MFW2812SSLASHES datasheet(4/9 Pages) INTERPOINT

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B4-6

MFW SERIES

70 WATT

DC/DC CONVERTERS

CALCULATING MAXIMUM AMBIENT TEMPERATURE

The MFW 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

dissipation (PDISS vs. ambient temperature for various heat sink

thermal resistances. PDISS may be calculated as:

PDISS = POUT / efficiency – POUT

The efficiency for all combinations of POUT and VIN for the various

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

Example:

Converter = MFW2815D, TAMB = 70°C,

VIN = 28 VDC, POUT = 45 watts

Efficiency = 85% (From Figure 7)

PDISS = (45 / 85) – 45 = 7.95 watts

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 = MFW2805S, VIN = 28 VDC, POUT = 45 W.

Thermal Resistance = 3°C / watt.

Efficiency = 83.5% (From Figure 3)

PDISS = (45 / 0.835) – 45 = 8.89 watts.

From Figure 1 we can see that the maximum allowed ambient

temperature is approximately 75°C.

HEAT SINK RECOMMENDATIONS

An MFW 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 approxi-

mately 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

° 40° 60° 80° 100° 120° 140°

U=4

°C/W

U=3

°C/W

U=2

°C/W

U=0

°C/W

Internal Power Dissipation (max)

vs. Ambient Temperature

FIGURE 1

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