No. 7047-4/6

STK402-070

Thermal Design Example

The thermal resistance,

θc-a of the required heat sink for the power dissipation, Pd, within the hybrid IC is determined as

follows.

Condition 1: The IC substrate temperature, Tc, must not exceed 125°C.

Pd

× θc – a + Ta < 125°C ......... (1)

Ta: Guaranteed ambient temperature for the end product.

Condition 2: The junction temperature, Tj, of each power transistor must not exceed 150°C.

Pd

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

N: Number of power transistors

θc-a: Thermal resistance per power transistor

However, the power dissipation, Pd, for the power transistors shall be allocated equally among the N transistors.

The following inequalities results from solving equations (1) and (2) for

θc-a.

θc – a < (125 – Ta) /Pd .................. (1)’

θc – a < (150 – Ta) /Pd – θj – c/N ......... (2)’

Values that satisfy these two inequalities at the same time represent the required heat sink thermal resistance.

When the following specifications have been stipulated, the required heat sink thermal resistance can be determined from

formulas (1)’ and (2)’.

• Load resistance value — RL

• Guaranteed ambient temperature — Ta

[Example]

of the power dissipation based on this type of continuous signal. (Note that the factor used may differ depending on the

safety standards used.)

That is:

The number of power transistors in the audio amplifier block of these hybrid ICs, N, is 4, and the thermal resistance per

transistor is 3.2°C/W. Therefore, the required heat sink thermal resistance for a guaranteed ambient temperature of 50°C

will be as follows.

From formula (1)’

θc – a < (125 – 50) /42

< 1.78

From formula (2)’

θc – a < (150 – 50) /42 – 3.2/4

< 1.58

Therefore, 1.58°C/W is the required heat sink thermal resistance.

Note that this thermal design example assumes the use of a constant-voltage power supply, and is therefore not a verified

design for any particular user's end product.