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When things hot up

Thermal management in CompactPCI systems. By Ken DuBois and David Bowring.

Whilst CompactPCI systems are increasingly the control platform of choice in data acquisition, instrumentation, control systems, process automation and telephony applications, cooling these systems has become a major issue.

Thermal modelling has shown that cooling via a typical three fan tray is inefficient once the heat load exceeds 500W – at 21 slots, this averages 23.8W per slot. However, CompactPCI (CPCI) systems are configured for redundancy and full hot swap; including the pcbs, the power supplies and the cooling system. This may result in the need to remove in excess of l.5kW from a system – at 21 slots, this averages 71.4W per slot. In any case, noise levels should be less than 55dBA.

Since some cpus alone can produce 100W, prudent system oems ask for thermal modelling prior to design release. But, in order for thermal modelling to be undertaken efficiently, the system design has to be complete. Thermal modelling can be costly and time consuming and generally requires a thermal specialist. Many companies have neither the money, the time nor the manpower to undertake such a complex task.

Electronic engineers often have to deal with mechanical and cooling issues as well as board design. Needless to say, it's not always their favourite subject. There is a need for a fast, inexpensive, easy to use and relatively accurate thermal management evaluation system which can assess cooling needs. RiTherm from Rittal can help users to validate the thermal characteristics of CPCI and other systems before the boards are designed.

Design problems
Designers can face a number of problems along the way and the following outline some more common ones.

* Mechanical design
A typical problem is the mechanical design of the system itself. EMC requirements often place restrictions on the air inlet and air exit features, which means airflow through the internal construction provides poor 'convection' cooling. Forced air devices – such as fans and blowers – cannot make good a limited (poor convection) airflow design. Instead, they create unnecessary turbulence, resulting in higher audible noise and inefficient cooling.

The solution is to select a mechanical design with good convection airflow characteristics. For example, the support mechanics (the guide rails, the horizontal support rails or the formed sheet metal supports) for each pcb at each 4HP slot must provide for maximum airflow. There is no standard for how wide such a support extrusion/support sheet metal should be.

Taking emc into account, the air intake and outlet should allow as much air to pass through as possible. Whilst the commonly used 4mm diameter holes provide for good emc protection, they only open up approximately 58% of the panel for air passage. On the other hand, the 6mm hexagon design opening provides similar emc protection, but increases air passage opening up to 72%. A further benefit is less noise when air passes through it.

* Axial fans
The popular tube axial fan (120mm x 120mm), commonly used for system cooling, has very low 'static pressure' performance – 7.6mm water gauge – and deals poorly with system back pressure. Typically, three such fans are mounted and wired onto a metal chassis and placed beneath the pcbs.

Ideally, when dealing with heat losses of more than 500W, a blower solution with a static pressure of around 40mm water gauge should be used. Unfortunately, the blowers which meet this requirement are large and noisy and even more difficult to install and replace.

System disturbance is the biggest causes of fan noise. When placing a fan in a system, great care should be taken in locating components. Turbulence, fan load, system vibration and fan speed all affect the system noise.
However, there are many limiting factors when designing a system and usually the selected fan(s)/blower(s) are placed in one of three common positions: below the pcbs and pushing air; above the pcbs and drawing air; or in the rear of the subrack, drawing air from front and exiting to rear. Thermal modelling will reveal the best solution for any specific system design.

Reducing the fan/blower speed by using thermal speed control can reduce system noise, simultaneously extending the life of the cooling device. However, the chosen fan(s)/blower(s) must provide enough airflow for adequate system cooling under reduced or failed fan/blower operation.

* Power supply
The compact design (8HP and increments thereof) of hot pluggable power supplies – in accordance with IEEE 1101.1 and CPCI R2.1 – allow for little air to pass, yet they often require a linear airflow of 400ft/min for cooling. Redundant, current sharing, hot pluggable power supplies are often mounted to the right side in a 6U CompactPCI system – either side by side (6Ux8HP) or above each other when 3U high – leaving little or no room for air to move. This can cause overheating.

Unfortunately, pluggable power supplies are like solid 'bricks' with little airflow passages within their defined slot widths. System designers often hunt for every bit of usable space and are mostly limited by the number of active pcb slots to be provided for. This means there are often poor airflow passages/conditions left for power supplies.
Theoretically, one power supply should never be mounted on top of another. The lower one heats the power supply mounted above it, and the upper one receives limited airflow. The issue of static pressure of the cooling device (fan or blower) now becomes even more important.

* Boards
A single slot wide pcb may accommodate multiple cpus, a disk drive and a mezzanine card, creating a mix of airflow blockages and hotspots. In the past, every pcb had a component side and a solder side; today's pcbs are loaded with components/devices on both sides. In many cases, solder side covers are required on component side 2 for electrostatic discharge (esd) protection and safety reasons. Since component side 2 mounted devices may require cooling, UL 94VO rated solder side covers with perforations may be needed. These provide for esd protection, yet allow any heat build up to escape.

* Chimney effect
The 'chimney effect' is typically found in systems with unoccupied slots or when a pcb is removed from a fully loaded system. Since air will always flow along the path of least resistance, these empty slots form perfect chimneys and there is a considerable drop in system cooling efficiency. Measurements have shown that one empty slot without the blockage filler panel can reduce system cooling efficiency by up to 20%.
To prevent the chimney effect, the guide area of empty system slots should be blocked over in such a way that 'slot blockers' prevent the insertion of a pcb, but can be removed when the slot is required. A filler panel may also be required for emc reasons.

* Current draw
To avoid electrical noise interference and current consumption, fans/blowers should not be connected to system supply voltages. If there is adequate current available for dc fans/blowers to be powered by the system supply, then noise reducing filters may be added. Otherwise, a separate output from the system power supply or a separate power supply altogether may be required. Allowance should also be made for the fan/blower start up spikes.

Author profiles: Ken DuBois and David Bowring are with Rittal (www.rittal.com)

Author
Graham Pitcher

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