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AN-1032

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Performance Restrictions Associated with 3.5 Watts SO-8 Power MOSFETs

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Advanced Power Products

Application Note 1032

July, 1999

Performance Restrictions Associated with

3.5 Watts SO-8 Power MOSFETs

Alan Li, Sr. Applications Engineer

Inapplicationsrequiringhighcurrentandsmallfootprintelectronic

In order to calculate power handling rather than rely on datasheet

specifications, users should be familiar with the parameter R_θJA

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.

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_θJAat a given condition are known. To

facilitate users in proper thermal design, Fairchild Semiconductor

publishes different R_θJAspecifications at various conditions.

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.

Figure 1. Typical Condition

R_θJA= 125 ^oC/W ¡ P_D= 1 W

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.

Figure 2. 1 in²2oz Copper PAD

R_θJA(10sec) = 36 ^oC/W ¡ P_D(10sec) = 3.5 W

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).

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.

AN1032 Rev. A

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