How do I estimate the junction temperature?

The junction temperature Tj can be obtained by the following formula.

Tj = Rth(j-t) * Pd + Ttab

Rth(j-t): Thermal resistance of Audio power IC,
Pd: Power dissipation
Ttab: Temperature of the audio power IC's heatsink.

Fig. 1 Thermal resistance measurement of HZIP type
Fig. 1 Thermal resistance measurement of HZIP type

The temperature of the heatsink of the audio power IC, Ttab, is measured using a thermocouple as shown in Fig. 1.

Insert the thermocouple between the heatsink of the audio power IC and the external heat sink. Tighten the screws securely.
(Use a thermocouple as thin as possible for measurement, and ensure adhesion to the external heat sink by applying silicone grease, etc.)

The thermal resistance of IC, Rth(j-t), is shown in the datasheet. And refer to another FAQ “How do I calculate the current consumption and power dissipation?” for the calculation of the power dissipation Pd.

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