How to achieve heat dissipation design of PXIe/PXI chassis
The PXIe/PXI system is the best platform for measurement and automated test systems that seek stability and harsh environments. The design of system heat dissipation plays an important role in the stability of the system, including the planning of air flow and flow field. How to avoid unnecessary heat sources from air flow channels? How to achieve the optimal balance of heat dissipation hole under the restriction of safety regulations; How do I configure my fans for optimal performance? And how to plan the space configuration of the power supply module to provide independent flow field? All of these factors will affect the heat dissipation of the PXIe/PXI system. This article will take you through the secrets of system thermal design to pick the best PXIe/PXI system for you.
Limitations of PXIe/PXI system design
The design of the PXIe/PXI chassis mainly considers the orientation of the PXIe/PXI module configuration. The direction of a module card directly affects the air flow. The most common PXIe/PXI chassis in the general market is a 4U high PXIe/PXI chassis equipped with 3U PXIe/PXI module cards, which are mostly vertically inserted and configured in a 4U PXIe/PXI chassis. In such a limited space, the design of heat dissipation to maintain the stability of the system is a major challenge for system designers.
Fan configuration considerations
Generally speaking, there are two positions for the fan configuration in the PXIe/PXI chassis, which are called “down mode” or “back mode”. Most traditional fans are installed at the lower part of the chassis. However, if the fan is installed at the lower part of the chassis, the air flow distribution is uneven, which affects the overall heat dissipation quality of the system. For example, the temperature of the slot between the two groups of fans may be higher than that of the other slots because of the uneven air flow. This is not conducive to the planning and configuration of the system. Therefore, the new design of fan configuration in the rear of the chassis came into being. The design of fan configuration in the rear of the chassis can bring the average air flow and improve the problem of uneven temperature.
However, there is still a different approach to configuring the fan behind the chassis. One is the air flow “from back to front suction”, because the fan can directly inhale air, the wind speed can be higher, but the control of the air flow is more difficult, plus according to the PXI specification defined, the air flow must be from the bottom up through the PXI module, so that the heat dissipation planning is bound to be achieved through the open hole above the chassis, which is a strict environment with higher requirements. Is a less favorable design (see Figure 3). In contrast, if the wind flow is a “forward-to-back suction type”, although the distance between the fan and the inhaled air is longer, the wind speed is lower, but the performance of the wind flow can be more stable and easy to control.
Flow channel planning
In addition, the configuration of the fan also needs to consider the design of the flow channel, how to avoid the heat source that the customer may encounter when using, and the smooth export of cold air is a very important consideration when designing the chassis. In the context of the PXIe/PXI chassis, most customers will use a hybrid test system to install the PXIe/PXI system in the cabinet. In this way, the heat source consideration will not only be the PXIe/PXI system itself, but also include the complete hybrid test system. Take the first type just mentioned, a chassis with a rear-to-front suction fan configuration will draw air into the chassis body and then direct it to the front of the chassis. However, because the air brought in from the rear of the cabinet is easy to mix with the hot air generated by other mixed systems, it will also cause the heat generated by the backplane of the PXIe/PXI chassis. It is also brought into the PXIe/PXI module in the front, which is not conducive to the heat dissipation of the overall system. In the design of the second new PXIe/PXI chassis, the rear fan is used to suction from the rear, and the “clean” cold air in the front is directed to the rear of the chassis through the PXI module to discharge, so as to avoid the above situation (see Figure 4).
Figure 4: The case design of the forward suction type provides a cleaner flow path to avoid unnecessary heat sources.
Optimize the design of the opening
In order to guide air flow optimization, the planning and design of chassis openings are also important. How to strike a balance between the safety restrictions and the limits of the mechanism also tests the skill of designing the chassis. The new PXIe/PXI chassis not only has holes in the corresponding positions of the front and rear to enhance heat dissipation, but also has a maximized opening design on both sides and the front panel. First of all, due to the limitation of PXIe/PXI specification, the air flow must be dissipated from the bottom up, so the position that can be opened can only be at the bottom of the chassis. In terms of height, considering the height limitation of PXIe/PXI module, the upper edge of the opening should not exceed the lower edge of PXIe/PXI module. The new PXIe/PXI chassis not only has a lot of tiny open holes in the front and lower edges of the chassis, but also utilizes possible space to maximize the open hole design on the lower edges of the chassis
Balance of fan performance and back pressure
Due to the limitation of the space environment of the chassis, it is necessary to find out the optimal configuration of fan back pressure and fan flow. The ideal situation is to have a smooth curve distance and a long distance, so that the performance of the fan can achieve PQ curve optimization. However, due to the space limitation of the PXIe/PXI chassis, the optimal slope of the chassis backplane must be obtained in continuous calculation and adjustment to show the best performance of the fan.
Power module configuration considerations
The selection and configuration of power modules is also a learning. In past system designs, it was easy to overlook the heat source generated by the power supply module itself, which also affected the performance of the PXIe/PXI chassis. In the traditional design, the power supply module is not separated from the heat flow of the chassis itself, and the power supply module’s forced exhaust function will cause the flow field of the chassis to be confused. Therefore, the new chassis must be able to block the heat flow of the power supply module and the fan itself, and even design independent openings for the power supply module to provide independent air flow. This is all for change