AN533

Through-hole packages

7/22

Figure 4.

1.1.7

Forced cooling

For high power or very high power applications, a forced-air or liquid cooling heatsink may

be required. Heatsink manufacturers give a coefficient depending on the air or liquid flow.

However, in some applications like vacuum cleaners, dissipated power is only a few watts

and air flow cooling is available. This allows a very small heatsink to be used, very often a

flat aluminium heatsink. In this case it is necessary to measure the case temperature in the

worst case scenario and to check the following formula:

1.2

Thermal impedance

In steady state, a thermal equivalent circuit can be made only with thermal resistances.

However, for pulse operation it can be useful to consider the thermal impedance, especially

when the component is on during a time lower than the time to reach the thermal resistance.

The thermal impedance value versus pulse duration is given in the datasheets (see an

For example, BTA08-600SW is able to dissipate

curve (in the case of this product the ratio is 0.06 as seen in Figure 5) and multiplying the

Rth(h-a)

100

50

30

20

10

1

2

3

5

STEEL

Thickness of

the plate in mm

0

2

8

14

4

10

16

6

12

18

20

0.5

1

2

5

Rth(h-a)

100

50

30

20

10

1

2

3

5

(°C/W)

COPPER

Thickness of

the plate in mm

(cm)

0

2

8

14

4

10

16

6

12

18

20

Length

0.5

1

2

5

Rth(h-a)

100

50

30

20

10

1

2

3

5

(°C/W)

ALUMINIUM

Thickness of

the plate in mm

(cm)

0

2

8

14

4

10

16

6

12

18

20

Length

0.5

1

2

5

Length

(°C/W)

(cm)

- Semiconductor device

in the center

- Bare convector (no ventilation)

- Vertical position

Thermal model for calculation

based on square heatsink

L

e

P =

Tjmax -Tamax

Zth(j-a) (1 s)

P =

125 - 25

60 x 0.06

= 27.5 W