Datasheet SI7489DP (Vishay) - 10

Application Note AN821
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PowerPAK® SO-8 Mounting and Thermal Considerations THERMAL PERFORMANCE Introduction
Because of the presence of the trough, this result suggests A basic measure of a device’s thermal performance a minimum performance improvement of 10 °C/W by using is the junction-to-case thermal resistance, R a PowerPAK SO-8 in a standard SO-8 PC board mount. thJC, or the junction-to-foot thermal resistance, RthJF This parameter is The only concern when mounting a PowerPAK on a measured for the device mounted to an infinite heat sink and standard SO-8 pad pattern is that there should be no traces is therefore a characterization of the device only, in other running between the body of the MOSFET. Where the words, independent of the properties of the object to which standard SO-8 body is spaced away from the pc board, the device is mounted. Table 1 shows a comparison of allowing traces to run underneath, the PowerPAK sits the DPAK, PowerPAK SO-8, and standard SO-8. The directly on the pc board. PowerPAK has thermal performance equivalent to the
Thermal Performance - Spreading Copper
DPAK, while having an order of magnitude better thermal performance over the SO-8. 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
TABLE 1 - DPAK AND POWERPAK SO-8
performance for a given area of spreading copper.
EQUIVALENT STEADY STATE PERFORMANCE
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.
PowerPAK Standard DPAK
The two internal layers and the backside layer are solid
SO-8 SO-8
copper. The internal layers were chosen as solid copper to Thermal 1.2 °C/W 1 °C/W 16 °C/W model the large power and ground planes common in many Resistance RthJC applications. The top layer was cut back to a smaller area and at each step junction-to-ambient thermal resistance
Thermal Performance on Standard SO-8 Pad Pattern
measurements were taken. The results indicate that an area Because of the common footprint, a PowerPAK SO-8 above 0.3 to 0.4 square inches of spreading copper gives no can be mounted on an existing standard SO-8 pad pattern. additional thermal performance improvement. A The question then arises as to the thermal performance subsequent experiment was run where the copper on the of the PowerPAK device under these conditions. A back-side was reduced, first to 50 % in stripes to mimic characterization was made comparing a standard SO-8 and circuit traces, and then totally removed. No significant effect a PowerPAK device on a board with a trough cut out was observed. underneath the PowerPAK drain pad. This configuration restricted the heat flow to the SO-8 land pads. The results Rth vs. Spreading Copper are shown in figure 5. (0 %, 50 %, 100 % Back Copper) 56 Si4874DY vs. Si7446DP PPAK on a 4-Layer Board SO-8 Pattern, Trough Under Drain 60 ) st 51 t a w 50 / C ) ( st e t c a n 46 w 40 a / d C( e Si4874DY p e c mI n 30 a d 100 % e 41 p Si7446DP mI 0 % 20 50 % N NOTE 10 36 O 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Spreading Copper (sq in) 0 Fig. 6 Spreading Copper Junction-to-Ambient Performance 0.0001 0.01 1 100 10000 Pulse Duration (sec) Fig. 5 PowerPAK SO-8 and Standard SO-0 Land Pad Thermal Path Revision: 16-Mai-13
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