The Quest to Keep Electronics Cool
When I/O connectors block airflow, a toolbox of solutions is required to keep today’s fast, dense, and hot systems cool.
Lithium ion batteries are not the only source of excessive heat in electronic devices. A nasty combination of dense system packaging and increased power consumption tends to concentrate heat inside the box, causing component stress and premature failure. Demand for devices that deliver greater speed and functionality in smaller envelopes put heat-generating devices in closer proximity, making it more difficult to keep system temperatures below acceptable limits. Thermal management in electronic systems is not a new problem, though. In 1985, the Cray 2 supercomputer kept its components cool by immersing PCBs in circulating fluorinert liquid, which was a costly but effective solution. We’ve been looking for better solutions ever since.
Today, conventional system designs often use convection or forced air to keep their cool. However front panel space that has traditionally been available for venting the system is now being consumed with an increasing number of pluggable I/O connectors that block airflow. In response, manufacturers of I/O connectors are being forced to juggle three key issues: signal speed, packaging density, and thermal management.
A variety of solutions have evolved, each offering a balance of efficiency, consumed space, and cost.
In some cases, the addition of a passive heat sink is able to transfer heat to the ambient environment and away from critical semiconductor devices.
When convective air circulation is not sufficient, equipment manufacturers have used a series of fans to simply blow the heat out of the box. However, the efficiency of forced-air cooling declines as the temperature of the incoming air increases. The traditional solution for this problem has been to increase the number and speed of the fans, but this can sometimes result in sound levels that exceed NEBS and OSHA acoustic noise limits. In large server farms and data centers, the cost of supplying refrigerated air can be the highest operating expense.
Individual heat-generating devices, such as processors, may be fitted with a custom heat sink and integrated fan to maintain safe junction temperatures.
Due to the prevalence of both the heat problem and potential solutions, the use of thermal analysis tools capable of identifying hot components and predicting the effectiveness of the chosen thermal strategy has become a required step in the new product design process.
Embedded high-performance computing systems used in military, avionic, and industrial control applications are often environmentally sealed, eliminating the passive or active air circulation strategy. In these applications, cold plates and/or heat pipes transfer heat to cooling fins built into the surface of the box. Recirculating liquid cooling has been used to remove the heat created by the thousands of watts consumed by high-performance computing racks, but results in a significant increase in initial, operating, and maintenance costs, as well as system complexity.
Connectors That Help Keep Electronics Cool
Connector manufacturers have recognized the thermal management challenge and made changes to many of their products in response. In some cases, the height of interfaces has been reduced to minimize the obstruction of cooling airflows. Connector housings have also been modified to allow air to circulate around the contacts, and new, higher conductivity copper alloys used in power contacts can reduce losses and resulting heat.
Thermal management at the I/O panel has become especially challenging, as the demand for ever-greater throughput has resulted in panels that are solid blocks of connectors instead of a combination of connectors and cooling vents.
Pluggable connectors that range from SFP to QSFP56 modules have become a standard high-density interface. Up to 72 of these modules can be mounted on a 1 RU switch, resulting in many heat-generating devices in close proximity. Thermal output for optical modules can range from 0.5 watts for a SFP to 32 watts for a CFP.
One solution is to simply allow ambient air to circulate around the module. Pluggable cage assemblies may be vented to improve cooling effectiveness.
When this solution is not adequate, cages may feature an opening designed to allow a secondary heat sink to make direct contact with the surface of the module.
This can work well, but the relatively tall cooling fins limit the number of connectors that can be mounted on the I/O panel.
Micro QSFP pluggable connectors from TE Connectivity feature integrated thermal heat sink fins on the modules and a vented PCB cage assembly. TE is also promoting their thermally enhanced zQSFP+ cages that provide improved airflow through the cage, resulting in more efficient heat dissipation.
The Octal Small Form factor Pluggable (OSFP) is a new pluggable module that supports 400G optical data links inside data centers, campuses, and other mid-reach applications. These modules feature integrated open- or closed-top heat sink fins as well as ventilation holes, as defined in the specification, which also includes recommendations for airflow and pressure.
When packaging density threatens to overcome the thermal limits of the system, a more active solution may include thermoelectric cooling. Optical pluggable connectors used in longer reach applications are a particular challenge. Optical SFP connectors can have a rated maximum operating temperature of 70°C, while ambient temperatures can reach 90°C.
A thermoelectric cooler uses the Peltier effect to transfer heat from one side of the semiconductor device to the other. The cooler consists of a thermal gasket, a heat spreader, and the active device, which clamps to the top of a pluggable cage and is capable of lowering the temperature by as much as 30°C. Where traditional passive and active cooling solutions only lower the temperature to the ambient, a thermoelectric cooler can reduce transceiver temperatures below the surrounding temperature. Additionally, reversing the polarity of thermoelectric device, a transceiver can be heated can heat transceivers used in cold environments. Since these devices have no moving parts, they are particularly effective in remote outdoor locations where long-term reliability is essential. The evolution toward 5G networks may generate a host of new applications for active transceiver coolers in equipment that must survive extreme outdoor environments.
Keeping high-performance electronics within a critical range of operating temperatures has become an integral part of the design process. As engineers continue to cram more functions running at higher speeds into smaller spaces, a toolbox of passive and active solutions is required to enable electronic products keep their cool.
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