AHF28XX Series
Application Information
Inhibit Function
Connecting the enable input (Pin 1) to input common (Pin 7)
will cause the converter to shut down. It is recommended
that the enable pin be driven by an open collector device
capable of sinking at least 400 µA of current. The open
circuit voltage of the enable input is 15 ±1 VDC. If the inhibit
function is not used, this input can be left unconnected
because it is internally pulled-up.
Hence, if TAMBIENT = +25°C, the DC/DC converter case
temperature will be approximately 100°C if no heat sink
or air flow is provided.
To calculate the heat sink area required to maintain a
specific case temperature rise, the above equation may
be manipulated as follows:
Thermal Management
−1.43
∆Τ
A
=
Assuming that there is no forced air flow, the package tem-
perature rise above ambient (∆T) may be calculated using
the following expression:
− ΑPKG
HEAT SINK
Ρ 0.85
80
As an example, if a maximum case temperature rise of
50°C rise above ambient is desired, then the required
effective heat sink area is:
∆T ≈ 80 A –0.7p 0.85 (°C)
where A = the effective surface area in square inches (in-
cluding heat sink if used), P = power dissipation in watts.
−1.43
50
− 4.9 = 3.75in.2
The total surface area of the AHF package is 4.9 square
inches. If a worse case full load efficiency of 78% is as-
sumed, then the case temperature rise can be calculated
as follows:
ΑHEATSINK
=
80(3.4)0.85
1
1
P = P
−1 =12
−1 = 3.4W
OUT
Eff
0.78
∆T = 80 (4.9)-0.7 (3.4)0.85 = 74°C
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INFINEON [ Infineon ]
