Compatible Systems RISC 2800i Bedienungsanleitung PDF herunterladen (Seite 47)

MPC7447A RISC Microprocessor Hardware Specifications, Rev. 5

Freescale Semiconductor 47

System Design Information

Shin-Etsu MicroSi, Inc. 888-642-7674

10028 S. 51st St.

Phoenix, AZ 85044

Internet: www.microsi.com

Thermagon Inc. 888-246-9050

4707 Detroit Ave.

Cleveland, OH 44102

Internet: www.thermagon.com

The following section provides a heat sink selection example using one of the commercially available heat

sinks.

9.8.3 Heat Sink Selection Example

For preliminary heat sink sizing, the die-junction temperature can be expressed as follows:

= T

+ T

+ (R

θJC

+ R

θint

+ R

θsa

) × P

where:

is the die-junction temperature

is the inlet cabinet ambient temperature

is the air temperature rise within the computer cabinet

θJC

is the junction-to-case thermal resistance

θint

is the adhesive or interface material thermal resistance

θsa

is the heat sink base-to-ambient thermal resistance

is the power dissipated by the device

During operation, the die-junction temperatures (T

) should be maintained less than the value specified in

Table 4. The temperature of air cooling the component greatly depends on the ambient inlet air temperature

and the air temperature rise within the electronic cabinet. An electronic cabinet inlet-air temperature (T

)

may range from 30° to 40°C. The air temperature rise within a cabinet (T

) may be in the range of 5° to

10°C. The thermal resistance of the thermal interface material (R

θint

) is typically about 1.5°C/W. For

example, assuming a T

of 30°C, a T

of 5°C, an HCTE package R

θJC

= 0.1, and a typical power

consumption (P

) of 18.7 W, the following expression for T

is obtained:

Die-junction temperature: T

= 30°C + 5°C + (0.1°C/W + 1.5°C/W + R

θsa

) × 18.7 W

For this example, a R

θsa

value of 2.1°C/W or less is required to maintain the die junction temperature below

the maximum value of Table 4.

Though the die-junction-to-ambient and the heat-sink-to-ambient thermal resistances are a common

figure-of-merit used for comparing the thermal performance of various microelectronic packaging

technologies, one should exercise caution when only using this metric in determining thermal management

because no single parameter can adequately describe three-dimensional heat flow. The final die-junction

operating temperature is not only a function of the component-level thermal resistance, but the

system-level design and its operating conditions. In addition to the component's power consumption, a

number of factors affect the final operating die-junction temperature—airflow, board population (local

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