Vicor VI-J00 Family DC-DC Converters and Configurable Power Supplies - Manual (Page 67)

Design Guide & Applications Manual
For VI-200 and VI-J00 Family DC-DC Converters and Configurable Power Supplies
VI-200 and VI-J00 Family Design Guide Rev 3.5 vicorpower.com
Page 66 of 98
Apps. Eng. 800 927.9474 800 735.6200
DEFINITIONS
T
max
= maximum baseplate temperature
(Available from converters data sheet, which can
be found on vicorpower.com.)
T
a
= ambient temperature
η = efficiency = P out
(Assume efficiencies of 81%
P
in
for 5 V outputs and 85%
for 12 V out and above.)
θ
bs = baseplate-to-heat sink thermal resistance
(From thermal impedance tables)
θ
sa
= heatsink-to-air sink thermal resistance
(From thermal impedance tables)
THERMAL EQUATIONS
P diss = dissipated power = P out
( 1
– 1
)
η
Airflow (LFM) =
(
CFM
)
Area
Maximum output power =
T max – T a
θ
sa
( 1
– 1
)
η
Maximum thermal impedance =
T
max
– T
a
P
out
( 1
– 1
)
η
Maximum ambient temperature = T max –
θ
sa x P out
( 1
– 1
)
η
Temperature rise =
θ
sa
x P
out
( 1
– 1
)
η
Thermal drop =
θ
bm x P out
( 1
– 1
)
η
TYPICAL EXAMPLES
Example 1. Determine the maximum output power for a
100 W, VI-200 converter, no heat sink, delivering 5 V in
400 LFM at a maximum ambient temperature of 45°C.
Maximum output power =
T
max
– T
a
θ
sa
( 1
– 1
)
η
T max = 85°C
T
a
= 45°C
θ
sa = 1.8°C/W
η = 81% = (0.81)
Maximum output power =
85 – 45
1.8
( 1
– 1
)
0.81
= 95 W max.
Example 2. Determine the maximum thermal impedance
of a 50 W, VI-J00 converter, no heat sink, delivering 24 V
at 45 W in free air convection at 55°C ambient.
Maximum thermal impedance =
T
max
– T
a
P
out
( 1
– 1
)
η
T max = 100°C
T a = 55°C
P
out
= 45 W
η = 85% = (0.85)
Maximum thermal impedance =
100 – 55
45
( 1
– 1
)
0.85
= 5.7˚C/W
20. Thermal and Module Mounting Considerations