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HUFA76404DK8T Scheda tecnica(PDF) 7 Page - Fairchild Semiconductor |
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HUFA76404DK8T Scheda tecnica(HTML) 7 Page - Fairchild Semiconductor |
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7 / 12 page  HUFA76404DK8T Rev. B www.fairchildsemi.com 7 Thermal Resistance vs. Mounting Pad Area The maximum rated junction temperature, TJM, and the thermal resistance of the heat dissipating path determines the maximum allowable device power dissipation, PDM, in an application. Therefore the application’s ambient temperature, TA ( oC), and thermal resistance RθJA (oC/W) must be reviewed to ensure that TJM is never exceeded. Equation 1 mathematically represents the relationship and serves as the basis for establishing the rating of the part. In using surface mount devices such as the SO8 package, the environment in which it is applied will have a significant influence on the part’s current and maximum power dissipation ratings. Precise determination of PDM is complex and influenced by many factors: 1. Mounting pad area onto which the device is attached and whether there is copper on one side or both sides of the board. 2. The number of copper layers and the thickness of the board. 3. The use of external heat sinks. 4. The use of thermal vias. 5. Air flow and board orientation. 6. For non steady state applications, the pulse width, the duty cycle and the transient thermal response of the part, the board and the environment they are in. Fairchild provides thermal information to assist the designer’s preliminary application evaluation. Figure 21 defines the RθJA for the device as a function of the top copper (component side) area. This is for a horizontally positioned FR-4 board with 1oz copper after 1000 seconds of steady state power with no air flow. This graph provides the necessary information for calculation of the steady state junction temperature or power dissipation. Pulse applications can be evaluated using the Fairchild device Spice thermal model or manually utilizing the normalized maximum transient thermal impedance curve. Thermal resistances corresponding to other copper areas can be obtained from Figure 21 or by calculation using Equation 2. The area, in square inches is the top copper area including the gate and source pads. The transient thermal impedance (ZθJA) is also effected by varied top copper board area. Figure 22 shows the effect of copper pad area on single pulse transient thermal impedance. Each trace represents a copper pad area in square inches corresponding to the descending list in the graph. Spice and SABER thermal models are provided for each of the listed pad areas. Copper pad area has no perceivable effect on transient thermal impedance for pulse widths less than 100ms. For pulse widths less than 100ms the transient thermal impedance is determined by the die and package. Therefore, CTHERM1 through CTHERM5 and RTHERM1 through RTHERM5 remain constant for each of the thermal models. A listing of the model component values is available in Table 1. (EQ. 1) PDM TJM TA – () RθJA ------------------------------- = (EQ. 2) RθJA 79.9 15 0.14 Area + ------------------------------- + = 100 150 200 0.001 0.01 1 10 50 0.1 Figure 21. Thermal Resistance vs Mounting Pad Area RθJA = 79.9 + 15/(0.14+Area) AREA, TOP COPPER AREA (in2) Figure 22. Thermal Impedance vs Mounting Pad Area 0 40 80 120 160 10-1 100 101 102 103 t, RECTANGULAR PULSE DURATION (s) COPPER BOARD AREA - DESCENDING ORDER 0.020 in2 0.140 in2 0.257 in2 0.380 in2 0.493 in2 |
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