STK730C

No. 4472-4/6

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

θc-a can be expressed as follows.

Pd = (PT + PF)

(W)

Tc – Ta

θc-a =

(°C/W)

Pd

Tc:

Substrate temperature (105°C, maximum)

Ta:

IC ambient temperature

The junction temperature, Tj, of each element can be expressed as follows.

Tj = PD

× θj-c + Tc

(°C)

PD:

Power loss for each element (PT, PF)

θj-c: The junction/case thermal resistance of each element

Thermal design consists of deriving the heat sink thermal resistance

θc-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.

IC power dissipation

IC power dissipation

θc-a – S

IC power dissipation