SI-8000HFE/HD
24
---(8)
The TjMAX is an inherent value for each product, therefore it must be strictly observed. For this purpose, it
is required to design the heat sink in compliance with PdMAX, TaMAX (determination of θfin). The heat
derating graphically describes this relation.
The designing of the heat sink is carried out by the following procedure:
1) The maximum ambient temperature Ta MAX in the set is obtained.
2) The maximum power dissipation PdMAX is obtained.
VIN
VOUT
IoVf
x
IoVOUTPd 11
100
---(9)
* ηx= efficiency (%), Vf= diode forward voltage
3) The size of heat sink is determined from the intersection of the heat derating.
The required thermal resistance of the heat sink can be also calculated. The thermal resistance required for
the heat sink is obtained by the following equation:
---(10)
An example of heat calculation for using SI-8008HFE under the conditions of VIN = 15V, Io = 4A and Ta =
85°C is shown below. Where efficiency η = 80% , Vf = 0.5V from the typical characteristics,
WPd 67.3
15
5
145.01
80
100
45 ≒
WCfini /9.55
75.2 85125 ゚≒
As a result, the heat sink with the thermal resistance of 5.9°C /W or less is required. As described above,
the heat sink is determined, but the derating of 10 - 20% or more is used. Actually, heat dissipation effect
significantly changes depending on the difference in component mounting. Therefore, heat sink
temperature or case temperature should be checked with the heat sink mounted.
The maximum rating Tjmax of the SI-8008HFE is 150°C, but it is recommended to design the heat sink at
TjMAX < 125°C since the thermal shutdown circuit may be operated at 130°C or higher.
4-5-2 Installation to Heat sink
Selection of silicon grease
When the SI-8000HFE is installed to the heat sink, silicon grease should be thinly and evenly coated
between the IC and heat sink. Without coating, thermal resistance θi is significantly increased because of
contact failure due to micro concavity/convexity between the backside of the IC and the surface of the heat
sink to accelerate the heating of the IC, resulting in shorter life of the IC.
In some kind of silicon grease to be used, oil component may be separated to penetrate into the IC,
resulting in the deformation of packages or the adverse effect on built-in elements.
Any other silicon grease than one based on the modified silicon oil shall not be used.
The recommended silicon greases are as follows: