As embedded computing systems become more powerful, so are the challenges to protect and cool the payload. In the past few years, we have seen the power of a single board increase in most cases to over 100W per slot. To further challenge the designers, these systems are being deployed in rugged environments with a push to use COTS (commercial off-the-shelf) products. Recently, liquid-cooled systems have been developed to combat these However, there are some challenges with liquid cooling that can make this technology prohibitive. For example, not all boards are available in conduction- cooled format, or there may not be an external chiller/pump available to implement the liquid approach. So how does a designer handle an environment where there is no liquid coolant available, ambient temperatures hover around 55°C, the enclosure has a payload of 500W, and the client wants the system to operate on numerous rugged platforms (ground vehicle, rotary wing, UAV, etc.)? Oh, and the enclosure has to be sealed to protect the COTS boards from the harsh environments and EMI concerns. And with all of this, there is a desire to monitor the temperatures/ health of the system to protect the expensive payloads.
Add Monitoring to ATR
One approach to this design challenge is to integrate an air-to-air heat exchanger into a standard ATR package with a monitoring system. We will look at this approach in a little more detail with the specifications as follows:
- Top-load Enclosure
- COTS air-cooled payload dissipating up to 500W
- Ambient temperature up to 55°C
- Harsh environment to meet MILSTD- 810
- EMI — Designed to meet MIL-STD 461 (CE101, CE102)
- Front-panel access to all power and I/O connectors
- An additional electronics package, dissipating more than 100 W, is mounted inside of the controlled environment of the enclosure. In addition to these requirements, there is a concern that the accumulation of fine dust particles on the boards would prevent proper cooling, and larger particles would cause abrasion of the boards and other electronic components.
Exterior Mechanical Design
The first challenge is providing a rugged outside housing for the payload. In this solution, we chose to go with an ATR-style (see Figure 1) form factor because it is a common platform that has had a proven record for many years. The other advantage to this style of form factor is that it will easily mount into many existing applications, and there are a number of readily available shock-isolated trays on the market that can help meet the rugged vibration environments. It is also important to find that optimal balance between the weight and ruggedness required. A designer could go with a brazing approach, but this typically adds unacceptable cost and lead time to the program. The more economical approach would be to go with a welded/bolted-together construction method. This still provides significant strength, but also reduces the weight, cost, and lead time. It is also very important to include a rugged military finish or paint on the outside surfaces to further protect the enclosure from the harsh environments. The paint for this solution was chosen for its UV reflective properties to reduce the heat load generated by external solar radiation. In addition to the structural integrity and resistance to the environments, serviceability plays a big factor in the final design. The air intake filters on this unit are important to get the air into the chassis, but just as important is the ability to remove these filters and service them in the field, eliminating costly depot maintenance time.
The second challenge is the requirement that the internal air be isolated from the external environment. The COTS air-cooled cards are not robust against accumulation of airborne sand and dust, and must be protected against this environmental threat. In addition, many board sets can generate substantial EMI, which must not only be contained within the unit, but must also be prevented from interfering with sensitive components in the electronics package (see Figure 2). Directly beneath the card cage is a small volume approximately 1.5" high, which can be used for additional payload. This space is EMI-isolated by aluminum walls and a conductive plane on the backplane. Airflow into this space is provided by ventilation holes in the backplane on the pressurized side of the recirculation fan.
To meet the environmental isolation requirement, an air-to-air heat exchanger was designed to allow the unit to shed heat, while preventing interchange of internal and external air. In order to minimize total system volume, reduce weight, and increase structural stiffness of the chassis, the heat exchangers and exhaust air ductwork are used to form the side walls of the chassis. The third and probably most important challenge was cooling the payload (see Figure 3). Four fans are used to pull external air through the external side of the heat exchangers, while internal air is re-circulated through the inner air passages of the heat exchangers. Additional cooling is provided for the electronics package by applying four smaller fans to pull air across the electronics package heat sink. The heat exchanger is a dual-passage counter-flow design where the internal air flows in the opposite direction of the external air.
This heat exchanger design is built as a brazement of aluminum plates and folded fin stock used to increase the surface area available for heat transfer. The recirculating air fan and the four fans on the electronics package are uncontrolled, and run directly from the 28V DC nominal input power. The four exhaust fans are speed-controlled to reduce audible noise when full cooling capability is not required. When run at highest fan speed, enabling internal payload power dissipation of over 500W, the cooling system is able to maintain internal air temperatures low enough to operate the system up to 55°C ambient. Thermal modeling shows that the recirculating air exiting the heat exchanger is kept within 10°C of the ambient air temperature.
In RF Mil-Spec systems, system monitoring falls into five major categories: temperature monitoring, fan monitoring, voltage monitoring, remote access, and other options. Temperature monitoring is becoming more critical as the value of the payload continues to rise. To address this, strategically located thermistors feed temperature values to a system monitor. The monitor can then evaluate the temperatures and increase/decrease the fan speed as needed. If a specified temperature is reached, a warning can be sent, and more importantly, if a temperature level is reached, the system monitor can inhibit the power to the backplane, shutting down and protecting the boards. It is also important to monitor other functions such as the health of the fans or system voltages. If one of these should fail, the system could be jeopardized.
In addition to the basic monitoring functions, new system requirements are generated every day. As the cost of processing boards rises, users are more interested in alerts that may not require system shutdown, which is moving customers towards boards and systems that can self-monitor the performance of the electronics package in addition to environmental factors.