Intermediate bus converters (IBCs) were developed to implement the Intermediate Bus Architecture (IBA), which promised greater efficiency and cost/space savings over the Distributed Power Architecture (DPA) for server, telecom, and network applications. The IBA is built on the premise of a multi-stage power conversion approach.
For example, a bus converter steps down a bus voltage, typically 48 V, that’s often generated from an ac front end (some applications distribute higher voltages) to an intermediate voltage. Buck converters step down and tightly regulate the voltage to the levels needed at the load.
While 48 V is an excellent choice for a distribution bus level for a number of reasons, it makes it difficult to achieve high performance in today’s systems with non-isolated buck converters, where applications require multiple low (think 3.3 V, 1.8 V, and 1.2 V) supply voltage levels, with high current demands.
Dropping the 48-V distribution bus at the board to an intermediate bus level of a nominal 9.6 V or 12 V enables the system designer to generate the device supply levels very close to the point of load with high-efficiency non-isolated point-of-load (niPOL) buck regulators while decreasing the total system power and cost. A recent trend has been to focus on power density at the card level as OEMs try to add more functionality per card, and real estate at the board level commands a premium.
All IBCs must provide isolation from the 48-V input bus and to transform the voltage to a lower level (the 12-V or 9.6-V levels mentioned earlier). One of the useful ways to differentiate IBCs is by their ability to deliver the specified output voltage over the full range of the nominal 48-V distribution bus input.
If the intermediate bus is fairly well regulated, the IBC topology takes advantage of it to hold its output to a range that is acceptable to the niPOLs. If the distribution bus has a wider range, then the IBC must provide some regulation so the intermediate bus level is acceptable to the point of load regulators. Not all IBCs provide regulation.
A design engineer wants to be certain that the IBC will always provide a suitable voltage level to the downstream regulators under all possible operating conditions. The engineer also needs to select an IBC that optimizes cost, efficiency, density, and stability. The choice may not always be obvious. You can look at IBCs as three basic types: regulated, quasi-regulated, and fixed ratio depending on the regulation characteristics of the distribution bus they are designed to work with (see the figure).
Fixed Ratio IBC
The fixed ratio IBC provides an output voltage that’s a fixed fraction of the input voltage. The IBC provides no regulation and offer up to a dramatic 98% efficiency (with only 2% conversion losses) if it’s using the design that employs sine amplitude conversion technology, which also leverages the highest power in a quarter-brick or eighth-brick package.
These advantages come with the caveat that the output voltage range is defined by the “turns ratio” of the device times the input voltage range. For instance, in a system where the point-of-load regulators will accept an intermediate bus level from 9 V to 15 V, the input of the IBC with a fixed ratio of 4:1 must be maintained in the range of 36 to 60 V. The simplicity of operation of this IBC makes it the leader in power per package size, power density, efficiency, and cost.
The regulated IBC is a very common category and identical in function to an isolated dc-dc converter. A regulated IBC can handle the widest range of voltage input and is most often used when the distribution bus is not anticipated to be well regulated. A typical specification for the regulated IBC might be a nominal 48-V input with 36 to 75 V as the min/max limits.
While this IBC category can handle the widest input voltage range and provides the tightest regulation of the intermediate bus level, it also has some limitations. It does not provide the highest-efficiency, lowest bill-of-materials (BOM) part count or the lowest cost. Of all IBC types, this category will exhibit the lowest power density.
The semi-regulated IBC is also designed to handle a wide input range, but it differs from the regulated IBC since the output is not regulated over the entire range of input voltages. The semi-regulated converter can use a regulating input stage that only operates when the input voltage rises above a certain limit.
For example, a semi-regulated 4:1 converter could be specified over an input range of 36 to 72 V. As the input rises from 36 to 60 V, the converter loosely regulates the output, providing an output that rises with the input voltage. As the input continues to rise from 60 to 72 V, one technique provides regulation that clamps the output, keeping it within a suitable range for the downstream devices.
This type of operation improves on the efficiency of the regulated IBC while covering a wide range of input voltages. However, the semi-regulated converter still does not reach the efficiency of the fixed ratio converter or compare in cost.
Putting It Together
There are important factors to consider when making the decisions associated with implementing an IBA solution. The IBC that accepts the widest range of input voltage may not be the best part for your design based on the power limitations of each IBC, and the ability to achieve a competitive edge by providing more functionality on the line card.
The efficiency advantage of the fixed ratio IBC may not be applicable in applications requiring the standard 36- to 72- V telecom requirements (36 to 75 V in some cases) because it provides insufficient voltage to the point-of-load regulators under low line conditions or excessive voltage under high line conditions.
The optimal system level solution could point back to the distribution bus variability. Maintaining the regulation of the distribution bus so the simple fixed ratio converters provide an acceptable range at the intermediate bus may be the best system level solution, or it may be beyond your control. The design engineer will need to match the IBC capabilities and tradeoffs with the system design goals to select and implement the optimal system level solution.