Architectural Attitudes On Power

Advanced Telecom Computing Architecture (ATCA) is the newest and most complete open standard for the implementation of telecom computing and data-centre systems. Conor Quinn looks at the power architecture options available for ATCA-compliant boards.

ATCA addresses the needs of telecom and datacom applications with an open standard documented in PICMG 3.0. It aims to provide high levels of availability via redundancy and hot-swapping techniques, high packaging density, reduction of development cost and time to market for system developers and an environment for standardised hardware support from multiple suppliers.

The physical structure for ATCA consists of chassis or shelf units available in several capacities to fit either 19in EIA racks or 600mm ETSI equipment racks. As shown in Figure 1, each chassis contains a backplane that interconnects vertically-mounted pluggable board or blade assemblies. High-speed interconnection technology addresses the needs of the next-generation high-performance processors. The blades or boards are 8U (357mm) high and 280mm deep with a board to board spacing of 30.5mm (1.2 in). Each board can support up to 200W of power dissipation on its 903cm2 (140 sq in) of area. EIA shelves can accept up to 14 boards and ETSI shelves up to 16.

The high power density is achieved by means of a standardised forced-air cooling system that is part of the architectural definition. Each chassis takes in ambient air at the bottom front of the unit through cooling fans (not shown in Figure 1). The air is then routed with a plenum to flow vertically up through the pluggable boards. Another plenum at the top of the chassis then redirects the air to exhaust at the back.

POWER ARCHITECTURE
The dominant input power source for ATCA systems will be existing telecom -48Vdc battery power plants, which allows ATCA systems to be mixed with existing equipment at telecom sites. ATCA shelves are configured to accept dual redundant -48V feeds, referred to as A and B feeds. Some systems further segment the power feeds to provide additional redundancy within the ATCA shelf. The -48V feeds are distributed to the individual boards or blades through zone 1 of the shelf backplane (the blue connectors in Figure 1). Zone 2 (immediately above the blue connectors) is allocated for board-to-board signal interconnection, and zone 3 (not shown) provides board cabling access from the rear of the enclosure.

The board-level power architecture is characterised by flexibility, but there are several functions mandated by the ATCA specification. These generally occur prior to any power-conversion circuitry and include:

  • Fusing
  • ORing of A and B power feeds
  • Hardwired sequencing
  • Noise filtering
  • Inrush current control
  • Hold-up capacitance

Figure 2 shows a generalised depiction of these board-level functions. Each power feed and return must be individually fused. ORing diodes ensure that the loss or shorting of any individual feed or its return will not disrupt power to the board, and are normally installed prior to the conversion function as shown. Alternatively, power feeds A and B can be fed direct to individual DC/DC converters, the outputs of which are then diode ORed. This provides redundancy in the power-conversion function, but incurs additional cost, board space and efficiency overheads. Conversion redundancy can still be achieved with the arrangement shown in Figure 2, either by duplicating conversion and load circuitry on the board or by providing full redundancy with dual pluggable boards.

Different length pins on the board connector facilitate basic sequencing as shown in Figure 2. The input power returns mate first, and can be connected to an optional pre-charge network to charge converter input capacitance prior to the connection of operational power.

The -48V A and B power feeds connect next, followed by the short pins, which generate an enable signal for the board-level power-control circuitry that handles all subsequent power sequencing and monitoring; this may involve negotiation with shelf-level power-control functions.

Each board is required to have sufficient capacitance on the combined 48V power feed to provide at least 5 ms of hold-up in the event of supply disruption. During this time, the 48V board power must remain above the lower operating input voltage limit of the converters.

There is some uncertainty over the interpretation of the ATCA specification regarding the assumed input voltage prior to disruption. Some interpret it to be a normal operating voltage (i.e. 48 to 53V). Others assume it to be the worst case operating voltage of about 43V. Using the most conservative interpretation of 43V and the maximum ATCA board power of 200W, the size of hold-up capacitor can be estimated as follows:

Starting voltage = 43V, Converter lower limit = 36V, Delta V = 7V

Average voltage during hold-up = 40V, Average capacitor current = 200W/40V = 5A

C = (I) (dt) / (dV) = (5A) (5 x 10-3 sec) / (7V) = 3600 mF

This is a substantial capacitor with significant stored energy and is one of the reasons that the ATCA specification requires inrush current limiting and hot-swap capability.

