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SI7478DP Fiches technique(PDF) 9 Page - Vishay Siliconix |
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SI7478DP Fiches technique(HTML) 9 Page - Vishay Siliconix |
9 / 12 page Vishay Siliconix AN821 Document Number 71622 28-Feb-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 θ jc, or the junction-to-foot thermal resistance, R θ jf. 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 DPAK, PowerPAK SO-8, and stan- dard SO-8. The PowerPAK has thermal performance equivalent to the DPAK, while having an order of magni- tude better thermal performance over the SO-8. Thermal Performance on Standard SO-8 Pad Pattern Because of the common footprint, a PowerPAK SO-8 can be mounted on an existing standard SO-8 pad pat- tern. The question then arises as to the thermal perfor- mance of the PowerPAK device under these conditions. A characterization was made comparing a standard SO-8 and a PowerPAK device on a board with a trough cut out underneath the PowerPAK drain pad. This configuration restricted the heat flow to the SO-8 land pads. The results are shown in Figure 5. Because of the presence of the trough, this result sug- gests a minimum performance improvement of 10 °C/W by using a PowerPAK SO-8 in a standard SO-8 PC board mount. The only concern when mounting a PowerPAK on a standard SO-8 pad pattern is that there should be no traces running between the body of the MOSFET. Where the standard SO-8 body is spaced away from the pc board, allowing traces to run underneath, the Power- PAK sits directly on the pc board. Thermal Performance - Spreading Copper Designers may add additional copper, spreading cop- per, 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 cop- per. Figure 6 shows the thermal resistance of a PowerPAK SO-8 device mounted on a 2-in. 2-in., four-layer FR-4 PC board. The two internal 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 applications. 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 0.3 to 0.4 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 significant effect was observed. TABLE 1. DPAK and PowerPAK SO-8 Equivalent Steady State Performance DPAK PowerPAK SO-8 Standard SO-8 Thermal Resistance R θ jc 1.2 °C/W 1.0 °C/W 16 °C/W Figure 5. PowerPAK SO-8 and Standard SO-0 Land Pad Thermal Path Si4874DY vs. Si7446DP PPAK on a 4-Layer Board SO-8 Pattern, Trough Under Drain Pulse Duration (sec) 0.0001 0 1 50 60 10 10000 0.01 40 20 Si4874DY Si7446DP 100 30 Figure 6. Spreading Copper Junction-to-Ambient Performance Rth vs. Spreading Copper (0 %, 50 %, 100 % Back Copper) 0.00 56 51 46 41 36 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 0 % 50 % 100 % |
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