XFP光学无线电收发机的元器件及板级热建模技术

Component and Board-Level Thermal Modeling Techniques for XFP Optical Transceivers

 

摘要Abstract

在10吉比特小型插塞式光学收发器的多源协议中，定义对于最多只包含16个功率为1.5,2.5和3.5瓦模块的单板结构，温度应保持在70摄氏度以下。The Multi-Source Agreement (MSA) for the 10 Gigabit Small Form Factor Pluggable (XFP) optical transceiver specifies that case temperature remain below 70 ºC for single board configurations of up to sixteen 1.5, 2.5, and 3.5 W modules.由于温度作用涉及很多不同的设计因素，如空气的流速，流向，散热片的设计方式以及其它元器件的布局等等，因此这一定义是设计方面的重大挑战。This specification can present a significant design challenge, as case temperature is a strong function of many varied design elements such as flow velocity, flow direction, heat sink design, and the placement of other components.热分析软件可快速详尽地探知光学收发器内部机械装置以及它与板内其它元件交互的热传递情况，从而大大的简化了热设计的过程。Thermal analysis software can greatly simplify the thermal design process by providing fast, detailed insight into the mechanisms of heat transfer for the optical transceiver and its interaction with other components on a board.本文主要介绍在XFP光学收发机元器件及板级分析中所采用的热建模技术。重点讲述建立详细紧凑的热模型的方法。This paper covers thermal modeling techniques for component and board-level analyses of XFP optical transceivers. A focus is placed on detailed and compact form modeling methodologies.

 

介绍Introduction

为光缆网络公司发布新一代产品的市场机会的评估在数月内，有时在几星期内就可完成。The window of opportunity for launching new generation products is measured in months, sometimes weeks for optical networking companies.与此同时，在规范产品性能上所带来的压力就成为工程设计者要面对的新的更加严峻的挑战。At the same time the pressure to push the limit on product performance introduces new, increasingly severe engineering challenges.

将10G比特率以太网部件和系统投放市场的潮流使那些提供热设计的实际建模软件对这种高级热设计技术进行了再投资。The rush to introduce 10 Gbps Ethernet components and systems to the market has placed a premium on advanced design techniques for thermal design such as those offered by virtual prototyping software.热分析主要集中在其中一个部件上，XFP光学收发器。但是，XFP有一个70摄氏度的温度限制。This thermal study focuses on one of these components, the XFP optical transceiver. The XFP, however, has a case temperature limit of 70 °C.由于电路板印刷管脚更为细小，可并排放置16个XFP模块，因此要对这种排放进行细致分析以确保不超过70摄氏度的温度限制。With a smaller board footprint, the ability to place up to 16 of these modules in a row must be carefully analyzed to ensure that the limiting temperatures are not exceeded.

FLOTHERM热分析软件可用于测试环境下的XFP光学接收器的建模。FLOTHERM thermal analysis software was used to model the XFP optical transceiver within the test environment.FLOTHERM为XFP建立了两种不同的模型表述。Two separate representations of the XFP were created in FLOTHERM.建立并测试的第一种模型是XFP的详细表述模型，它包括了内部器件的详细描述，诸如电路板，元器件以及激光组件等等。The first model created and tested is considered a detailed representation of the XFP, including interior details such as the board, components, and laser assemblies.第二种XFP模型是在同等温度条件下对详细模型的简化描述。通过改变气流速度，测试电路板构型和功率给出两种模型的比较结果。The second model of the XFP is a compact representation that is designed to reproduce equivalent case temperatures as the detailed model. Results involving comparisons between the detailed and compact models are presented for varying airflow rates, test board configurations, and powers.

 

 

建模Modeling

无论是XFP的详细模型还是简化模型，都必须能够对给定的气流环境中的温度情况进行预测。The detailed and compact models of the XFP must both serve the same purpose, to predict accurate case temperatures in a given flow environment.但是，简化模型在一些方面要优于详细模型。其中最重要的一个优势是它没有表现出任何的自身敏感性信息。这就使简化模型可以满足任何对XFP热模型的要求。However, there are several advantages that the compact model has over the detailed model. The most important advantage is that the compact model does not reveal any sensitive proprietary information. This enables a compact model representation to be given to anyone who requires a thermal model of the XFP.

使用简化模型的另外一个显著优势是可以减少问题解决所需的时间。任何分析软件对复杂几何构型的分析总比对简单几何构型的分析花费的时间多。用简化模型可以在很大程度上节约时间。Another large advantage for using compact models is reducing solution time. In any analysis software, more complex physical geometry will take longer to solve than simple geometry. By using a compact model, significant amounts of time.

