Thermal/Mechanical Reference Design
Assume the datasheet TDP is 85 W and the case temperature specification is 68 °C. Assume as well that the system airflow has been designed such that the local processor ambient temperature is 45°C. Then the following could be calculated using equation
Equation 2-5.ΨCA = (TCASE – TLA) / TDP = (68 – 45) / 85 = 0.27 °C/W
To determine the required heatsink performance, a heatsink solution provider would need to determine ΨCS performance for the selected TIM and mechanical load configuration. If the heatsink solution was designed to work with a TIM material performing at ΨCS ≤ 0.05 °C/W, solving for equation
Equation 2-6.ΨSA = ΨCA − ΨCS = 0.27 − 0.05 = 0.22 °C/W
If the local processor ambient temperature is assumed to be 40°C, the same calculation can be carried out to determine the new
Equation 2-7.ΨCA = (TCASE – TLA) / TDP = (68 – 40) / 85 = 0.33 °C/W
It is evident from the above calculations that, a reduction in the local processor ambient temperature has a significant positive effect on the
2.4.3Chassis Thermal Design Considerations
2.4.3.1Chassis Thermal Design Capabilities and Improvements
One of the critical parameters in thermal design is the local ambient temperature assumption of the processor. Keeping the external chassis temperature fixed, internal chassis temperature rise is the only component that can affect the processor local ambient temperature. Every degree gained at the local ambient temperature directly translates into a degree relief in the processor case temperature.
Given the thermal targets for the processor, it is extremely important to optimize the
chassis design to minimize the air temperature rise upstream to the processor (Trise), hence minimizing the processor local ambient temperature.
The heat generated by components within the chassis must be removed to provide an adequate operating environment for both the processor and other system components. Moving air through the chassis brings in air from the external ambient environment and transports the heat generated by the processor and other system components out of the system. The number, size and relative position of fans, vents and other heat generating components determine the chassis thermal performance, and the resulting ambient temperature around the processor. The size and type (passive or active) of the thermal solution and the amount of system airflow can be traded off against each other to meet specific system design constraints. Additional constraints are board layout, spacing, component placement, and structural considerations that limit the thermal solution size.
In addition to passive heatsinks, fan heatsinks and system fans, other solutions exist for cooling integrated circuit devices. For example, ducted blowers, heat pipes and liquid cooling are all capable of dissipating additional heat. Due to their varying attributes, each of these solutions may be appropriate for a particular system implementation.
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