Converting the -48V input power source to the circuit voltage levels can be achieved in a variety of ways, provided there is at least one isolation barrier. Figure 2 shows how either an intermediate bus architecture (IBA) or distributed power architecture (DPA) can be used for generating the circuit voltages. The IBA solution has the advantage in that it only involves one isolated conversion function, which is provided by the Intermediate Bus Converter (IBC).

The output of this converter is an intermediate voltage level, typically 12V, which is then distributed to relatively inexpensive and efficient non-isolated point-of-load (POL) converters located near their respective loads. These POL converters are optimised to provide excellent voltage regulation and transient response for loads with highly dynamic current demands. IBA will typically be a lower-cost solution.

Another advantage of IBA is that the intermediate bus voltage is usually appropriate for powering PCI Mezzanine Cards (PMC) and Advanced Mezzanine Cards (AMC) in architectural extensions of ATCA.

BETTER CONVERSION EFFICIENCY
In the case of DPA, multiple isolated DC/DC converters (bricks) are needed. This results in only a single conversion stage for each output voltage and sometimes produces a slightly better overall system conversion efficiency, depending on the output voltages. Conversion from 48V directly to a low voltage such as 1.2V is not always the most attractive option. Sometimes non-isolated POL converters can also be used with DPA, as described later. The downside to DPA is that the duplication of the isolation function adds cost and size to the total power solution. It is expected that many ATCA board designs will use elements of both IBA and DPA, using an IBC in conjunction with both non-isolated and isolated DC/DC converters. The flexibility to optimise board power architecture by selecting the best attributes of each approach is a very useful tool for the ATCA board designer.

IMPLEMENTATION OF BOARD POWER FOR ATCA
The following examples illustrate how ATCA board power systems can be configured with standard off-the-shelf conversion modules, and how different architectures can be used and mixed depending upon application needs. Due to space constraints, these examples are simplified and do not show details such as the 48V input circuitry, decoupling capacitors and control components. Artesyn Technologies' conversion components—such as those shown in Figures 3 and 4—are used for illustration and sizing purposes, but the concepts also apply to products from other suppliers.

Shown in Figure 5a is a system implemented with a pure IBA approach. The total output power is 149.3W, and the 3.3V control voltage shares the bus converter with the other outputs. This system can be easily modified to use an isolated converter powered from 48V for the control voltage, as shown in Figure 5b. All of the specified power modules are low-profile open-frame designs. The overall conversion efficiency of this design, from the 48V input to the system operating voltages, is 88.1%, which is quite good in spite of the double conversion stages. The board surface area required for the power conversion modules is just 6.48 in2, which is only 4.6% of the total ATCA board area. IBA approaches will typically need less board area than pure DPA systems with multiple isolated converters.

Figure 5c shows a DPA-based system that also uses some 'spot power' derived from non-isolated POL converters. An independent isolated DC/DC converter is used to generate the 3.3V control voltage. The bulk 3.3V power is provided by a dedicated DC/DC converter operating well within its rated output current. The 5V DC/DC converter's output is shared between the system 5V demand and the POL converters that generate the 1.5V and 1.8V levels.

The input power for this system is 182.8W. Allowing perhaps 10W for power losses in the ORing diodes and other front-end functions, this system is approaching the maximum allowable power on an ATCA board. An overall conversion efficiency of 89.8% is achieved, which is marginally higher than the IBA system. All of the conversion modules used are low-profile open-frame designs with a maximum above-board height of 0.35 in. Note that the area required for the power-conversion modules is increased to 8.46 in2.

Both DPA and IBA can be used to create efficient, easily configurable power solutions for ATCA boards. Although each approach involves trade-offs, both are supported with readily available and reliable converters. As ATCA becomes more widely used, it is expected that additional products will be developed to address the needs of the board power designer. One possibility would be the integration of much of the 48V front-end functionality, such as ORing diodes, inrush control and filter components into the intermediate bus converter.

ATCA is likely to prove the most attractive system architecture for telecom and datacom for the foreseeable future. Its open nature allows easy and flexible interconnection of hardware from multiple vendors, facilitating fast development and highly available systems. Artesyn Technologies is fully committed to ATCA, offering a range of products that directly address the needs of the standard. The TQW intermediate bus converter, for example, is specified over the ATCA input worst-case voltage range in addition to the normal telecom voltage range. Furthermore, the company intends to introduce additional power products in 2005 that further enhance the ATCA product set and make the power designer's task easier.

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