Heat sink sizing for LDMOS
Your dissipated power numbers are the &quot;Total Device Dissipation @ TC = 25°C&quot; from the datasheets.
This is not a power you can achieve in practice. It is like saying &quot;the Bugatti Chiron can go at 500km/h&quot;. Yes, but you need a special road for that, which is not easy to find. To achieve &quot;Total Device Dissipation @ TC = 25°C&quot; you need a special heat sink that is not easy to find, since it can maintain the case at 25°C no matter what the power is. Since there is thermal resistance between the case and the sink, say 0.1°C/W, this means at 690W the sink must be is -69°C below the 25°C case. So bring on the liquid nitrogen.
Basically, &quot;Total Device Dissipation @ TC = 25°C&quot; is purely the result of a mathematical formula. Its use is to tell the designer &quot;If your power goes near that, there is no way, you need another transistor.&quot; And your question shows that is true: if you use this power, you need a magical heat sink.
So your formula is correct, but the power you use is wrong. You must use the power that it will actually dissipate in your application (which is not your RF power).