takes care of power factor correction (often from single or three phase lines) on a universal input voltage line, and provides an isolated output. Similarly, the niPOL comprises a standard, off-the shelf single or multiphase Synchronous Buck converter, which can deal with a relatively wide input range while providing accurate Load regulation. The bus converter is a fixed ratio electronic transformer device that interfaces the 48V backplane with the 9.6V or 12V (nominal) intermediate bus. In terms of total life-cycle cost, all three stages have achieved extremely high performances, with commodity level acquisition costs, and peak efficiencies well in excess of 90%. In order to minimize intermediate bus losses, the IBC is placed as close as possible to the niPOL it supplies.

Intermediate Bus Architecture

Figure 1: IBA 48V Telecom System

The dotted lines in Figure 1 also show where the various blocks are located: DC backup and AC-DC PEM are normally stand-alone systems, sized to supply an entire rack through the distribution backplane. IBCs and niPOLs are either discrete or modular devices that are soldered on each unit Mainboard. Further points to note include:

  • Having larger, stand-alone AC-DC converters allows lower cost and higher peak efficiency to be achieved.
  • IBCs are available as open-frame, through-hole devices with power ratings ranging from few hundred Watts to one kiloWatt; as such, they can effectively be placed close to one or a few niPOL regulators.
  • niPOL regulators are standard synchronous buck converters that are tailored to each specific load.

The major drawback of this configuration is that blocks at rack level are generally oversized, which implies an added initial cost and higher energy cost because they do not operate at their peak efficiency.

A possible FPA for a typical system is shown in Figure 2. It still consists of three stages of power conversion; while the Adaptive Cell PFM first stage is functionally equivalent to the AC-DC PEM block in the IBA, the load regulation function is now accomplished by a non-isolated converter, the PRM. This converter works with an input line range of 36V to 55V, and outputs a “factorized bus” that can range from 0V up to 55V.

FPA for a Typical System

Figure 2: FPA for a Typical System

The VTM stage is enabled by the same topology used in the IBC (in effect, an electronic “DC transformer”), and provides both effective current multiplication (by a fixed factor), and isolation. Compared to IBC, FPA offers significant system advantages:

  • The factorized bus is normally maintained above 40V, reducing losses and conductor cross-section requirements on the Mainboard.
  • The Load voltage that can be regulated has a wider range, given the factorized bus range and the availability of various VTM transformer ratios.
  • Just like IBCs, PRMs and VTMs are power components that can be directly soldered on the Mainboard PCB
  • The PFM is also a power component that can be mounted on PCB.

In this case also, the dotted lines show where the various components are ideally located. It is clear that FPA offers the possibility of integrating the entire power supply within each unit, potentially avoiding any 48V distribution at rack level.

Given the ever increasing power density of DC backup systems, it is conceivable to integrate that block at Unit level as well, rather than at Rack level. This approach offers a number of advantages;

  • Each Switch or Server unit is fully autonomous and does not depend on infrastructure, even just at rack level
  • The entire supply system, being more granular, can be tailored more closely to actual power levels of use, therefore maximizing conversion efficiency and minimizing acquisition costs
  • Low voltage levels are generated within, and confined to, close proximity of the Load, minimizing distribution requirements and losses.

FPA flexibility is also evident when a high voltage DC supply is available: in this case, by simply substituting the Adaptive Cell PFM block with a High Voltage Bus Converter module (BCM), the same system can be efficiently powered, as shown in Figure 3.

FPA High Voltage Bus Converter Module

Figure 3: FPA High Voltage Bus Converter Module

Other Relevant Posts

Is IBA the Right Architecture for your Application? (Part 1)

Background to Factorized Power Architecture

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2 Responses to:
Intermediate Bus Architecture (IBA) versus Factorized Power Architecture (FPA)

  1. Gabriel on September 6, 2012 at 5:02 AM

    thank you for this article, it’s very interesting.
    But I have to submit you a slightly different problem:

    In medical application a common choiche is to use a mains transformer for step down the AC input at a voltage under 50Vac. In such a way the mains transformer keep in charge a lot of safety relevant problem, like a 5KV I/O isolation. Without the transformer the Power Entry Module will be a custom and expensive module.

    Now the problem wich arises is:
    are the market offering a commercial 48Vac input Switching power supply?

    • Marco Panizza on September 18, 2012 at 4:43 AM

      I understand your issue. Medical equipment have either a 1:1 insolation transformer, followed by a conventional AC switchmode P/S, or a step down transformer that reduces the voltage to the SELV range 50V. In this specific case, I would say that, rectifying the 50Vac with a full wave bridge should not be a complicated issue. After that, as the resulting DC voltage is around 70V, it is possible to implement an FPA architecture by using a PRM connected to the 70V rail, followed by VTM that steps down to the required system voltage.

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