Thermal Resistance Information

Air Flow Rate

Natural Convection

20 ft. / min (0.1 ms)

7.12 °C/W

100 ft./min (0.5 ms)

6.21 °C/W

200 ft./min (1.0 ms)

5.17 °C/W

300 ft./min (1.5 ms)

4.29 °C/W

400 ft./min (2.0 ms)

3.64 °C/W

500 ft./min (2.5 ms)

2.96 °C/W

600 ft./min (3.0 ms)

2.53 °C/W

700 ft./min (3.5 ms)

2.37 °C/W

800 ft./min (4.0 ms)

2.19 °C/W

Derating Curve

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20

25

30

35

40

45

50

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20

30

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50

60

70

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90

100

Natural Convection

20 ft./min. (0.1 m/s)

100 ft./min. (0.5 m/s)

200 ft./min. (1.0 m/s)

300 ft./min. (1.5 m/s)

400 ft./min. (2.0 m/s)

500 ft./min. (2.5 m/s)

600 ft./min. (3.0 m/s)

700 ft./min. (3.5 m/s)

800 ft./min. (4.0 m/s)

Ambient Temperature ,T (

Maximum Power Dissipation vs Ambient Temperature and Air Flow without heatsink

a

27-Feb-14

Application Notes

QSB300

Example

1.Calculate power dissipated

= [Power in – Power out] = [(5V*45A)/90% efficiency – 5V*45A] = 25 W

2.Use de-rating curve to establish airflow

Using 25 W dissipated power and 35 °C ambient, airflow is 600 ft/min

(3.0 m/s)

3.Use table to establish typical thermal resistance Rca

Airflow of 600ft/min gives typical Rca of 2.53 °C/W

4.Check that airflow is adequate to limit case temperature to 100 °C

maximum

Case temperature = Temperature rise + Ambient temperature

Temperature rise = Power dissipated * Typical thermal resistance Rca

= 25 W* 2.53 °C/W = 63.25 °C

Case temperature = 63.25 °C + 35 °C = 98.25 °C i.e. <100 °C

Air Flow Rate

Natural Convection

20 ft. / min (0.1 ms)

3.00 °C/W

100 ft./min (0.5 ms)

1.44 °C/W

200 ft./min (1.0 ms)

1.17 °C/W

300 ft./min (1.5 ms)

1.04 °C/W

400 ft./min (2.0 ms)

0.95 °C/W

Power Dissipated vs Ambient Temperature and Air Flow with XP part ‘ICH HEATSINK’

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Natural C onvection

20 ft./min. (0.1 m/s)

100 ft./min. (0.5 m/s)

200 ft./min. (1.0 m/s)

300 ft./min. (1.5 m/s)

400 ft./min. (2.0 m/s)

a

Example

1.Calculate power dissipated

= [Power in – Power out] = [(12V*20A)/90% efficiency – 12V*20A]

= 26.27 W

2.Use de-rating curve to establish airflow

Using 26.27 W dissipated power and 65 °C ambient, airflow is

200 ft/min (1.0 m/s)

3.Use table to establish typical thermal resistance Rca

Airflow if 200 ft/min gives typical Rca of 1.17 °C/W

4.Check that airflow is adequate to limit case temperature to 100 °C

maximum

Case temperature = Temperature rise + Ambient temperature

Temperature rise = Power dissipated * Typical thermal resistance Rca

= 26.67 W* 1.17 °C/W = 31.2 °C

Case temperature = 31.2 °C + 65 °C = 96.2 °C i.e. <100 °C

Airflow required for QSB30048S05 at 45A output current and 35°C ambient

Airflow required for QSB30048S12 at 20A output current and 65 °C ambient