1

AN1032 Rev. A

Application Note 1032

Performance Restrictions Associated with

3.5 Watts SO-8 Power MOSFETs

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).

Alan Li, Sr. Applications Engineer

© 1999 Fairchild Semiconductor Corporation

July, 1999

Figure 1. Typical Condition

¡ P

¡ P

¡ P

Equation 1.

T

θJA

where:

T

P

In order to calculate power handling rather than rely on datasheet

Since T

calculated assuming the maximum allowable temperature, ambi-

facilitate users in proper thermal design, Fairchild Semiconductor

Be Aware That Under “Typical” Conditions, SO-8 Power MOSFETs Only Handle About One Watt.

Advanced Power Products