光学收发器是根据MSA中提供的信息建立起来的。这些信息包括部件的整体尺寸和构形，放置散热器的空间和测试环境的结构。The optical transceiver was constructed based on the information provided in the MSA. This included the overall size and configuration of the cage, the space available for the heat sink, and the configuration of the test environment.散热器是10 x 4的齿状结构。XFP详细模型的外观图如图1所示。The heat sink was constructed as a 10 by 4 pin fin heat sink. The outer view of the detailed XFP is shown in Figure 1.箱体，散热器和模块的传导率可从早先研究概括在MSA中的数据直接获得。The conductivities of the cage, heat sink and module were taken directly from a previous study summarized in the MSA.内部结构的详细情况如图2，假设有一块主板，板上有三个部件以及一组激光组件。The interior details, shown in Figure 2, were constructed by assuming a single main board with 3 components as well as estimates of typical laser assemblies.

 

建立简化模型是为了去掉内部细节，简化箱体设计。箱体连接器集成在一起，给出一个传导系数来表示等量热阻。箱的底部同样简化成具有等量热阻的单一模块。散热器以一种FLOTHERM软件提供的简化方式来表示。XFP的简化模型整观图如图3所示。The compact model was constructed in order to remove the interior details, and simplify the cage design. The cage connector tabs were lumped together and given a conductivity that represented an equivalent thermal resistance. The cage bottom was also simplified to a single block with an equivalent resistance. The heat sink was replaced with a compact representation that is available within the FLOTHERM software. An overall view of the compact XFP is shown in Figure 3.

Figure 3: Overall view of the compact XFP.

测试模型在MSA提供的信息基础上创建。其结构如图4所示。The test configuration was created based on information provided by the MSA. This configuration is shown in Figure 4.测试板的尺寸为406.4mmx 279.4mm。8个部件置于板上，散出的热量使空气温度上升10摄氏度。所有空气在40摄氏度时从风道一端进入由另一端排出。尖刃处没有空隙，空气不会散失。The test board has a size of 406.4 mm (16 in) by 279.4 mm (11 in). Eight components were also placed on the board and given a heat dissipation that resulted in a 10 ºC temperature rise of the air. All of the air enters at 40 °C at one end of the wind tunnel, and exits out the other end. There are no openings in the bezel to allow the air to escape.

Figure 4: Wind tunnel test configuration showing eight XFP’s.

 

基本模型模拟结果Baseline Result

构建简化模型完全依据从详细模型的基本解决方案中所获取的结果。The construction of the compact model is entirely dependent on the results obtained from a baseline solution involving the detailed model.详细模型必须提供热量由内部器件导入箱体的位置以及在每一位置上的热传导量。The detailed model must provide the locations that heat is conducted into the case from the interior components, as well as the amount of heat conducted at each location.

初始的详细模型解决方案包括：将一个XFP放置在测试板上。散失功率1.5瓦。通过风道的气流速度为1.016米/秒。温度由箱体上的四个离散点确定。这四点分别是组合部件的顶部，两侧和尾部。The baseline detailed model solution involved placing a single XFP on the test board. The detailed model dissipated 1.5 W. The flow through the wind tunnel was 1.016 m/s (200 lfm). Case temperatures were determined at four discrete points on the cage. The cage temperatures determined were the top, the two sides and rear of the cage assembly.

内部器件和箱体外部的热传递路径应该是确定的。用像FLOTHERM这样的分析软件对这些位置的热量传导进行测量是非常容易的。The conduction paths between the heat generating interior components and the outer cage should be well defined. In analysis software such as FLOTHERM, the amount of heat being conducted into the cage at these locations can be measured quite easily.例如，在分别位于XFP板上和箱体顶部的两个器件之间放置一个隔离垫。因而箱体和隔离垫的接触面就是主要的热传导位置，通过这个面的热传导量很容易度量。For example, a gap pad has been placed between two of the components on the XFP board and the top side of the cage. Therefore the interface between the cage and gap pads is a primary conduction location, and the amount of heat conducting through that face is easily measured. Based on this test condition, the case temperatures ranged from 49.2 to 51.0 °C. The flow of air through the wind tunnel is shown in Figure 5. The high conductivity of the cage, along with the large surface area of the heat sink both contribute to a large percentage of the heat being convected to the air instead of conducted to the board.

Figure 5: Flow of particles past a single detailed XFP.

The most important piece of information, the heat conducted into the cage, was also determined. Using this information, a compact model can now be completed. Instead of using interior heat generating components as in the detailed model, non-physical heat sources can now be created at the same locations. This includes the gap pad/cage interface, and the laser assembly/module interface. The remaining heat that did not directly conduct into the cage is spread throughout the interior volume in order to convect to the cage.

