STK415-140-E

No. A1501-9/12

[Thermal Design Example for STK415-140-E (RL = 8Ω)]

The thermal resistance,

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

follows.

Condition 1: The hybrid 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

θj-c: Thermal resistance per power transistor

However, the power dissipation, Pd, for the power transistors shall be allocated equally among the number of power

transistors.

The following inequalities result 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)'.

• Supply voltage

VH, VL

• Load resistance

RL

• Guaranteed ambient temperature

Ta

[Example]

When the IC supply voltage, VH=±52V, VL=±32V and RL is 6Ω, the total power dissipation, Pd, within the hybrid

IC, will be a maximum of 156W at 1kHz for a continuous sine wave signal according to the Pd-PO characteristics.

For the music signals normally handled by audio amplifiers, a value of 1/8PO max is generally used for Pd as an

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

depending on the safety standard used.)

This is:

Pd

≈ 63.0W

(when 1/8PO max. = 15W, PO max. = 120W).

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

transistor,

θj-c, is 1.5°C/W. Therefore, the required heat sink thermal resistance for a guaranteed ambient temperature,

Ta, of 50

°C will be as follows.

From formula (1)'

θc-a < (125 − 50)/63.0

< 1.19

From formula (2)'

θc-a < (150 − 50)/63.0 − 1.5/4

< 1.21

Therefore, the value of 1.19

°C/W, which satisfies both of these formulae, is the required thermal resistance of the heat

sink.

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.