stk732c Sanyo Semiconductor Corporation, stk732c Datasheet - Page 4

• Manufacturer Part Number: stk732c
• Description: 15 V Single Output Mos Chopper Regulator
• Manufacturer: Sanyo Semiconductor Corporation

Thermal Design
The power dissipating sections of a power supply block consist of the power transistor (PTR), the flywheel diode
(FWD), the choke coil, and the current detection resistor. Of these, the components that are incorporated in the hybrid IC
itself are the PTR and the FWD.
Taking PT to be the power dissipated in the PTR and PF to be the power dissipated in the FWD, the power dissipation
Pd for the whole hybrid IC and the heat sink thermal resistance c-a can be expressed as follows.
Pd = (PT + PF)
The junction temperature, Tj, of each element can be expressed as follows.
Tj = PD
Thermal design consists of deriving the heat sink thermal resistance c-a that satisfies the two thermal conditions, i.e.,
the maximum IC substrate temperature Tc max (105°C) and the maximum junction temperature Tj max for each
semiconductor device, and then implementing that thermal resistance. Since thermal dissipation is greatly influenced by
the ambient temperature, the structure of the equipment itself, and other factors, ample margins must be included in the
thermal design to take them into account.
The figure below left shows the relationship between area and thermal resistance when an aluminum plate is used in the
thermal design. The radiation characteristics of an aluminum plate can be improved by painting the surface in black. This
can reduce the thermal resistance by 20% for a given surface area.
c-a =
j-c and Tj max for the STK732C
FWD (FRD)
PTR (FET)
Parameter
Tc – Ta
Pd
Tc: Substrate temperature (105°C, maximum)
Ta: IC ambient temperature
PD: Power loss for each element (PT, PF)
j-c: The junction/case thermal resistance of each element
j-c + Tc
3.5°C/W
6.9°C/W
Heat sink area, S – cm
j-c
c-a – S
Tj max
[W]
[°C/W]
[°C]
150°C
150°C
2
STK732C
Output current, I
Pd – I
O
O
– A
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