POWER DERATING
The operating case temperature range of the QBR series is -40°C to +105°C. When operating the QBR series, proper derating or cooling is needed.
The maximum case temperature under any operating condition should not exceed + 105°C.
Forced Convection Power De-rating without Heat Sink
Example (without heatsink):
AIR FLOW RATE
TYPICAL Rca
Power Dissipated vs Ambient Temperature and Air Flow without heatsink
Natural Convection
20ft. /min. (0.1m/s)
30
28
26
24
22
20
18
16
14
12
10
8
10.1 ℃/W
Natural Convection
20 ft./min. (0.1 m/s)
100 ft./min. (0.5m/s)
200 ft./min. (1.0m/s)
300 ft./min. (1.5m/s)
400 ft./min. (2.0m/s)
8.0 ℃/W
5.4 ℃/W
4.4 ℃/W
3.4 ℃/W
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)
6
4
2
0
0
10
20
30
40
50
60
70
80
90
100
Ambient Temperature ,Ta(Deg. C)
What is the minimum airflow necessary for a QBR101S5-12 operating at nominal line voltage, an output current of 12A, and a maximum ambient
temperature of 50°C?
Solution:
Given:
Vin=110Vdc, Vo=5Vdc, Io=12 A
Determine Power dissipation (Pd):
Pd =Pi - Po=Po(1-η)/η
Pd =5 × 12 × (1-0.91)/0.91=5.93Watts
Determine airflow:
Given: Pd = 5.93W and Ta=50°C
Check Power Derating curve:
Minimum airflow= 100 ft./min.
Verify:
Maximum temperature rise is
∆T = Pd × Rca=5.93W×8.0=47.44°C.
Maximum case temperature is
Tc=Ta+∆T=97.44°C <105°C.
Where:
The Rca is thermal resistance from case to ambient environment.
Ta is ambient temperature and Tc is case temperature.
September 18 2017 B.03
Page 5 of 15
DATEL [ DATEL, Inc. ]
