External Circuit Diagram

No. 4830-7/10

STK400-020

Heat Radiation Design Considerations

The radiator thermal resistance

θc-a required for total substrate power dissipation Pd in the STK400-020 is determined as

follows:

Condition 1: IC substrate temperature Tc not to exceed 125°C.

Pd x

θc-a+Ta <125°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 the thermal resistance per power transistor chip.

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

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

θc-a to yield:

θc-a<(125–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

is a maximum of 41 W (Fig. 1). In general, when this sort of continuous signal is used for estimation of power