Using this compact model, it was placed in the exact same wind tunnel as before. In this test, the cage temperatures ranged from 50.0 to 50.8 °C, which is not significantly different than the detailed model results. The largest percentage difference in the change in temperature was less than 9%. A comparison between the detailed and compact model results is shown in Table 1. The largest advantage, however, can be seen in the solution time. Where the detailed model took over 10 hours to converge, the compact model was able to converge in 29 minutes, over an order of magnitude less. The reduction in grid count was also significant. While the detailed model had over 413,000 grid cells, the compact model had only 115,000 grid cells.

Top	Bottom		Left		Right
Detail	49.2	51.0		50.3	49.8
Compact	50.0	50.8		50.6	50.3
% Change	8.50	1.89		3.40	5.18

 

Table 1: Case temperature comparison for a single XFP in 1.016 m/s airflow.

Parametric Studies

While an initial comparison of the cage temperatures between a detailed and compact representation of the XFP were very close, there is no guarantee that this would be true in different test conditions. The advantage gained from the reduction in solve time would be greatly diminished if every different test condition required a detailed model baseline result. The best situation is if the result gained from the initial baseline test could be used in different conditions. Common differences between separate test configurations would include the flow rate of air, the number of XFP’s, and the test board. Therefore a parametric study was conducted to investigate the effect of these changes.

In each test configuration, both compact and detailed XFP modules were tested. This ensures that the cage temperatures for the compact model would always have a detailed result to compare with. However, the compact models that are used are based entirely on the heat conduction data obtained from the baseline case. Ideally, a detailed model test situation would only have to be solved for once, and using that result, the compact model can be placed in a variety of different test configurations to present reliable cage temperatures.

Parametric Study 1

The first parametric study involved altering the flow rate through the wind tunnel. The baseline test condition was 1.016 m/s (200 lfm). Two additional flow rates were tested at 1.778 m/s (350 lfm) and 2.54 m/s (500 lfm). The compact model was based entirely on the data obtained from the initial baseline case.

For the 1.778 m/s case, the case temperatures for the detailed model ranged from 46.6 to 48.9 °C. The compact model resulted in case temperatures that ranged from 47.9 to 49.0 °C. A comparison is shown in Table 2. Again, the compact model solution is a conservative representation of the case temperature, but still not much different from the detailed model result.

Top	Bottom		Left		Right
Detail	46.6	48.9		48.0	47.6
Compact	47.9	49.0		48.8	48.5
% Change	19.74	1.55		9.93	11.77

 

Table 2: Case temperature comparison for a single XFP in 1.778 m/s airflow.

For the 2.54 m/s case, the case temperatures for the detailed model ranged from 46.4 to 48.6 °C. The compact model resulted in case temperatures that ranged from 46.6 to 47.8 °C. These results are outlined in Table 3. While the compact solutions are not all on the conservative side, on a percentage basis they are still within 10% of the detail model solution.

Top	Bottom		Left		Right
Detail	46.4	48.6		47.7	47.3
Compact	46.6	47.8		47.5	47.3
% Change	3.72	9.71		2.49	1.29

 

Table 3: Case temperature comparison for a single XFP in 2.54 m/s airflow.

Parametric Study 2

The second parametric study focused on the number of XFP’s that are placed on the test board. All of the previous tests were configured with only one module. A series of tests involving eight transceivers was computed using two different airflow rates, 1.016 m/s and 2.54 m/s (200 and 500 lfm).

The eight transceivers were placed in a single row, with a pitch of 46 mm. This pitch distance was taken directly from the MSA. While results for all 8 transceivers are available, only the case temperatures of the sixth transceiver will be presented here for brevity’s sake. The flow of air particles is shown in Figure 6.

Figure 6: Flow of particles past eight detailed XFP’s.

At a flow rate of 1.016 m/s, the sixth detailed transceiver had case temperatures ranging from 60.5 to 62.0 °C. For the compact models, the sixth transceiver had case temperatures ranging from 61.2 to 61.9 °C. These results are summarized in Table 4. As can be seen, even though the compact model is still based on the baseline result, the results for an XFP placed in the interior of a row of eight devices still present very accurate results. The percentage difference in case temperatures are all less than 4%.

Top	Bottom		Left		Right
Detail	60.5	62.0		61.3	61.0
Compact	61.2	61.9		61.7	61.5
% Change	3.56	0.13		1.98	2.45

 

Table 4: Case temperature comparison for the sixth XFP in 1.016 m/s airflow.

