AN-1025 Datasheet, PDF (1/11 Pages) Fairchild Semiconductor – Maximum Power Enhancement Techniques for SuperSOTTM-3 Power MOSFETs

AN1025
April, 1996
Maximum Power Enhancement Techniques for SuperSOT^TM-3 Power
MOSFETs
Alan Li, Brij Mohan, Steve Sapp, Izak Bencuya, Linh Hong
1. Introduction
As packages become smaller, achieving efficient thermal performance for power applications
requires that the designers employ new methods of meliorating the heat flow out of devices.
Thus the purpose of this paper is to aid the user in maximizing the power handling capability of
the SuperSOT^TM-3 (SOT-23) Power MOSFET offered by Fairchild Semiconductor. This effort
allows the user to take full advantage of the exceptional performance features of Fairchild’s
state-of-the-art Power MOSFET which offers very low on-resistance and improved junction-to-
case (R_θJC) thermal resistance. Ultimately the user may achieve improved component perfor-
mance and higher circuit board packing density by using the thermal solution suggested below.
In natural cooling, the method of improving power performance should be focused on the opti-
mum design of copper mounting pads. The design should take into consideration the size of the
copper and its placement on either or both of the board surfaces. A copper mounting pad is
important because the drain lead of the Power MOSFET is mounted directly onto the pad. The
pad acts as a heatsink to reduce thermal resistance and leads to improved power performance.
D
S
G
Figure 1. SuperSOT^TM-3 Power MOSFET has the same package dimensions as the SOT-23 but the
maximized copper lead frame reduces the junction-to-case thermal resistance R_θJC to 75^oC/W.
2. Theory
When a device operates in a system under the steady-state condition, the maximum power
dissipation is determined by the maximum junction temperature rating, the ambient tempera-
ture, and the junction-to-ambient thermal resistance.
P_Dmax = (T_Jmax - T_A)/ R_θJC (2.1)
The term junction refers to the point of thermal reference of the semiconductor. Equation 2.1
can also be applied to the transient-state:
P_Dmax (t) = [ T_Jmax - T_A] / R_θJC (t) (2.2)
1
Rev B, August 1998