Technical Information Site of Power Supply Design

2019.02.21 DC/DC

# Heat Calculation for Package Selection: Example 2

Examination of Losses

In this article, in succession to the previous "Heat Calculation for Package Selection: Example 1", we consider, as "Heat Calculation Example 2", measures to enable use of a desired package.

Heat Calculation for Package Selection: Example 2

We first review the previous loss calculations and calculation results, and then indicate the heat calculation results under those conditions.

Total of power losses for the power supply IC

Conditions of Use

Heat Calculation Equation

Tj＝Ta＋θja×P

Ta：Ambient temperature (℃)
θja：Junction to ambient thermal resistance (℃/W)
P： Power loss (W)

Mounting Board, Thermal Resistance, Power Dissipation (PD）, Tjmax

Heat Calculation Results for Condition ①

Tj＝Ta＋θja×P　⇒　85℃＋189.4℃/W×1.008W＝275.9℃　→ exceeds Tjmax＝150℃, FAIL

The above is a summary of the previous article; in actuality, for a Tjmax＝150℃, the temperature is a complete FAIL at 275.9℃, rendering further study unnecessary.

In light of this result, here calculations are performed for the other board condition ② described above.

②: 4-layer PCB (2 inner layers copper foil area of PCB, copper foil area on the backside of PCB:　74.2×74.2mm)

Condition②:θja＝40.3℃/W

Tj＝Ta＋θja×P　⇒　85℃＋40.3℃/W×1.008W＝125.6℃　→ lower than Tjmax＝150℃, PASS

Under condition ②, through the heat dissipation effect of a 4-layer board, the thermal resistance was reduced from 189.4°C/W to 40.3°C/W, or nearly 1/5 the original value. Hence even when Ta=85°C, a margin with respect to Tjmax of about 24°C was achieved. This can also be confirmed by the fact that the point of intersection of the 1.008 W line, indicated in red in the above graph of allowable losses, and the line at which Ta=85°C, is inside the allowable loss curve for condition ②.

With this, it is found that the HTSOP-8 package that was chosen can in fact be used, but that as the board, a 4-layer board must be employed.

While these examples are somewhat extreme, experience with such calculations enables one to immediately understand the general conditions that are necessary. However, in order to reach a conclusion, proper calculation of the power losses should not be neglected.

#### Key Points:

・Losses are determined in order to perform heat calculations.

・Heat dissipation measures are taken so as to ensure that Tj does not exceed the absolute maximum rating.