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SQS840EN Datasheet(PDF) 11 Page - Vishay Siliconix |
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SQS840EN Datasheet(HTML) 11 Page - Vishay Siliconix |
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11 / 14 page  Vishay Siliconix AN822 Document Number 71681 03-Mar-06 www.vishay.com 3 THERMAL PERFORMANCE Introduction A basic measure of a device’s thermal performance is the junction-to-case thermal resistance, R θjc, or the junction to- foot thermal resistance, R θjf. This parameter is measured for the device mounted to an infinite heat sink and is therefore a characterization of the device only, in other words, independent of the properties of the object to which the device is mounted. Table 1 shows a comparison of the PowerPAK 1212-8, PowerPAK SO-8, standard TSSOP-8 and SO-8 equivalent steady state performance. By minimizing the junction-to-foot thermal resistance, the MOSFET die temperature is very close to the tempera- ture of the PC board. Consider four devices mounted on a PC board with a board temperature of 45 °C (Figure 4). Suppose each device is dissipating 2 W. Using the junc- tion-to-foot thermal resistance characteristics of the PowerPAK 1212-8 and the other SMT packages, die temperatures are determined to be 49.8 °C for the Pow- erPAK 1212-8, 85 °C for the standard SO-8, 149 °C for standard TSSOP-8, and 125 °C for TSOP-6. This is a 4.8 °C rise above the board temperature for the Power- PAK 1212-8, and over 40 °C for other SMT packages. A 4.8 °C rise has minimal effect on rDS(ON) whereas a rise of over 40 °C will cause an increase in rDS(ON) as high as 20 %. Spreading Copper Designers add additional copper, spreading copper, to the drain pad to aid in conducting heat from a device. It is helpful to have some information about the thermal performance for a given area of spreading copper. Figure 5 and Figure 6 show the thermal resistance of a PowerPAK 1212-8 single and dual devices mounted on a 2-in. x 2-in., four-layer FR-4 PC boards. The two inter- nal layers and the backside layer are solid copper. The internal layers were chosen as solid copper to model the large power and ground planes common in many appli- cations. The top layer was cut back to a smaller area and at each step junction-to-ambient thermal resistance measurements were taken. The results indicate that an area above 0.2 to 0.3 square inches of spreading copper gives no additional thermal performance improvement. A subsequent experiment was run where the copper on the back-side was reduced, first to 50 % in stripes to mimic circuit traces, and then totally removed. No signif- icant effect was observed. TABLE 1: EQIVALENT STEADY STATE PERFORMANCE Package SO-8 TSSOP-8 TSOP-8 PPAK 1212 PPAK SO-8 Configuration Single Dual Single Dual Single Dual Single Dual Single Dual Thermal Resiatance RthJC(C/W) 20 40 52 83 40 90 2.4 5.5 1.8 5.5 Figure 4. Temperature of Devices on a PC Board 2.4 °C/W 49.8 °C PowerPAK 1212 20 °C/W 85 °C Standard SO-8 PC Board at 45 °C 52 °C/W 149 °C Standard TSSOP-8 40 °C/W 125 °C TSOP-6 |
Similar Part No. - SQS840EN_15 |
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Similar Description - SQS840EN_15 |
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