Electronic – Is 150mW a lot to dissipate from a TO-92 in 50°C free air without heat sink

power-dissipationtransistors

I'm designing a circuit in which a 2N3096 PNP BJT in a TO-92 package will drop 15V with \$I_C\$ of 10mA worst case, for 150mW heat dissipation. Unfortunately, the worst case is when the circuit is idle, which could happen for long stretches if left on continuously, overnight, etc. The internal case air temperature can get to 50°C and there is no fan or case vents. Cooling is through conduction via the aluminum enclosure. The equipment is designed for use in living areas, like a lab bench, so it's unlikely to see external air temperatures above say 35°C

The thermal resistance, Junction-to-Ambient, \$R_\text{$\theta$JA}\$, stated on the datasheet is 200°C/W. I'm taking that to mean the device could get to \$50 + (200*0.15) = 50 + 30 = 80°C = 176°F\$, which is definitely hotter than I want to touch with my finger.

This strikes me as a bit hot to be running a TO-92 package on a fairly extended basis.

Am I being over-conservative or should I really think about making a change here, either to the current or the package?

Best Answer

According to the datasheet it can dissipate up to 0.5W at 50˚C ambient temperature, which would make the maximum temperature about 150˚C. This leaves plenty of room for derating in long-term use, so you shouldn't have problems. Additionally, you could keep the leads short and use the PCB for a bit of power dissipation.