Hi,
Everyone got there before me about the current bypassing transistor and stuff about TL431. Interesting how both TL431 and LM317 internal reference have approx 50mV droop with temperature [same thing, one for reference, other for power = TL431 is Vref x (1 + (R1/R2)) + (Iref x R1) and LM317 is Vref x (1 + (R2/R1)) + (Iref x R2)].
Had a play on an approximation to the circuit for the benefit of xymox and out of personal curiosity... Important aspect is that LM317 model used appears to have no temperature drift (!), other than that is seems to work normally/credibly. I was unable to find a model of the heavy-duty -250V, -15A, -1.2V NJW0302G PNP and got sick of spending time looking for one so used a tiny -3A device instead... I found a suitable inductor value by trial and error (it oscillates a lot with other values but looks acceptable with the 100 uH, so that'll do seeing as we're guessing. The source resistance of VG1 (impedance is not available, only source resistance can be defined in the sim tool) is 1 Ohm.
View attachment 176816
Interestingly, even for an approximation, temperature run at -50ºC, +50ºC, +150ºC shows 'considerable' Vout droop (14.1V down to 13.77V, from memory). I'd wondered if there was some theory about TL431 and LM317 and maybe even the BJT Veb leading to temperature-compensated Vout, seems not here, maybe the respective TL431 and LM317 internal reference droops do cancel out in reality to keep Vout constant across temperature range.
Dana's comment about TL431 stability vs capacitance is spot-on, the graphs in the datasheet clearly show 100nF is a risky choice...
Ahhh (...what
good and often
embarrassingly bad memories), this takes me back several years to the 7805 being the only voltage regulator I had used, and I even think Crutschow helped me understand how to implement the more sophisticated version:
View attachment 176819
As we all know, I'm not great at this stuff and have lagoons in my knowledge, but IMO, the circuit looks like 'a bit naïve, cheap and crummy, e.g. down to skipping 2 cents-worth of regulator protection diodes, etc., but who cares because the end user will never know', which is odd given the wasteful use of the TL431 which does seem to be intended to improve Vout regulation (the TL43 - which considered globally, is about as accurate as the LM317 internal reference anyway...). Frankly, I could have made this, which is patently no endorsement of the quality... LM317 datasheet has several schematics with a voltage reference on the adjust pin, by the way.
Xymox, I really do recommend suffering the learning curve of becoming well-versed in using LTSpice or any other simulation tool, you will actually save a lot of time in the long run, plus you can look at a wide range of signals especially under different conditions that it is impossible to reproduce at home, for example.
And here are the pertinent datasheets, for anyone interested: