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What is the thermal resistance of 2SJ355 on FR4 (no heat sink)?

Latest Updated:09/02/2013

Question:

The total power dissipation of the 2SJ355 is specified as 2.0 W (at 25℃) when mounted on the prescribed ceramic printed circuit board (PCB). What is the total power dissipation when mounted on an FR4 printed circuit board without a heat sink? How about when the ambient temperature is 60℃? Does thermal resistance decrease with increasing FR4 board area?

Answer:

The thermal resistance at saturation point (no heat sink) on an FR4 PCB is 283 ℃/W (reference value). Total power dissipation can be calculated from thermal resistance using the following formula.

Pd = (Tj(max) - Ta)/Rth
Pd: Total power dissipation; Ta: Ambient temperature; Rth: Thermal resistance

The total power dissipation at an ambient temperature of 60℃ is as follows.

(150 - 60)/283 ≒ 0.32 W

Since FR4 PCBs generally have inferior heat dissipation, the total power dissipation will likely not change much even if board area is increased.

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