AFS090 datasheet(258/334 Pages) MICROSEMI

258 / 334 page

DC and Power Characteristics

3- 8

R e v ision 2

Theta-JA

Junction-to-ambient thermal resistance (

θJA) is determined under standard conditions specified by

JEDEC (JESD-51), but it has little relevance in actual performance of the product. It should be used with

caution but is useful for comparing the thermal performance of one package to another.

A sample calculation showing the maximum power dissipation allowed for the AFS600-FG484 package

under forced convection of 1.0 m/s and 75°C ambient temperature is as follows:

EQ 4

where

EQ 5

The power consumption of a device can be calculated using the Microsemi power calculator. The

device's power consumption must be lower than the calculated maximum power dissipation by the

package. If the power consumption is higher than the device's maximum allowable power dissipation, a

heat sink can be attached on top of the case, or the airflow inside the system must be increased.

Theta-JB

Junction-to-board thermal resistance (

θJB) measures the ability of the package to dissipate heat from the

surface of the chip to the PCB. As defined by the JEDEC (JESD-51) standard, the thermal resistance

from junction to board uses an isothermal ring cold plate zone concept. The ring cold plate is simply a

means to generate an isothermal boundary condition at the perimeter. The cold plate is mounted on a

JEDEC standard board with a minimum distance of 5.0 mm away from the package edge.

Theta-JC

Junction-to-case thermal resistance (

θJC) measures the ability of a device to dissipate heat from the

surface of the chip to the top or bottom surface of the package. It is applicable for packages used with

external heat sinks. Constant temperature is applied to the surface in consideration and acts as a

boundary condition. This only applies to situations where all or nearly all of the heat is dissipated through

the surface in consideration.

Calculation for Heat Sink

For example, in a design implemented in an AFS600-FG484 package with 2.5 m/s airflow, the power

consumption value using the power calculator is 3.00 W. The user-dependent Ta and Tj are given as

follows:

From the datasheet:

EQ 6

θJA = 19.00°C/W (taken from Table 3-6 on page 3-7).

= 75.00°C

TJ = 100.00°C

TA = 70.00°C

θJA = 17.00°C/W

θJC = 8.28°C/W

Maximum Power Allowed

TJ(MAX) TA(MAX)

–

---------------------------------------------

Maximum Power Allowed

100.00°C 75.00°C

–

19.00°C/W

----------------------------------------------------

1.3 W

TJ TA

–

-------------------

100°C 70°C

–

17.00 W

------------------------------------

1.76 W

AFS090 Datasheet(PDF) 258 Page - Microsemi Corporation

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