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SI7633DP Scheda tecnica(PDF) 10 Page - Vishay Siliconix |
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SI7633DP Scheda tecnica(HTML) 10 Page - Vishay Siliconix |
10 / 13 page PowerPAK® SO-8 Mounting and Thermal Considerations Application Note AN821 www.vishay.com Vishay Siliconix Revision: 16-Mai-13 3 Document Number: 71622 For technical questions, contact: powermosfettechsupport@vishay.com THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 THERMAL PERFORMANCE Introduction A basic measure of a device’s thermal performance is the junction-to-case thermal resistance, RthJC, or the junction-to-foot thermal resistance, RthJF 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 standard SO-8. The PowerPAK has thermal performance equivalent to the DPAK, while having an order of magnitude 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 pattern. The question then arises as to the thermal performance 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. Fig. 5 PowerPAK SO-8 and Standard SO-0 Land Pad Thermal Path Because of the presence of the trough, this result suggests 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 PowerPAK sits directly on the pc board. Thermal Performance - Spreading Copper Designers may 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 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. Fig. 6 Spreading Copper Junction-to-Ambient Performance TABLE 1 - DPAK AND POWERPAK SO-8 EQUIVALENT STEADY STATE PERFORMANCE DPAK PowerPAK SO-8 Standard SO-8 Thermal Resistance RthJC 1.2 °C/W 1 °C/W 16 °C/W 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 Rth vs. Spreading Copper (0 %, 50 %, 100 % Back Copper) Spreading Copper (sq in) 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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