B4-36

HR700 SERIES

70 WATT

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

models may be obtained from the graphs on the preceding pages.

Example:

Efficiency = 85% (From Figure 7)

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:

Thermal Resistance = 3°C / watt.

Efficiency = 83.5% (From Figure 3)

From Figure 1 we can see that the maximum allowed ambient

temperature is approximately 75°C.

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.

THERMAL MANAGEMENT

Degrees Centigrade

20

° 40° 60° 80° 100° 120° 140°

20

15

10

5

U=4

°C/W

U=3

°C/W

U=2

°C/W

U=0

°C/W

Internal Power Dissipation (max)

vs. Ambient Temperature