STK672-080
<Determining the Size of the Hybrid IC Heat Sink>
Determine θc-a for the heat sink from the average power loss determined in the previous item.
Tc max: Hybrid IC substrate temperature (°C)
Tc max – Ta
θc-a = —————— [°C/W]
Ta: Application internal temperature (°C)
Pd
EX
PdEX: Hybrid IC internal average loss (W)
Determine θc-a from the above formula and then size S (in cm2) of the heat sink from the graphs shown below.
The ambient temperature of the device will vary greatly according to the air flow conditions within the application.
Therefore, always verify that the size of the heat sink is adequate to assure that the Hybrid IC back surface (the aluminum
plate side) will never exceed a Tc max of 105°C, whatever the operating conditions are.
θc-a — S
θc-a — Pd
20
16
12
2
Tc max – Ta
Pd
Vertical
θc-a=
(°C/W)
standing type
Convection
cooling
Tc max=105°C
10
7
5
8
3
2
4
0
No. Fin 25.5(°C/W)
No. Fin 25.5(°C/W)
1.0
10
0
2
4
6
8
10
12
14
16
2
3
5
7
2
3
5
100
2
IC internal average power dissipation, Pd — W
Heat sink area, S — cm
Next we determine the usage conditions with no heat sink by determining the allowable hybrid IC internal average loss
from the thermal resistance of the hybrid IC substrate, namely 25.5 °C/W.
105 – 50
For a Tc max of 105°C at an ambient temperature of 50°C
Pd = ———— = 2.15 W
EX
25.5
105 – 40
For a Tc max of 105°C at an ambient temperature of 40°C
Pd = ———— = 2.54 W
EX
25.5
This hybrid IC can be used with no heat sink as long as it is used at operating conditions below the losses listed above.
(See ∆Tc – P curve in the graph on page 17.)
d
<Hybrid IC internal power element (MOSFET) junction temperature calculation>
The junction temperature, Tj, of each device can be determined from the loss Pds in each transistor and the thermal
resistance θj-c.
Tj = Tc + θj-c × Pds (°C)
Here, we determine Pds, the loss for each transistor, by determining Pd in each excitation mode.
EX
Pds = Pd/4
Since the average loss includes the loss of the current detection resistor, we take that voltage drop into consideration in
the calculation.
Vsat = I · Ron + I · Rs
OH
OH
Vdf = Vdf + I · Rs
OH
The steady-state thermal resistance of a power MOSFET is 15.6°C/W.
No. 6507-15/17
SANYO [ SANYO SEMICON DEVICE ]
