KEPCO KFD 6-25-28W - Manual (Page 12)

8
KFD 62528W/060801
the fins oriented along the width). Natural convection is the air flow produced when air in contact with a
hot surface is heated. An open environment is required with no external forces moving the air. The Figures
apply when the power module is the only source of heat present in the system.
18.5 USE OF GRAPHS
First determine the amount of power that is to be dissipated as heat, as well as the ambient operating tem-
perature. Plot the data on the graph, and note the intersection point; the point indicates the appropriate
heat sink to use. For instance, if P
D
=20 W and Ta=30°C, a 0.5 inch (12.7mm) heat sink with fins oriented
along the width, would keep the module within its operating temperature rating
18.6 THERMAL MODELS
The curves in Figure 14 are plots of thermal resistance against air velocity, for various types of heat sinks;
with fins oriented along the width and with fins oriented along the length. The plots are determined experi-
mentally without a heat sink and with the heat sinks illustrated in Figures 9 and 10 . The highest values on
the curves represent natural convection. In a system with free flowing air and other heat sources, there
may be additional air flow.
The following two examples illustrate how the curves can be used to determine thermal performance under
various air flow and heat sink configurations.
Example 1: To determine the air flow required to maintain T
c,max
(case temperature)=95°C for the KFD
150W D.C. to D.C. Converter (the KFD 6-25-28W) without a heat sink, consider the following:
The KFD DC to DC Converter (150W) operates at Io=30 Amps and T
A
=50° without a heat sink. The power
dissipated by the unit can be determined from the difference between the input power and output power
and the efficiency of the converter. It can be noted that the unit has a power dissipation of 35.2 watts. The
thermal resistance that is necessary to maintain a 95°C case temperature is determined from the equa-
tions that follow:
The total thermal resistance of the unit is defined as the maximum case temperature rise divided by the
power dissipation of the module:
ϑ = Total thermal resistance
∆Τ c max = Maximum case temperature rise
P
D
= Power dissipated as heat
From Figure 14 the required air flow necessary to maintain a 95°C case temperature then is greater than
500 ft./min. (2.5 m/s)
Example 2: How to determine the case temperature for a specific operating environment. Say for exam-
ple, that only an air flow of 150 ft./min is available and it is required to determine the case temperature
θ
Tc max
,
TA – ( )
PD
---------------------------------------------- =
θ 95 50 – ( ) 35.2 ) ÷ 1.3
° C
W
------- = =