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The Effect of PCB Design on the Thermal Performance of SIMPLE SWITCHER® Power Modules The Effect of PCB Design on the Thermal Performance of SIMPLE SWITCHER® Power Modules Summary The SIMPLE SWITCHER® Power Modules use a TO-PMOD package similar to a TO-263. This application note focuses on the low current modules which come in a 7 Lead, 10.16 x 4.57 x 9.81 mm package. This package has excellent thermal performance enabled by an exposed pad, which can be soldered to the PCB. The key thermal characteristics are: • θJC = 1.9°C/W • θJC 21.6°/W (On a 4-layer thermal board) National Semiconductor Application Note 2026 Lianxi Shen February 8, 2010 Power Module better in thermal performance than other package types. For example, LGA packages have a θJC of about 5C/W or larger for the similar package size, depending on the copper and thermal vias in its substrate. Parametric Study In order to optimize the PCB design to get the best thermal performance out of the SIMPLE SWITCHER® Power Module and to understand the effect of environmental conditions, this application note analyzes how some factors affect the thermal performance of a PCB or the θJA of a package mounted on it. These factors include: 1. Size of direct thermal attachment pad 2. Copper layers (2 or 4 layers) 3. PCB size 4. Air flow 5. Heat sink Figure 1 shows these factors schematically. For the parametric study, the above factors were varied as follows: 1. The sizes of copper area on top and bottom layers include: Copper Area = DAP size (8.5x5.4mm) Copper Area = Package body size (10x10mm) Copper Area = 2 X package body size (20x20mm) Copper Area = Full copper layer (4 solid copper layers) 2. 2 layer and 4 layer boards 3. The PCB size varies from 4”x3” (102x76mm) to 1.5”x1.5” (38x38mm) 4. The air flow includes Natural Convection, 200LFPM, and 400LFPM 5. The heat sink may be on the package top or on the PCB bottom side What Determines θJA In order to understand how a PCB’s thermal performance determines the thermal resistance (θJA) of a Power Module mounted on the PCB, a brief analysis for θJA is given as follows. There are two heat dissipation paths, i.e., JunctionPCB-Ambient and Junction-PKG surface-Ambient. Because the two paths are in parallel, θJA can be expressed as θJA = (θJCA x θJTA)/(θJCA + θJTA) θJCA is the thermal resistance from junction to ambient through the PCB and θJTA is the thermal resistance through the package surface to ambient (mainly package top). For the situation where no heat sink is applied on the package top, 95% or more of the power dissipates through the PCB, meaning that θJA is dominated by θJCA (also meaning that θJTA is much bigger than θJCA). As a result, θJA can be simply expressed as θJA = θJCA - RJTA = θJC + θCA - RJTA θCA is the thermal resistance from package bottom case to ambient through the PCB. It is mainly dependent on the thermal conductivity of the PCB and the thermal connection between the package and the PCB. RJTA gives a small reduction of θJA caused by the power dissipation through the package top. So, it is seen from the equation above that on any given board, the small θJC and large exposed thermal pad should make the 30112901 AN-2026 FIGURE 1. Thermal Management of the SIMPLE SWITCHER® Power Module on a 4-Layer PCB © 2010 National Semiconductor Corporation 301129 www.national.com AN-2026 Thermal Measurement and Simulation The thermal performance of the module on a 4-layer evaluation board is measured. This is used to validate our thermal model for the parametric study. The 4-layer evaluation board is 3”x1.75” with a thickness of 1.6mm and 4 solid copper lay- ers of 1oz thickness. Thermal simulations are carried out using CFD software Flotherm, where ambient temperature is 25°C and power dissipation is 1.82W. The thermal model is validated by comparing measured and simulated data. Finally, a parametric study for the previously mentioned five factors is done using the validated simulation model. The results are plotted in Figures 2-6. Note: θJC is the junction-to-case thermal resistance, which characterizes the thermal performance of package itself, and can be used to rate different packages. Note: θJA is the junction-to-ambient thermal resistance, which is used to evaluate the thermal performance of a package in an application environment. 30112902 FIGURE 2. Effect of Cu Area on Top and Bottom Layers of 4-Layer PCB www.national.com 2 AN-2026 30112903 FIGURE 3. Effect of PCB Size of 4-Layer PCB 3 www.national.com AN-2026 30112904 FIGURE 4. Effect of Cu Area on Top and Bottom Layers of 2-Layer PCB www.national.com 4 AN-2026 30112905 FIGURE 5. Effect of Airflow for Two PCBs 5 www.national.com AN-2026 30112906 FIGURE 6. Effect of Heatsink Conclusion The TO-PMOD package has excellent thermal performance, as demonstrated by its low θJA and θJC. The thermal performance of any package strongly depends on its application environment. But how well a package can take advantage of a high thermal conductivity PCB is determined by the package itself, that is, its θJC and its exposed pad size. The TO-PMOD package module has been optimized on both sides, giving excellent thermal performance. For a specific application, users of the SIMPLE SWITCHER® Power Module can refer to the results of the parametric study plotted in Figure 2 Figure 6 to quickly estimate the real θJA and evaluate the maximum power dissipation that the device can handle. Note that the effect of other heating sources on the same PCB is not considered in this thermal analysis. So, a system level simulation may be needed when other complicated factors are involved. For this purpose, a Flotherm model is available upon request. www.national.com 6 AN-2026 7 www.national.com The Effect of PCB Design on the Thermal Performance of SIMPLE SWITCHER® Power Modules Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Amplifiers Audio Clock and Timing Data Converters Interface LVDS Power Management Switching Regulators LDOs LED Lighting Voltage References PowerWise® Solutions Temperature Sensors PLL/VCO www.national.com/amplifiers www.national.com/audio www.national.com/timing www.national.com/adc www.national.com/interface www.national.com/lvds www.national.com/power www.national.com/switchers www.national.com/ldo www.national.com/led www.national.com/vref www.national.com/powerwise WEBENCH® Tools App Notes Reference Designs Samples Eval Boards Packaging Green Compliance Distributors Quality and Reliability Feedback/Support Design Made Easy Design Support www.national.com/webench www.national.com/appnotes www.national.com/refdesigns www.national.com/samples www.national.com/evalboards www.national.com/packaging www.national.com/quality/green www.national.com/contacts www.national.com/quality www.national.com/feedback www.national.com/easy www.national.com/solutions www.national.com/milaero www.national.com/solarmagic www.national.com/training Applications & Markets Mil/Aero PowerWise® Design University Serial Digital Interface (SDI) www.national.com/sdi www.national.com/wireless www.national.com/tempsensors SolarMagic™ THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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