STK400-030

No. 5703—4/6

Heatsink Design Considerations

The heatsink thermal resistance,

θc-a, required to cover

the hybrid IC’s total power dissipation, Pd, is determined

as follows:

Condition 1: Hybrid IC’s substrate temperature not to

exceed 125

°C.

Pd

× θc-a + Ta < 125°C ........................................ (1)

where Ta is the guaranteed maximum ambient tempera-

ture.

Condition 2: Power transistor junction temperature, Tj, not

to exceed 150

°C.

Pd

× θc-a + Pd/N × θj-c + Ta < 150°C ................. (2)

where N is the number of power transistors and

θj-c is the

thermal resistance per power transistor. Note that the

power dissipated per transistor is the total, Pd, divided

evenly among the N power transistors.

Expressions (1) and (2) can be rewritten making

θc-a the

subject.

θc-a < (125 − Ta)/Pd............................................. (1)′

θc-a < (150 − Ta)/Pd − θj-c/N .............................. (2)′

The heatsink required must have a thermal resistance that

simultaneously satisfies both expressions.

The heatsink thermal resistance can be determined from

(1)

′ and (2)′ once the following parameters have been

defined.

• Supply voltage

• Load resistance

• Guaranteed maximum ambient temperature

: Ta

=

±23V and R

When estimating the power dissipation for an actual audio

signal input, the rule of thumb is to select Pd correspond-

sine wave input. For example,

The hybrid IC has 6 power transistors, and the thermal

resistance per transistor,

θj-c, is 2.1°C/W. If the guaran-

teed maximum ambient temperature, Ta, is 50

°C, then the

required heatsink thermal resistance,

θc-a, is:

From expression (1)

′: θc-a < (125 − 50)/31

< 2.41

From expression (2)

′: θc-a < (150 − 50)/31 − 2.1/6

< 2.87

Therefore, to satisfy both expressions, the required heat-

sink must have a thermal resistance less than 2.41

°C/W.

From expression (1)

′: θc-a < (125 − 50)/36

< 2.08

From expression (2)

′: θc-a < (150 − 50)/36 − 2.1/6

< 2.42

Therefore, to satisfy both expressions, the required heat-

sink must have a thermal resistance less than 2.08

°C/W.

This heatsink design example is based on a constant-volt-

age egulated power supply, and should be verified within

your specific set environment.