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Most major telecom companies now regard Advanced Telecom Computing Architecture (AdvancedTCA) as the way of the future. Based on an open-standard architecture, AdvancedTCA facilitates the use of commercially available off-the-shelf (COTS) technology. Industry analysts predict that by 2009, the AdvancedTCA market will be worth in excess of $7.5 billion. The latest market report (available at www.modt.se/AdvancedTCA.html) indicates that AdvancedTCA will comprise 50% of the wireless payload server market in the next five to seven years, and that the European share alone will be worth $3 billion in 2010.
The AdvancedTCA specification is documented in PICMG 3.0. This specification defines a compact, standardized shelf structure containing up to 16 hot-swappable boards, or blades, each of which can nominally demand up to 200 W of power and must accommodate dual voltage sources. The entire system must be capable of running from one or more conventional -48-Vdc telecom power sources and provide high availability through redundancy. The challenges this presents to power-system designers are significant, especially in terms of power-conversion efficiency, physical size and product reliability.
The physical structure for AdvancedTCA consists of chassis or shelf units available in several capacities to fit either 19-in. Electronic Industries Alliance (EIA) racks or 600-mm European Telecommunications Standards Institute (ETSI) equipment racks (Fig. 1). Each chassis contains a backplane that uses high-speed interconnection technology to interconnect vertically mounted, pluggable board or blade assemblies. The blades are designed to occupy 8 U of chassis space, and the boards themselves are 12.68 in. high × 11 in. deep with a blade-to-blade spacing of 1.2 in. Each board can nominally support up to 200 W of power dissipation on its 139.5-sq-in. area. EIA shelves can accept up to 14 blades, while ETSI shelves can accept up to 16.
Many of the powerful AdvancedTCA blades now appearing on the market make full use of their space allocation. A number of the latest-generation carriers already support up to four Advanced Mezzanine Cards (AdvancedMCs) for expansion purposes, and the AdvancedTCA specification allows for as many as eight. As a consequence, little physical space is available on each board for power-conversion technology (Fig. 2), resulting in a demand for high-efficiency dc-dc converters with exceptionally small footprints.
The high power density of the overall AdvancedTCA chassis is achieved via a standardized forced-air cooling system, which is part of the architectural definition. Each chassis takes in ambient air at the bottom front of the unit through cooling fans. The air is then routed with a plenum to flow vertically up through the pluggable boards. Another plenum at the top of the chassis directs the air to exhaust at the back.
IBAs for AdvancedTCA
Despite the comprehensive nature of the AdvancedTCA specification, the choice of on-board power architecture is left to the board designer's discretion. PICMG 3.0 merely stipulates that the board must be capable of accepting two voltage-supply feeds of -48 Vdc. Both feeds must actually accommodate operating voltages in the range of -43 Vdc to -72 Vdc, but will typically support even lower voltages. PICMG 3.0 also contains a set of rigorously defined interface specifications for the board, covering overcurrent protection, current inrush, EMI and transients (including holdup).
However, PICMG 3.0 does not dictate the method for combining the dual redundant -48-Vdc feeds. The most cost-effective method involves combining the two feeds through some form of ORing function — implemented using diodes or FETs — before directing them to a single dc-dc converter. The ORing function must ensure that the loss or shorting of any individual feed or its return will not disrupt power to the board, and is located prior to the conversion function. Alternatively, power feeds A and B can be fed directly to individual dc-dc converters, the outputs of which are then ORed; this provides redundancy in the power-conversion function, but incurs additional cost, board space and efficiency penalties.
While many power-conversion architectures are permitted, it is anticipated that most designers will favor an intermediate bus architecture (IBA) using a 12-V intermediate bus voltage (Fig. 3). An intermediate bus converter (IBC) is usually preferable to traditional brick-type dc-dc converters simply because distributing 48 Vdc across the board is not deemed a good design practice, and 12 V is used in preference to 5 V because of the power levels involved on AdvancedTCA boards. In any case, the AdvancedMCs require 12 V for their payload power.
Design Goals and Test Results
Artesyn has leveraged its existing IBC technology to develop a high-power-density converter optimized for high-availability telecom applications such as AdvancedTCA blades.
The ultimate goals of the design were to provide a robust, reliable and cost-effective power solution while minimizing the footprint. In addition to the footprint of the converter module itself, a further design consideration was the footprint of ancillary components such as the holdup energy storage elements.
The result is the ATC210 (Fig. 4). This converter measures just 2.3 in. × 1.8 in., has a rated output power of 210 W and employs conservative derating criteria to achieve a typical mean time between failure (MTBF) in excess of 2 million hours, in accordance with Telcordia SR-332.
This module implements the features desired for an AdvancedTCA blade implementing an IBA as illustrated in the block diagram of Fig. 5.
The most obvious method for minimizing package footprint is to make effective use of the available height. The form factors defined by AdvancedTCA have reversed the recent trends of smaller card pitch in most telecom systems. The PICMG 3.0 specification allows for components approximately 0.8 in. high. This permitted a proven IBC technology to be stacked on a lower board. The additional functionality required of an AdvancedTCA power-conversion solution can then be implemented on the lower board without significantly increasing the solution footprint. This stacked approach can be easily seen in Fig. 4.
Another key design criterion that has a significant effect on pc-board footprint is the holdup time. Meeting the PICMG 3.0 specification is no simple task. Energy storage is required to allow for continuous operation of a blade even if the input-voltage sources are pulled to 0 V for 5 ms. This type of event may start with a short-circuit fault on an adjacent blade and end when that blade's fuses open and isolate the fault. The ATC210 implements a voltage-clamp circuit, which enables capacitors with lower voltage ratings — and therefore smaller physical dimensions — to be used to achieve the same level of energy storage. The holdup capacitance is not included on the converter module, but is chosen by the blade designer based on actual power usage. This allows optimization of the amount, size and placement of the capacitance.
In addition to minimizing the converter's footprint using the aforementioned techniques, it was essential that this be accomplished while maintaining robustness and reliability for operation in carrier-grade environments.
For example, Artesyn chose to implement the necessary ORing function of the ATC210's dual inputs using diodes rather than FETs, to ensure robustness and reliability and minimize component cost. It also should be noted that the voltage-clamp circuit described in this article is very simple with a low parts count contributing to a reliable design.
The ATC210 features a built-in serial bus interface for monitoring and alarms, which further contributes to overall system availability and enables end users to easily integrate the IBC into their distributed power schemes.
Ultimately, designers must make their own decisions about on-board power-conversion architectures. Some AdvancedTCA boards will not require the densest solutions. But many will, including the carrier board example here. The information presented in this article offers designers an off-the-shelf innovation that addresses the design goals of a high-density, rugged and reliable solution. It also offers designers who may be unfamiliar with -48-Vdc telecom requirements a solution that minimizes risk when time to market is critical.