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QSB300 Datasheet(PDF) 4 Page - XP Power Limited |
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QSB300 Datasheet(HTML) 4 Page - XP Power Limited |
4 / 4 page Thermal Resistance Information Air Flow Rate Typical Rca 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 Rca = Thermal resistance from case to ambient Derating Curve 0 5 10 15 20 25 30 35 40 45 50 0 10 20 30 40 50 60 70 80 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 ( °C) 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 Typical Rca 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’ 0 5 10 15 20 25 30 35 40 45 50 0 10 20 30 40 50 60 70 80 90 100 Ambient Temperature, Ta (°C) 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 |
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