L11

01/09/26 Tue 8:18am

Heat Sinks

A transistor with $V_c=20V$ and $I_c=1A$ has a $1\text{celsius}$ per watt junction to case thermal resistance. If the value of $Q_{cs}=0.4\text{celsius}$ calculate the thermal resistance for the heatsink that will keep the maximum junction temperature at $90\text{celsius}$ when the ambient temperature is $25^{\circ }C$.

soln:
$P_{Dmax}=20\times 1=20$

$$Q_{SA}+Q_{JC}+Q_{CS}=\frac{T_J-T_A}{P_{Dmax}}$$ $$\boxed{Q_{SA}+1+0.4=\frac{90-25}{20}⇝Q_{SA}=1.85}$$

A Power transistor has a thermal resistance of $200\text{celsius}perwatt$ calculate the maximum permissible power dissipation when the $T_{Jmax}=90\text{celsius}$ and $T_A=25\text{celsius}$

$$\boxed{\frac{90-25}{200}=0.325}$$