Is IBA the Right Architecture
for your Application? (Part 2)

April 26, 2012
By
Director Applications Engineering

Tom Curatolo, Director Applications Engineering

In part one we talked about the advantages of the IBA, and some of the considerations that need to be borne in mind when considering implementing this approach to power. In part 2 we continue to look at the limitations and issues with this approach.

Chip geometries, and therefore voltages, have reduced, and voltage rails have proliferated.  At the same time supply rail voltages have gone up, with a backplane typically carrying 48 Vdc, compared with the earlier VMEbus 5 V supply. This has meant that designers have had to convert from higher source voltages to lower load voltages, demanding much higher currents within a more densely populated environment. Efficiency, I²R losses and thermal management have become critical issues. The problems confronting power systems designers such as system speed, power and density continue to increase, leading to a need to supply many low voltages, which in turn leads to a proliferation of converters, increasing cost and board space demand. With very low load voltages – now below 1 V – currents must be correspondingly high, so the need to derive these low voltage, high current supplies close to the POL to avoid unsustainable I²R losses across the PCB or unacceptable trace widths comes to forefront:

  • If the system DC rail is 48 V, high duty cycle losses will be sustained in a traditional DC-DC converter due to the wide voltage differential, shown as the 2% duty cycle with sub 1 Volt requirements like 0.8V.
  • Microprocessors demand fast step changes in power so board real estate must be expended on capacitors to store readily available energy
  • Space and expense must also be devoted to filters to protect the load from converter noise and harmonics
  • Efficiency and heat dissipation become critical issues

The duty cycle step down limitations of the buck converter is dramatically demonstrated here.

Understanding the operation and limitations of  IBA power systems is a good place to start.

The IBA power distribution system revolves around Intermediate Bus Converters that isolate and step down voltage from the power source, typically a semi-regulated 48V bus.  NiPOLs, consisting of single stage or if needed, multi-stage “interleaved” synchronous buck converters, are located in close proximity to the loads.  The intermediate bus provides the common voltage source from which a multiplicity of niPOLs are powered to regulate their respective loads. The “intermediate” voltage level, typically 12V, is chosen to “bridge the gap” between the input distribution bus, at 48V, and a typical load at, … say, 3V.

The intermediate bus architecture “underlying concept” is to step the voltage down to a level higher than… but sufficiently close to… typical POL voltages to enable a multiplicity of inexpensive buck regulators to finish off the job of supplying a multiplicity of devices.  The buck regulator, through its output inductor, delivers a voltage to the load equal to the “average” voltage at the common node between its top and bottom switches. This is equal to the “duty cycle” of the top switch times the intermediate bus voltage.

Inherent in this approach is a tension between conflicting requirements.

First of all, the niPOLs of IBA have an inductor in the “wrong place”. Bulk storage capacitance is therefore required to contain the output voltage within acceptable bounds while the inductor current takes its time to either slew up or slew down. As we all know, current flow through an inductor changes at a slew rate dependent on the applied voltage. At low output voltages, under a load dump, the output current can only be slewed at a rate limited by the output voltage.

As the output voltage gets to be a smaller and smaller fraction of the intermediate bus voltage, it takes longer and longer to apply the brakes on the output current…never mind going into reverse!

Upstream energy storage, necessary to provide filtering and maintain a source impedance low enough to facilitate stability, does little or nothing in terms of load bypass.  And the interposed buck regulator inductor creates inertia where agility is needed. 

The net effect is significant business for vendors of bulk capacitors taking up precious real estate on system boards.

To find out more about IBA and the other power architectures available listen to the rebroadcast of our recent web seminar, “The Three Approaches to Power”.  Tom Curatolo, our director of applications engineering, also talks about another option, which retains all the desirable attributes of CPA, DPA and IBA.  Listen to how, by disintegrating the classic functions of DC-DC converters, Factorized Power (FPA) and its novel power conversion building blocks, PRMs and VTMs, are capable of providing efficient power system solutions from the wall plug to the processor core.

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