Power Distribution

Power Management: It’s in the System

The recent escalation in power density could dramatically affect system operation and capabilities. Several bus architectures reveal significant new approaches to power distribution and system management.


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It is fair to say that no single aspect of system design will have the potential to radically change the operation and capability of future systems as much as the dramatic escalation in power density. Until recently, system voltage distribution has continued to follow a downward trend, which is the result of low voltage signaling methods such as LVDS. However, to support a distributed 3.3v power bus means very high current levels. Therefore, to achieve higher power levels, system designers have resorted to bussed 12, 24 and 48 VDC, which is then downconverted on each board as required for the 3.3v components. At the same time, the focus of many new embedded architectures is to support the communication needs of both the public network infrastructure and industrial networking infrastructure applications where 48 VDC battery power is provided. Therefore, there is an additional compelling reason for designers to choose 48 VDC for power distribution, as in AdvancedTCA (ATCA).


Developers of the ATCA specification intended systems based on the spec to operate in today’s typical unmanned central office environment, while remaining under closely coordinated control of a remote system manager. PICMG 2.9 is therefore an important subset of the 450-page specification. It includes the use of an Intelligent Platform Management Interface (IPMI) between various system components. This results in the ability to control fan speeds and board power consumption by the local system manager or a remote management entity in response to local environmental factors or the requirements of other system resources.

AdvancedTCA utilizes the concept of field replaceable units (FRUs). These could be node or switch boards, fan or blower assemblies, power inlet modules, power bricks or individual mezzanine modules on a carrier card. Power is typically supplied to an ATCA chassis through a power entry module (PEM), which is another FRU. This component provides filtering to control conducted emissions, over-current protection and transient fault protection. If an IPMI controller were added, it would make sense to also have a current or voltage monitor.

The ATCA specification provides for two 48 VDC power circuits: “A” and “B.” Power distribution schemes are defined that allow both power circuits to be fed separately. Alternately, if one battery feed fails a special circuit, it will allow the single remaining battery source to feed both the A and B backplane power domains. In addition, wiring instructions are defined to allow the backplane to be supplied by separate power systems. This redundancy would let each switch slot in a dual star system be on different power circuits. It is also possible to have separate power domains for each node card of a pair. Using such power schemes, highly fault-tolerant systems can be constructed.


The MicroTCA specification had not been completed at the time this article was written. However, enough of its features have already been defined so that several detailed comments can be made with some confidence.

MicroTCA is a backplane architecture designed to support large numbers of removable AMC modules outside of an ATCA chassis. It defines the necessary wiring for a variety of different power source voltages. These power supply environments include: 48 VDC battery plants, 60 VDC battery plants, 24 VDC battery plants, 100V AC mains, 120V AC mains or 230V AC mains.

A modular power supply interface connector has recently been defined by the MicroTCA working group. It is intended that a variety of common modular power units will become commodity components so that any MicroTCA system can be equipped with the proper power supply modules for the power source environment where it is to be deployed.

The major difference between a MicroTCA chassis and an ATCA chassis is the power distribution approach. Unlike ATCA, which only provides 48 VDC on the backplane, a MicroTCA backplane provides 12 VDC and 3.3 VDC. The reason is that the AMC modules expect these voltages to be supplied and do not have provisions for being powered directly by 48 VDC.

In an ATCA system, the 12 VDC and 3.3 VDC are provided by the carrier card on which a given AMC module is supported. This means that in a MicroTCA system where the AMC modules plug directly into a common backplane, those voltages must be distributed to each slot by the backplane.

CompactPCI Express

CompactPCI Express is the natural migration path for cPCI systems. Each slot type has different power requirements. If all pins in a system slot were used to their maximum, the slot could draw about 480W. To keep power consumption under control, there is a stipulation that the combined current of all inputs for a system slot cannot exceed 45 amps. A Type 1 slot has a similar stipulation of 50 amps for the maximum combined power.

CompactPCI Express implements hot plug in accordance with the PCI Express Base Specification 1.1, and with the electrical requirements included in the PCI Express Card Electromechanical Specification Revision 1.1.

The CompactPCI Express specification permits power entry in accordance with PICMG 2.11, which uses the 47-pin Positronic power connector. This is the preferred implementation when a pluggable modular power supply must be used. The more typical implementation is pressfit power studs that are intended to be cabled to directly from a fixed power supply, using standard cabling, ring terminals, loc washer and hex nuts (Figure 1).

The electrical requirements for minimum decoupling capacitance, noise and ripple are tightly defined. For hot-plug boards there are additional requirements for maximum current slew rate, initial hot-plug capacitance, peak pre-charge current and maximum board capacitance.


The VITA 41 VXS standard is a convenient hybrid. It accommodates the established VME64x architecture, but provides a much higher center connector P0/J0 that supports four serial lanes of 3.1 to 6 Gbits/s for a cumulative bandwidth of 10 to 20 Gbits/s for each of two fabrics.

With the exception of the new MultiGig P0/J0 connector—which supports two high-speed fabrics, IPMI signals and a combined keying and guide module—the payload cards are unchanged. In the case of a dual star implementation, the two switch cards are implemented entirely with five new MultiGig connectors and a 6-pin Positronic power connector. This switch card power connector can supply 150W at 5 VDC.


VITA 46 VPX is the very latest VME-derived architecture. It may not be fair to judge this standard since it is not yet finished. VITA 46 references VITA 38 and requires that the two IPMB I2C circuits be implemented in the backplane. JTAG signals may also be present in the backplane as part of the VME64x signals if VITA 46.1 is implemented in the VPX J1 connector. However, modules are not required to implement VITA 38 and no details have been provided to make such an implementation practical.

At present, backplane power voltage is designated to be between 5 VDC and 50 VDC and held to within 10% of the nominal. Peak-to-peak ripple must not exceed 10%. There are six contacts provided for +Vs1 and an equal number of -Vs1 return contacts. Six contacts are also provided for +Vs2 and an equal number of -Vs2 return contacts. Each contact is rated at 8 amps. A VITA 46 backplane layout is shown in Figure 2.

With 48 VDC supplied to the backplane, this could provide for a whopping 4,608W of power for each slot. Even with Vs1 and Vs2 allocated to 3.3 VDC, there would be provisions for 422W of power per slot. Luckily, VITA 48 is a mechanical chassis system that provides for liquid-cooled modules. This could be the explanation for this unusually generous power allocation.

There are no features in this standard that presently define power inlet module function, power initialization function, system management command set, conduction emissions, negotiated power usage or N+1 power redundancy.

Not all new architectures implement the same provisions for either system management or power distribution (Figure 3). However, where power distribution has been addressed, there is a trend for 48V distribution. Even for systems that are intended to be powered off of 120 VAC or 230 VAC mains, 48 VDC distribution still offers an important advantage by allowing lower current levels at the backplane interface while still meeting higher power needs.

Elma Bustronic
Fremont, CA.
(510) 490-7388.