AN1703 Datasheet, PDF (1/22 Pages) STMicroelectronics – GUIDELINES FOR USING ST’S MOSFET SMD PACKAGES

AN1703
APPLICATION NOTE
GUIDELINES FOR USING ST’S MOSFET SMD PACKAGES
1. ABSTRACT
R.Gulino
The trend from through-hole packages to low-cost SMD-applications is marked by the improvement of
chip technologies. "Silicon instead of heatsink" is therefore possible in many cases. Many applications
today use PCBs assembled with SMD-technologies, the emphasis being on Power ICs in SMD packages
mounted on single-sided PCBs laminated on one side.
The printed circuit board (PCB) itself becomes the heatsink. In early fabrications a solid heatsink was
either screwed or clamped to the power package. It was easy to calculate the thermal resistance from the
geometry of the heatsink.
In SMD-technology, this calculation is much more difficult because the heat path must be evaluated: chip
(junction) - lead frame - case or pin - footprint - PCB materials (basic material, thickness of the laminate)
- PCB volume - surroundings.
As the layout of the PCB is a main contributor to the result, a new technique must be applied. Surface
mount board layout is a critical portion of the total design. The footprint for the semiconductor packages
must be the correct size to ensure proper solder connection interface between the board and the
package.
The power dissipation for a SMD device is a function of the drain pad size, which can vary from the
minimum pad size for soldering to a pad size given for maximum power dissipation. The measurements
achieved on SMD packages for different drain pad size show that by increasing the area of the drain pad
the power dissipation can be increased.
Although one can achieve improvements in power dissipation with this method, the tradeoff is to use
valuable board area.
Next we consider the common ST MOSFET SMD packages (D²PAK, DPAK, SOT-223, SO-8, PowerSO-
8™, PowerFLAT™ (5x5) and (6x5), TSSOP8 and PowerSO-10™) with their recommended footprints.
For each of these packages we will show the power dissipation for the minimum footprint and for a large
drain pad area (1in² or 600mm²) using the max measured R_THJ-PCB.
We will show the maximum allowable power dissipation versus drain pad area for different T_J-T_A values
as well.
Finally we will make a thermal performance comparison for all SMD packages analysed.
Information regarding the mechanical dimensions for each SMD package can be found on the related
datasheets.
2. THERMAL MEASUREMENTS
The most practical method of optimizing thermal performance is to characterize the MOSFET on the
PCB where it will be used. The basis of this method is to dissipate a known amount of power in the
MOSFET, and to measure the amount of temperature rise this causes in the junction, given the data
June 2003
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