STK4067

No. 4376-5/8

Sample STK4067 Heat Sink Design

The radiator thermal resistance

θc-a required for total substrate power dissipation Pd in the STK4067 is determined as:

Condition 1: IC substrate temperature Tc not to exceed 110°C.

Pd x

θc-a+Ta <110°C ······························· (1)

where Ta is set assured ambient temperature.

Condition 2: Power transistor junction temperature Tj not to exceed 150°C.

Pd x

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

where N is the number of power transistors and

θj-c is the thermal resistance per power transistor chip.

However, power transistor power consumption is Pd equally divided by N units.

Expressions (1) and (2) can be rewritten based on

θc-a to yield:

θc-a<(110–Ta)/Pd ······································(1)'

θc-a<(150–Ta)/Pd–θj-c/N··························(2)'

The required radiator thermal resistance will satisfy both of these expressions.

From expressions (1)' and (2)', the required radiator thermal resistance can be determined once the following

specifications are known:

•

Supply voltage

•

Load resistance

•

Assured ambient temperature

Ta

signal, is a maximum of 19.3W (Fig. 2).

The STK4067 has four power transistors, so the thermal resistance per transistor

θj-c is 1.6°C / W. With an assured

ambient temperature Ta of 50°C, the required radiator thermal resistance

θc-a would be:

From expression (1)'

θc-a <(110–50)/19.3

<3.1

From expression (2)'

θc-a <(150–50)/19.3–1.6/4

<4.78

To satisfy both, 3.1°C/W is the required radiator thermal resistance.

Pd max= 34.8W

From expression (1)'

θc-a <(110–50)/34.8

<1.72

From expression (2)'

θc-a <(150-50)/34.8–1.6/4

<2.47

To satisfy both, 1.72°C/W is the required radiator thermal resistance.

Pd max= 12W

From expression (1)'

θc-a <(110–50)/12

<5

From expression (2)'

θc-a <(150-50)/12–1.6/4

<7.93

To satisfy both, 5°C / W is the required radiator thermal resistance. This design example is based on a fixed voltage

supply, and will require verification within your specific set environment.