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Choosing the right PSU for a CompactPCI system
The increasing use of high-speed multicore processors in CompactPCI systems is placing extra demands on power supply units. It is therefore important to select the PSU carefully in order to avoid problems and system downtime, say Keith Reynolds, Oliver Kistner and Martin Traut.
The specifications for individual bus technologies vary considerably in the requirements they set out for system power supply. In CompactPCI systems, output voltages of 5V and 3.3V (for processors) and ±12V (e.g. for hard disk drives and fans) are required as a rule. Additionally, the processors used in the system place particular demands on the dynamic behaviour of the PSUs, and this also needs to be given careful consideration.
Where Pentium M processors are employed – mainly in industrial and process control applications – there is a somewhat increased initial current, but then the current remains relatively stable during operation, without peaks.
By contrast, newer multicore processors present significantly greater demands on the power supply system. Such processors are primarily found in applications involving high data rates, e.g. telecommunications, Wi-Fi, passenger information systems, and so on. Boards with multicore processors are generally provided with a power-saving mode, so the processor has to frequently rise from its sleep state to full power in a very short time, with a steep rise in current. While the initial current is very small, short current peaks of around 100 µs at up to 18A repeatedly occur during operation.
ATX PSUs
CompactPCI systems typically use one of two different types of power supply: an ATX unit or a 19in. plug-in PSU. ATX stands for 'Advanced Technology eXtended', and these power supplies generally offer about 350W of nominal output. Originally designed as a relatively low-cost unit for mass-market office PC systems, the ATX PSU is also suitable for many industrial applications.
The specifications for individual bus technologies vary considerably in the requirements they set out for system power supply. In CompactPCI systems, output voltages of 5V and 3.3V (for processors) and ±12V (e.g. for hard disk drives and fans) are required as a rule. Additionally, the processors used in the system place particular demands on the dynamic behaviour of the PSUs, and this also needs to be given careful consideration.
Where Pentium M processors are employed – mainly in industrial and process control applications – there is a somewhat increased initial current, but then the current remains relatively stable during operation, without peaks.
By contrast, newer multicore processors present significantly greater demands on the power supply system. Such processors are primarily found in applications involving high data rates, e.g. telecommunications, Wi-Fi, passenger information systems, and so on. Boards with multicore processors are generally provided with a power-saving mode, so the processor has to frequently rise from its sleep state to full power in a very short time, with a steep rise in current. While the initial current is very small, short current peaks of around 100 µs at up to 18A repeatedly occur during operation.
ATX PSUs
CompactPCI systems typically use one of two different types of power supply: an ATX unit or a 19in. plug-in PSU. ATX stands for 'Advanced Technology eXtended', and these power supplies generally offer about 350W of nominal output. Originally designed as a relatively low-cost unit for mass-market office PC systems, the ATX PSU is also suitable for many industrial applications.
CompactPCI system with ATX power supply unit
It is either housed in a simple metal casing or used as a PCB-based unit with heat sinks, then integrated into a system. As the mains power input and the outputs to the backplane are via cables, the ATX unit is tied into the system and cannot be easily or quickly removed and replaced when the need arises. Also, since these PSUs are not mounted in the defined airflow of the board cage, additional measures are often requiresd to cool them.
19in. plug-in PSUs
Pluggable PSUs with a 19in. form factor generally have a rated output of 250W. They are easy to integrate into a system, since they can be simply inserted into a suitable slot on the front, like any other plug-in module. No further wiring is necessary. This simple pluggability is an important feature for rapidly exchanging units and thus for ensuring high system availability.
19in. plug-in PSUs
Pluggable PSUs with a 19in. form factor generally have a rated output of 250W. They are easy to integrate into a system, since they can be simply inserted into a suitable slot on the front, like any other plug-in module. No further wiring is necessary. This simple pluggability is an important feature for rapidly exchanging units and thus for ensuring high system availability.
CompactPCI system with 19in. plug-in PSU
In addition, placing the PSU within the board cage is an advantage in terms of cooling. The system's thermal management, using fans or fan modules, is specifically designed for this arrangement of the boards, and so no additional cooling measures are necessary. However, the spatial dimensions of 19in. PSUs are fixed by the standards, so the designer is often obliged to use costly smaller components in order to maintain conformity.
Dynamic behaviour of the PSUs
Since ATX and 19in. plug-in PSUs have different designs of regulation system, they exhibit different dynamic behaviour patterns. The low-cost ATX units are less technologically sophisticated than 19in. plug-in units. An ATX power supply has, for example, only one converter, which must supply all four required voltages and at the same time evaluate the current status. Should a situation arise in which, say, the 5V or the 12V rail experiences a sudden change in load, the reaction time of the converter is somewhat slow.
