I was designing a 0-35v/6A power supply with keypad entry with ATMEGA32.The main idea is to output calculated output voltage by a 12 bit DAC and feed it to an op-amp configured as non inverting amplifier so the op-amp can take care of the control loop. The targeted design was working in real life with an op-amp powered at 40v as I was using NPN darlington(BD139+TIP35) as the output driver transistors.But To power the op-amp,a separate 40v regulator was needed.
Now i modified the design as in the schematic attached. I am using LT1014(for my previous design as it can handle as much as 44v).I have some queries listed below regarding the new output stage:-
1)The gain of Sziklai pair(for this circuit) is βQ4βQ1+ βQ4βQ2+βQ4βQ3. So depending on that,the current needed to drive the base of Q4 in order to get a max of 6A is 2.6ma as (6000/(30x25)x3).[typical gain of TIP35 is 25 and TIP42 is 30].Is my calculation right??
2)What is the purpose of R4?Is it simply to Pull up the base of Q4??How to calculate the value of it??
3)Is R7 used for current limiting?How it's value can be calculated??
I am attaching my circuit.Please feel free to suggest if anything is required to make the circuit more stable/functional.
Thanks in advance......