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Thermal Resistance vs. Mounting Pad AreaThe maximum rated junction temperature, TJM, and thethermal resistance of the heat dissipating path determinesthe maximum allowable device power dissipation, PDM, in anapplication. Therefore the application’s ambienttemperature, TA (oC), and thermal resistance RθJA (oC/W)must be reviewed to ensure that TJM is never exceeded.Equation 1 mathematically represents the relationship andserves as the basis for establishing the rating of the part.
In using surface mount devices such as the TO-263package, the environment in which it is applied will have asignificant influence on the part’s current and maximumpower dissipation ratings. Precise determination of PDM iscomplex and influenced by many factors:
1. Mounting pad area onto which the device is attached andwhether there is copper on one side or both sides of theboard.
2. The number of copper layers and the thickness of theboard.
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, theduty cycle and the transient thermal response of the part,the board and the environment they are in.
Fairchild provides thermal information to assist thedesigner’s preliminary application evaluation. Figure 21defines the RθJA for the device as a function of the topcopper (component side) area. This is for a horizontallypositioned FR-4 board with 1oz copper after 1000 secondsof steady state power with no air flow. This graph providesthe necessary information for calculation of the steady statejunction temperature or power dissipation. Pulseapplications can be evaluated using the Fairchild deviceSpice thermal model or manually utilizing the normalizedmaximum transient thermal impedance curve.
Thermal resistances corresponding to other copper areascan be obtained from Figure 21 or by calculation usingEquation 2 or 3. Equation 2 is used for copper area definedin inches square and equation 3 is for area in centimetersquare. The area, in square inches or square centimeters isthe top copper area including the gate and source pads.
.ENDSNote: For further discussion of the PSPICE model, consult A New PSPICE Sub-Circuit for the Power MOSFET Featuring Global Temperature Options; IEEE Power Electronics Specialist Conference Records, 1991, written by William J. Hepp and C. Frank Wheatley.
NOTES: UNLESS OTHERWISE SPECIFIED A) ALL DIMENSIONS ARE IN MILLIMETERS. B) REFERENCE JEDEC, TO-263, VARIATION AB. C) DIMENSIONING AND TOLERANCING PER ANSI Y14.5M - 1994. D) LOCATION OF THE PIN HOLE MAY VARY (LOWER LEFT CORNER, LOWER CENTER AND CENTER OF THE PACKAGE). E) LANDPATTERN RECOMMENDATION PER IPC TO254P1524X482-3N F) FILENAME: TO263A02REV6
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