A gift from decades of telecoms-industry development, bullet-proof models for high-availability power-supply systems and key standards documents are freely available online to tempt adventurous designers into exploring their own solutions for safeguarding processes that cannot afford unplanned downtime. Adopted in 2000 as the telecom industry’s standard for powering infrastructure equipment, the intermediate bus architecture (IBA) provides a stable yet flexible framework for design that’s attractive to a much wider audience. For instance, MicroTCA is designed from the ground up to appeal to markets way beyond its telecoms heritage, as evidenced by power-system options that span a single-module 300W supply to the n+1 redundancy that strives to ensure telecoms’ iconic ‘five-nines’ availability, or 99.999 percent uptime. Its format may appeal to designers as it is, or those looking for a template for custom hardware development.
The need for high availability in demanding circumstances that include meeting regulatory stipulations explains the popularity and dominance of modular power solutions. One digital power converter family delivers easily accessible efficiency gains as standalone analog-converter replacements, when applications will benefit from onboard intelligence that continually strives to maximize conversion efficiency. The difference between analog and digital power converter cores lies with replacing the analog error amplifier with an ADC that closes the loop and passes the digitized error signal through a digital-signal-processing chain that modulates the PWM generator (see figure 1).
But a digital power-converter properly comes alive when it connects with its peers and local board power manager logic such as a microcontroller that supervises operation in the target application, and/or with remote intelligence such as a PC running software to program and monitor the converters as the system runs.
Programmability can imply difficulty, which the PMBus protocols dispel. With a standard command language that is specifically designed for power-control applications, this industry-standard medium rationalizes programming compatible converters that would otherwise appear complex. The physical layer connectivity is based on SMBus, where the two serial data lines are similar to and electrically compatible with I2C. Four additional bus lines are available to serve applications that need features such as an interrupt signal for a maximum of six bus lines. Coupled with a rich but succinct command language that uses minimal resources to exchange message data, the minimal hardware footprint helps explain the popularity that PMBus enjoys to the extent of being indispensable for digital power-converter users.
IBA and down conversion – a closer look
Taking -48 VDC distribution-level power from an AC/DC front-end with an optional battery-backup system, “IBA” relates to dual-stage downconversion at board level. The primary stage comprises an intermediate bus converter (IBC) that isolates and downconverts the -48 VDC feed to an intermediate bus level of between 5 V and 12 VDC. This level supplies a number of non-isolated point-of-load converters (POLs) that power the load circuitry.
The 5 V to 12 VDC choice relates to the power that the bus must supply to service the POLs. A 12 VDC bus can provide more current than the 5 VDC option albeit at the expense of increased dissipation in the POLs. One might expect the 7 VDC difference the IBC faces to have some influence, but IBCs are sophisticated items and less sensitive to downconversion ratios than simple POLs.
Converter datasheets should help estimate the variances in power dissipation for fixed POL output currents at different input voltages. Turned around to examine the effect of variations in POL output currents suggests that as the output load falls, it would be advantageous to lower the intermediate bus voltage to save downconversion energy, and raise the level to ensure sufficient power is available when the output loading rises.
This is the principle behind dynamic bus-voltage adjustment, which is a technique for automatically adjusting the intermediate bus level to optimize efficiency in running systems. It is particularly applicable to demand-driven applications such as communications networks. The clue to success lies with uncovering a robust model of demand conditions over time that also accounts for unexpected events. The technique holds immense promise for energy savings in large-scale systems and is a key application for digital power converters.
Digital conversion improves efficiency
Digital converters substitute binary values for passive components in the feedback loop and are able to trim these control variables to compensate for line and load variations as the converter runs. Minimizing the dead-time period between the upper and lower MOSFETs switching is a key technique, when a small delay is essential to prevent mutual conduction that can destroy the MOSFET switches. This dead-time value is fixed in any representative analog converter, but adjustment can improve peak efficiency by one to two percent in an already-good design.
As a typical efficiency plot shows, the digital converter’s characteristic shape flattens to extend the area of efficient operation, which is helpful at the light-load end (Figure 2).
In some instances, moving from analog to digital conversion yields exceptional benefits. Comparing two highly developed quarter-brick analog IBCs with an equivalent-footprint digital design shows the digital solution from Ericsson sources as much as 396 W with ±2 percent voltage regulation and greater than or equal to 96 percent efficiency from approximately 10 percent of full output power upwards. These results comprehensively outperform its analog peers, one of which achieves ±2 percent regulation but only 204 W with 94.5 percent peak efficiency, and the loosely regulated solution sources 377 W but only manages +4/-9 percent regulation albeit with 96 percent peak efficiency.
It does not take an engineer to appreciate the significance of a first-generation product being able to match – let alone comfortably better in every respect – the achievements of well-refined forbears from the same design house. Factor in the unprecedented increase in functionality that the PMBus system delivers, the vast reduction in PCB real-estate that integral power-management functions permit, and numerous other factors such as second-generation developments featuring onboard energy optimization and it is hard to ignore the contribution that digital technology can bring to power conversion given sufficient R&D effort. From a modular power user’s perspective, products are available that are currently unmatched in performance, easy to apply, and pre-qualified to exacting telecoms-industry specifications.