DS_Q48SA52001_05142009

9

THERMAL CONSIDERATIONS

Thermal management is an important part of the system

design. To ensure proper, reliable operation, sufficient

cooling of the power module is needed over the entire

temperature range of the module. Convection cooling is

usually the dominant mode of heat transfer.

Hence, the choice of equipment to characterize the

thermal performance of the power module is a wind

tunnel.

Thermal Testing Setup

Delta’s DC/DC power modules are characterized in

heated vertical wind tunnels that simulate the thermal

environments

encountered

in

most

electronics

equipment. This type of equipment commonly uses

vertically mounted circuit cards in cabinet racks in which

the power modules are mounted.

The

following

figure

shows

the

wind

tunnel

characterization setup. The power module is mounted

on a test PWB and is vertically positioned within the

wind tunnel. The space between the neighboring PWB

and the top of the power module is constantly kept at

6.35mm (0.25’’).

Note: Wind Tunnel Test Setup Figure Dimensions are in millimeters and (Inches)

12.7 (0.5”)

MODULE

AIR FLOW

50.8 (2.0”)

FACING PWB

PWB

AIR VELOCITY

AND AMBIENT

TEMPERATURE

MEASURED BELOW

THE MODULE

Figure 18:

Wind tunnel test setup

Thermal Derating

Heat can be removed by increasing airflow over the

module. To enhance system reliability, the power

module should always be operated below the maximum

operating temperature. If the temperature exceeds the

maximum module temperature, reliability of the unit may

be affected.

Figure 17:

Circuit configuration for trim-down (decrease

output voltage)

If the external resistor is connected between the TRIM

and Vo(+), Vout decreases (Fig. 18). The external

resistor value required to obtain output voltage change

△

U is defined as:

)

(

7

.

55

51

*

51

Ω

−

Δ

=

−

K

U

R

down

trim

Ex. When Trim-down 15%， U

△

is

15%*Vnormal=0.15*52.5=7.875

)

(

6

.

274

7

.

55

875

.

7

51

*

51

Ω

=

−

=

−

K

R

down

trim

When using trim-up, the output voltage of the module

is usually increased, which increases the power output

of the module with the same output current.Care

should be taken to ensure that the maximum output

power of the module remains at or below the maximum

rated power.