Next we determine the usage conditions with no heat sink by determining the allowable hybrid IC internal average loss

from the thermal resistance of the hybrid IC substrate, namely 25.5 °C/W.

105 – 50

For a Tc max of 105°C at an ambient temperature of 50°C

25.5

105 – 40

For a Tc max of 105°C at an ambient temperature of 40°C

25.5

This hybrid IC can be used with no heat sink as long as it is used at operating conditions below the losses listed above.

(See

∆Tc – P

<Hybrid IC internal power element (MOSFET) junction temperature calculation>

The junction temperature, Tj, of each device can be determined from the loss Pds in each transistor and the thermal

resistance

θj-c.

Tj = Tc +

θj-c × Pds (°C)

Pds = Pd/4

Since the average loss includes the loss of the current detection resistor, we take that voltage drop into consideration in

the calculation.

The steady-state thermal resistance of a power MOSFET is 15.6°C/W.

No. 6507-15/17

STK672-080

4

0

8

12

16

20

0

2

4

6

8

10

12

14

16

2

1.0

3

7

5

10

2

10

2

3

5

7

100

2

3

5

No. Fin 25.5(

°C/W)

No. Fin 25.5(

°C/W)

40°C

50

°C

60°C

θc-a= Tc max – Ta (°C/W)

Tc max=105

°C

Pd

2 mm

thick

Al

plate

(with

no

surface

preparation)

(with

the

surface

painted

black)

Vertical

standing type

Convection

cooling

<Determining the Size of the Hybrid IC Heat Sink>

Determine

θc-a for the heat sink from the average power loss determined in the previous item.

Tc max: Hybrid IC substrate temperature (°C)

Ta: Application internal temperature (°C)

Determine

The ambient temperature of the device will vary greatly according to the air flow conditions within the application.

Therefore, always verify that the size of the heat sink is adequate to assure that the Hybrid IC back surface (the aluminum

plate side) will never exceed a Tc max of 105°C, whatever the operating conditions are.

Tc max – Ta

θc-a = —————— [°C/W]

IC internal average power dissipation, Pd — W

θc-a — Pd

θc-a — S