HR700 SERIES
70 WATT
DC/DC CONVERTERS
THERMAL MANAGEMENT
CALCULATING MAXIMUM AMBIENT TEMPERATURE
The HR700 Series of DC/DC converters has an upper operating
temperature of + 85°C at the baseplate of the case. The degree of
heat sinking required to remain within this limit may be determined
from Figure 1 which shows the maximum allowed internal power
HEAT SINK RECOMMENDATIONS
An HR700 Series converter in still air (other than convective
currents) and with no conductive cooling paths other than through
electrical connections at the pins will exhibit a thermal resistance of
approximately 4°C / watt. In cases where this value proves to be too
high it is recommended that additional heat sinking be supplied. The
simplest method of accomplishing this is to firmly attach the
converter to a PCB thereby providing a conductive thermal path.
Secondly it is recommended that airflow be provided over the
converter. Although each situation requires a thorough thermal
analysis these two measures can reduce the thermal resistance to
as low as 2°C / watt. If calculations indicate further heat sinking is
required it is recommended that additional thermal mass be
provided either under the base plate or on top of the converter's
mounting flanges or both.
dissipation (P
vs. ambient temperature for various heat sink
DISS
thermal resistances. P
may be calculated as:
DISS
P
= P
/ efficiency – P
DISS
OUT OUT
The efficiency for all combinations of P
and V for the various
IN
OUT
models may be obtained from the graphs on the preceding pages.
Example: Converter = HR702-2815, T = 70°C,
AMB
V
= 28 VDC, P
= 45 watts
IN
OUT
Efficiency = 85% (From Figure 7)
= (45 / 85) – 45 = 7.95 watts
P
DISS
From Figure 1 we can see that this situation will require thermal
resistance of approximately 4.5°C / watt.
Conversely we may also find the maximum ambient temperature
which can be tolerated if we know the heat sink thermal resistance.
Example: Converter = HR701-2805, V = 28 VDC, P
IN
= 45 W.
OUT
Thermal Resistance = 3°C / watt.
Efficiency = 83.5% (From Figure 3)
P
= (45 / 0.835) – 45 = 8.89 watts.
DISS
From Figure 1 we can see that the maximum allowed ambient
temperature is approximately 75°C.
20
15
10
5
20° 40° 60° 80° 100° 120° 140°
Degrees Centigrade
Internal Power Dissipation (max)
vs. Ambient Temperature
FIGURE 1
B4-36
CRANE [ Crane Aerospace & Electronics. ]
