STK730C Sanyo Semiconductor Corporation, STK730C Datasheet - Page 4

• Manufacturer Part Number: STK730C
• Description: 5V Signal 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 black. This
can reduce the thermal resistance by 20% for a given surface area.
c-a =
Tc – Ta
Tc:
Ta:
PD: Power loss for each element (PT, PF)
Pd
j-c: The junction/case thermal resistance of each element
j-c + Tc
Substrate temperature (105°C, maximum)
IC ambient temperature
IC power dissipation
Heat sink area, S – cm
Output current, I
c-a – S
O
– A
2
(W)
(°C/W)
(°C)
STK730C
IC power dissipation
IC power dissipation
Output current, I
Output current, I
O
O
– A
– A
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