AN533 datasheet(6/22 Pages) STMICROELECTRONICS

6 / 22 page

Through-hole packages

AN533

6/22

The formula to calculate the thermal resistance between heatsink and ambient air is the

following:

Where:

●

j is the junction temperature in °C

●

P is the maximum dissipated power in W

●

th(j-c) is the thermal resistance between junction and case in

°C/W

th(c-h) is the thermal resistance between case and heatsink in

°C/W, depending on the

contact case/heatsink.

Since the current alternates in a TRIAC, we have to consider the Rth(j-c) in alternating

current which is different to the Rth(j-c) in direct current.

This difference is due to the die of the TRIAC. The first half of the silicon die works when the

current is positive, the second when the current is negative. Because of the thermal coupling

between these two parts, this gives the following equation.

Rth(j-c)AC = 0.75 . Rth(j-c)DC

1.1.6

Choice of heatsink

Choosing a heatsink depends on several parameters; the thermal characteristic, the shape

and the cost.

However, in some applications a flat heatsink can be sufficient. Figure 4 shows the curve

Rth(h-a) versus the length of a flat square heatsink for different materials and thickness.

Some applications need heatsinks with an optimized shape where the thermal resistances

are not known.

For this, the best solution involves measuring the case temperature of the component in the

worst case scenario and keeping to the following formula:

Tc < Tjmax - P . Rth(j-c)

Where:

●

Tc is the case temperature

●

Tjmax is the maximum junction temperature

●

P is the dissipated power in the component

●

Rth(j-c) is the thermal resistance between junction and case.

Rth(h-a) =

Tj -Ta

- Rth(j-c) - Rth(c-h)

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