Hello once again. I still haven't managed to build this supply even though I designed my own PCB, which probably is still crap. Who cares.
The reason I post here today is because I finally want to finish the PSU and I am not sure what type of heat sink I should pick for Q2 (BD139). It only says "on a pretty big heat sink" in the parts list, which is a kind of vague assertion.
Some toughts on this topic (not necessarily correct or complete):
Q2 is used to drive the two power transistors Q4 and Q5. At the nominal 3A output current (IOUT=3A) the current gets shared between the two transistors and each transistor has to handle 1.5A of current.
I took a look at the OnSemi datasheet for the 2N3055 and the minimum DC current gain (B) seems to be 20 (worst case).
IB = IC / B
IB = 1.5A / 20 = 75 mA
Since we're driving two 2N3055s we have to supply two times the base current that I just calucated.
IBx = 2 x IB = 2 x 75mA = 150 mA
Since I sort of suck at electronics I have no idea how much voltage the transistor Q2 will see (worst case). However I need the voltage to calculate the total power dissipated by Q2, which then would allow me to calculate an appropriate heat sink.
Looking into the output transitor circuit more closely I think it kinda works like this:
US = 30V
Q2 Emitter Resitor Voltage:
UR16 = UR24 + UBE_Q5 = (IOUT/2 * R24) + UBE_Q5 = (1.5A * 0.33R) + 0.7V = 0.495V + 0.7V = 1.195V
Q4 and Q5 Collector Emitter Voltage:
UCE_Q4 = UCE_Q5 = US - UR24 = 30V - 0.495V = 29.505V
Q2 Collector Emitter Voltage:
UCE_Q2 = US - UR16 = 30V - 1.195V = 28.805V
Power dissipated in Q2:
P_Q2 = UCE_Q2 * IBx = 28.805V * 150mA = 4.32W
Thermal resistance of (heat) sink to ambient:
RthSA = to be calculated
Thermal resistance of case to (heat) sink (thermal grease layer):
RthCS = 0.3K/W (don't know where this number comes from, http://sound.westhost.com/heatsinks.htm#s7
suggest 0.25 for beryllium oxide which is kind of grease like (?))
Thermal resistance of transistor junction to case:
RthJC = 10K/W (from ST datasheet)
Maximum ambient temperature:
Ta = 55°C (selected, ambient temp in summer in north germany hardly ever gets over 35°C, 20°C safety margin included)
Maximum operating junction temperature of transistor:
Tj = 150°C (from ST Datasheet)
Tj = P_Q2 (RthJC + RthCS + RthSA) + Ta
(equation from http://irf.custhelp.com/app/answers/detail/a_id/91/~/heat-sink-selection-and-thermal-calculation
Solve for RthSA:
(Tj-Ta/P_Q2) - (RthJC + RthCS) = RthSA
RthSA = ([150°C - 55°C] / 4.32W) - (10K/W + 0.3K/W) = 11.69K/W
RthHS = RthSA * 0.80 = 11.69K/W * 0.80 = 9K/W (another 20% safety margin)
So the heat sink should have a thermal resistance of around 9K/W or lower.
Hope my calculations are ok, not entirely sure. I probably forgot to take the rectification factor into account and US is probably higher than 30V.
What type of heat sink did you guys use in your build of this PSU?
//Edit: Few typos corrected.