1032 Datasheet, PDF (1/2 Pages) Lattice Semiconductor – In-System Programmable High Density PLD

Advanced Power Products
Application Note 1032
July, 1999
Performance Restrictions Associated with
3.5 Watts SO-8 Power MOSFETs
Alan Li, Sr. Applications Engineer
In applications requiring high current and small footprint electronic
switches, a Power MOSFET packaged in an SO-8 is usually the
best choice. This is because the combination offers low on-
resistance and reasonable power handling. A second benefit is
the numerous suppliers for this product. As a result, SO-8 Power
MOSFETs are commonly found in applications such as notebook
computer DC-DC converters, battery chargers, and disk drive
motor controls.
SO-8 packaged Power MOSFETs handle about one Watt under
“typical” conditions (Figure 1), regardless of the silicon size. The
thermal capability of any semiconductor is strongly related to the
operating conditions, however users often overlook the impor-
tance of such criteria. In addition, the lack of an industry standard
means that MOSFET manufacturers may mislead the power
handling of SO-8 Power MOSFETs by two to four times their
typical thermal capability. Users should also remember that
current handling is proportional to power handling. By simply
manipulating the power rating conditions, MOSFET manufactur-
ers can tailor any current rating to any “non-typical” condition.
Of particular concern is a claim circulating that 3.5 Watt SO-8
Power MOSFETs exist in the industry. This is another twist in
addition to the existing ill-defined 2.5 Watt power rating. If users
carefully read the fine print of the disclaimer on the datasheets, it
reveals that 3.5 Watt power handling can only be achieved “when
mounted on a 1 inch square copper board; t < 10 sec.” Such
conditions are not only unrealistic, but also pose a potential
hazard for those who design parts with this information. As a
result, it is not uncommon to see SO-8 Power MOSFETs being
destroyed when they smoke during lab experiments.
The disclaimers are technically correct but unrealistic for two
reasons. First, one square inch of board space is somewhat
equivalent to the size of a typical CPU, (Figure 2). Realistically, no
one can afford such precious space for a MOSFET switch in any
portable electronics. Secondly, SO-8 Power MOSFETs reach
thermal equilibrium in a few minutes. That means a 10 second
operation is far from the worst case condition. As a general rule,
SO-8 Power MOSFETs handle about one Watt under “typical”
conditions (Figure 1).
In order to calculate power handling rather than rely on datasheet
specifications, users should be familiar with the parameter R_θJA.
Since T_J - T_A = P_D * R_θJA (Equation 1), power handling can be
calculated assuming the maximum allowable temperature, ambi-
ent temperature, and R_θJA at a given condition are known. To
facilitate users in proper thermal design, Fairchild Semiconductor
publishes different R_θJA specifications at various conditions.
Figure 1. Typical Condition
R_θJA = 125 ^oC/W ¡ P_D = 1 W
Figure 2. 1 in² 2oz Copper PAD
R_θJA (10sec) = 36 ^oC/W ¡ P_D(10sec) = 3.5 W
R_θJA = 50 ^oC/W ¡ P_D = 2.5 W
Equation 1.
T_J - T_A = P_D * R_θJA
where:
T_J = Junction Temperature
T_A =Ambient Temperature
P_D =Power Dissipation
Be Aware That Under “Typical” Conditions, SO-8 Power MOSFETs Only Handle About One Watt.
© 1999 Fairchild Semiconductor Corporation
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AN1032 Rev. A