The STK400-020 has six power transistors, so the thermal resistance per transistor

θj-c is 2.1°C / W. With an assured

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

θc-a would be as follows:

From expression (1)'

θc-a < (125–50)/23.5

< 3.19

From expression (2)'

θc-a < (150–50)/23.5 – 2.1/6

< 5.84

To satisfy both, 3.19°C/W is the required radiator thermal resistance.

From expression (1)'

θc-a < (125–50) – 26.5

< 2.83

From expression (2)'

θc-a < (150-50)/26.5 – 2.1/6

< 3.42

To satisfy both, 2.83°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.

No. 4830-8/10

STK400-020