Another negative factor here is the inductance of the long cables used in the ATX unit. The result is that the slow reaction of the PSU causes an excessive voltage drop to the processor. The processor board includes a voltage monitoring function, and whenever even one of the voltages is not correct, the processor receives a reset command. It can thus happen in some circumstances that a processor running a high-speed application and powered by an ATX power supply performs repeated resets, and the application is unable to run.
In 19in. plug-in PSUs there are generally two separate converters that act as a pair to supply the output voltages. Since the work is shared between the two converters, they can smooth out extreme dynamics more easily. In addition, in 19in. units the 12V rail is also monitored to check whether the voltage is still within the prescribed range. Thanks to these measures and a generally more advanced technology, even sudden load changes do not present a critical situation for these power supplies and the processors they supply.
Which power supply for which application?
In many industrial applications based on Pentium M processors, ATX power supply units can still provide a cost-effective solution. However, because they are hard-wired into the system, they cannot be used where redundancy is a requirement. In such cases, 19in. plug-in PSUs are the only practical option. A significant advantage of 19in. plug-in PSUs is that even where two of these units are fitted (for redundant operation), these require about one-third less space than a single ATX unit. Plug-in 19in. PSUs are also recommended for applications based on multicore processors, including existing CompactPCI systems that have been upgraded with new CPU boards featuring multicore processors. Here, the existing ATX PSUs should be replaced by 19in. plug-in units. The latest generation of CompactPCI Serial systems have utilised 19in. plug-in PSUs from the outset. In comparison to the system overall, the cost of the PSU is relatively small, but it nevertheless ensures the reliable operation of the system.
Keith Reynolds is technical manager of Schroff UK, Oliver Kistner is principal engineer for power electronics at Schroff GmbH, and Martin Traut is product manager for integrated systems at Schroff GmbH.
Dynamic behaviour of the PSUs
Since ATX and 19in. plug-in PSUs have different designs of regulation system, they exhibit different dynamic behaviour patterns. The low-cost ATX units are less technologically sophisticated than 19in. plug-in units. An ATX power supply has, for example, only one converter, which must supply all four required voltages and at the same time evaluate the current status. Should a situation arise in which, say, the 5V or the 12V rail experiences a sudden change in load, the reaction time of the converter is somewhat slow.
Another negative factor here is the inductance of the long cables used in the ATX unit. The result is that the slow reaction of the PSU causes an excessive voltage drop to the processor. The processor board includes a voltage monitoring function, and whenever even one of the voltages is not correct, the processor receives a reset command. It can thus happen in some circumstances that a processor running a high-speed application and powered by an ATX power supply performs repeated resets, and the application is unable to run.
In 19in. plug-in PSUs there are generally two separate converters that act as a pair to supply the output voltages. Since the work is shared between the two converters, they can smooth out extreme dynamics more easily. In addition, in 19in. units the 12V rail is also monitored to check whether the voltage is still within the prescribed range. Thanks to these measures and a generally more advanced technology, even sudden load changes do not present a critical situation for these power supplies and the processors they supply.
Which power supply for which application?
In many industrial applications based on Pentium M processors, ATX power supply units can still provide a cost-effective solution. However, because they are hard-wired into the system, they cannot be used where redundancy is a requirement. In such cases, 19in. plug-in PSUs are the only practical option. A significant advantage of 19in. plug-in PSUs is that even where two of these units are fitted (for redundant operation), these require about one-third less space than a single ATX unit. Plug-in 19in. PSUs are also recommended for applications based on multicore processors, including existing CompactPCI systems that have been upgraded with new CPU boards featuring multicore processors. Here, the existing ATX PSUs should be replaced by 19in. plug-in units. The latest generation of CompactPCI Serial systems have utilised 19in. plug-in PSUs from the outset. In comparison to the system overall, the cost of the PSU is relatively small, but it nevertheless ensures the reliable operation of the system.
Keith Reynolds is technical manager of Schroff UK, Oliver Kistner is principal engineer for power electronics at Schroff GmbH, and Martin Traut is product manager for integrated systems at Schroff GmbH.
Contact details for publication:
www.schroff.co.uk
schroff.uk@pentair.com
Tel: +44 (0)1442 218726
Press inquiries to:
Keith Reynolds
Schroff UK Ltd
keith.reynolds@pentair.com
Tel: +44 (0)1442 218726
or
Rick Bauling
RJB Communications
rbauling@rjbcoms.com
Tel: +44 (0)1234 782255
www.schroff.co.uk
schroff.uk@pentair.com
Tel: +44 (0)1442 218726
Press inquiries to:
Keith Reynolds
Schroff UK Ltd
keith.reynolds@pentair.com
Tel: +44 (0)1442 218726
or
Rick Bauling
RJB Communications
rbauling@rjbcoms.com
Tel: +44 (0)1234 782255
21st May 2012, Ref. SC341A/uk1208