When you want to build this power supply, you may encounter some difficulties while buying the components. Or maybe you want to build one that has different characteristics.
For transformer L1, I use a 30V transformer with an extra 20V winding. If you can't find a transformer like this, you can also use 2 transformers.
The maximum voltage across C5 will be 30V∙√2 - 1.4 = 41V. So I choose for G2 a B80C5000/3300. (80 = maximum voltage; 5000 = maximum peak current [mA]; 3300 = maximum continuous current [mA]). C5 should be able to sustain 50V.
The maximum power dissipation in R4 is 412/3k3 = 0.51W. Usually, you take the next value available, in this case 1W. However, in this case the voltage across R4 will not continuously be 41V, but just for short periods of time. So I took a 0.5W resistor.
T6 is a high power transistor. Make sure you cool it using a proper heatsink. I also attached T6 to the metal case of my power supply. Its minimum current gain is 20, so the maximum current in T3 is 2A/20 = 0.10A. The maximum power dissipation in T3 is 41V∙0.10A=4.1W. According to the datasheet, the maximum power dissipation without heatsink is just 2W, so this transistor needs a little heatsink. N.B. Do NOT use a TIP41, since its VCE,max = 40V. Use a TIP41A, B or C.
T1 can be any PNP transistor. T2 can be any NPN transistor. T4 and T5 can be any transistor where VCE,max >= 45V. So American users should take care when applying a 2N3904 here; the maximum VCE of this transistor is 40V. If anyone knows a good American replacement for it, please let me know. (I'm not sure if the BC-series are available in America.)
The maximum power dissipation in R14 is 32∙0.5 = 4.5W. The next available value is 5W.
For opamps U1 and U2 I used a CA3140. Do not use a cheap 741 or so; these are not suitable for this job.
If you want more than 30V output voltage, you need to change more than just the transformer. You also need higher voltage versions of C2, C5, T4 and T5. You also need to change the voltage deviders R19/R16 and (maybe) R9/R10. For example, if you want to build a 40V power supply, C2 = 10uF/50V and C5 = 10000uF/63V. T4 and T5 should be replaced with a BC546. Without any change, the maximum voltage on the non-inverting pin of U1 is 40V/2.8 = 14.2V. Although this is less than 15V, you'd better replace R10 with a 2k2 resistor. When VFB+ = 40V, Vp,U1 should (still) be 15V. This means that R19/R16 should devide the voltage by 40/15 = 2.67: R19 = 56k and R16 = 33k.
If you need less than 30V, you only need a lower voltage transformer and different values for R19 and R16. For example, if VOUT,max = 20V, R19/16 should devide the feeback voltage by 20/15 = 1.33: R19 = 18k and R16 = 56k.