The amount of time saved is also even more significant for this case. The detailed model of the eight XFP’s involved a grid cell count of 1,645,020 and a solve time of several days. The compact model, in contrast, had a grid cell count of 310,200 and a solve time of approximately 6 hours.

At a flow rate of 2.54 m/s, the sixth detailed transceiver had case temperatures ranging from 53.6 to 55.8 °C. For the compact models, the sixth transceiver had case temperatures ranging from 54.6 to 56.0 °C. As shown in Table 5, the comparison between compact and detailed models shows that the compact models give very accurate results.

Top	Bottom		Left		Right
Detail	53.6	55.8		54.8	54.5
Compact	54.6	56.0		55.5	55.3
% Change	7.05	0.93		4.87	5.51

 

Table 5: Case temperature comparison for the sixth XFP in 2.54 m/s airflow.

Parametric Study 3

In all of the test configurations discussed so far, the composition of the wind tunnel test board and its eight components has remained constant. In this study, the effect of changing this board was investigated with regards to the amount of heat being dissipated, and the composition of the board itself.

The power being dissipated by the other generic components on the test board had been adjusted in order to give an air temperature rise of 10 °C. Each of the eight components dissipated the same amount of heat. As a result, the test board itself would have been heated substantially, thereby reducing the amount of heat being conducted into the board from the transceivers. The amount of power being dissipated by the components was reduced to zero in order to determine the effect on the case temperatures.

The tests were performed with one XFP in 1.016 m/s airflow. For the detailed model, the case temperatures ranged from 45.4 to 45.8 °C. The effect of the component heat on the case temperature can be seen quite clearly. The case temperatures here are much cooler than the initial baseline case due to the reduced temperature of the board. For the compact model, the case temperatures ranged from 45.3 to 45.7 °C. As the results show in Table 6, the effect of having a cooler test board still resulted in the compact model giving very accurate case temperatures.

Top	Bottom		Left		Right
Detail	45.4	45.8		45.6	45.5
Compact	45.7	45.3		45.5	45.5
% Change	5.77	8.52		1.02	0.91

 

Table 6: Case temperature comparison with zero test board power dissipation.

The second part of this study was to alter the composition of the board. For all of the test cases, the board configuration was set to 10% copper and 90% FR4. For this test, the percentage was changed to 20% copper and 80% FR4. In this case, the detailed model case temperatures ranged from 51.3 and 54.0 °C. The compact model case temperatures ranged from 52.4 to 54.0 °C. Again as shown in Table 7, even though the compact model was created based on a different test board configuration, it still presents very accurate results.

Top	Bottom		Left		Right
Detail	51.3	54.0		53.0	52.4
Compact	52.4	54.0		53.6	53.2
% Change	9.20	0.42		4.20	6.30

 

Table 7: Case temperature comparison for different test board composition. Parametric Study 4

Up to this point the focus has been on the 1.5 W version of the XFP transceiver. The last parametric study is to determine if a 3.5 W version of the XFP transceiver can be similarly represented in a compact model with equally accurate results.

To create a compact representation of a 3.5 W transceiver, a detailed 3.5 W transceiver was first tested in a 2.54 m/s environment. The process to create the compact model was exactly the same as in the initial baseline case involving the 1.5 W transceiver.

For the 2.54 m/s flow over one XFP, the detailed model had case temperatures ranging from 51.4 to 54.7 °C. The compact model had case temperatures ranging from 52.1 to 53.3 °C. As can be seen in Table 8, the results still agree very well.

Top	Bottom		Left		Right
Detail	51.4	54.7		53.3	52.8
Compact	52.1	53.3		53.1	52.8
% Change	5.72	9.99		1.50	0.20

 

Table 8: Case temperature comparison for a single 3.5 W XFP.

The 3.5 W XFP was also tested with eight XFP’s in 2.54 m/s airflow. Again, only the sixth transceiver will be compared. For the detailed model, the case temperatures ranged from 61.1 to 64.5 °C. For the compact model, the case temperatures ranged from 64.3 to 65.8 °C. These results are summarized in Table 9.

Top	Bottom		Left		Right
Detail	64.5	62.9		62.7	61.1
Compact	65.8	65.4		65.2	64.3
% Change	14.71	5.09		10.71	11.23

 

Table 9: Case temperature comparison for the sixth 3.5 W XFP.

Conclusion

The viability of using compact models of XFP optical transceivers has been analyzed. The use of compact models hides all of the important proprietary information while dramatically reducing solve times. While the XFP optical transceiver is well suited for compact modeling due to its large heat sink and high conductivity cage design, the compact modeling procedures outlined here are applicable to other complex components as well. The most important element of creating such a compact model is to fully understand the conduction paths into the cage and board, and to represent those paths with equivalent thermal resistances