24V, 5 Amp Power Supply Circuit, comments required

[Ali Sajjad]

I have designed a 0-24V, 5Amp Power Supply.. I need your help to identify if i have left any error in the design.. or if the supply is not going to work.

The opamp provides stable 26 volts approx at its output (zener diode is of 3.3 Volts and the gain of opamp is 9). VR1 sets the voltage at Q2 base, and subsequently at Q4, Q5 emitter i.e. the ouput. Q6, Q1, D2 and R5 limit the output current: not to exceed 5 amps.

I will be glad to recieve your replies for modification in the circuits for better results.. Thanks to all.

- Ali Sajjad

 

[panic mode]

Why don't you put R5, Q6 and D2 right after the L1 (between C2 and R1)?
This way you would not get the fluctuations on the output when load changes. Since you don't have feedback (it's just open loop control),
your output will not be very stable anyway but 1V difference with or without load is too much for my taste (this is when you connect 5A load).
Since you want it for big current, I would rather make it switching power supply so your transistors will not sweat as much. In worst case this unit would create enough heat to cook stake...

 

[Exo]

I would put the fuse in front of the bridge rectifier. The way it is now the entire circuit is not protected, only the load

 

[Nigel Goodwin]

You would be far better off using feedback in the design, as you already have an opamp you wouldn't need to add one - simply use the one you already have.

A few extra points:

R4 seems very low at 100 ohms, at 24V output it will provide 250mA base current to the transistor - which seems rather high.

L1 - a 1H 5A inductor sounds expemsive, do you really need it?, it's not normally included in PSU's like this.

Output range!, at lower voltages, with high current, you are going to be dissipating a huge! amount of heat - the input is labelled 40V (presumably the AC rating of the transformer?). If we are conservative, and assume only 40V across the transistors - 40V*5A is 200W. If you really need to have this kind of range, try and arrange for lower incoming voltages from the transformer - you could do this by switching taps on the transformer, use a comparator fed from the output, and a relay to switch between windings.

 

[ante]

Don’t forget emitter resistors for the power transistors to even out the current!
Or else you are in trouble! :wink:

 

[Ali Sajjad]

Thankyou everybody who replied. That was the beginning and i m gonna improve it in the light of your comments. Perhaps i would now switch to a switch-mode supply becaz i dont want the power transistors to curse me for bearing that much heat ...... But soon i will be back with a new design for your comments. I hope you people will help and guide me further in this regard.......(and Nigel i like you man .. KEEEEEEEE YAAAAA) :lol: :shock: :evil: :twisted: :idea: :arrow: :wink:


- Ali Sajjad

 

[ante]

If you go switch mode, maybe you can try the MAX 724.

 

[Sebi]

Apply a pre-regulator...

 

[Ali Sajjad]

Thanks Sebi, would you also help me understand the circuit functioning and the specs for the transistor so thati can use suitable replacement for these if i could not found these trabsistirs easily in the market.

Thankyou very much.. i would condider this design as well as i wil try to design one myself..

- Ali Sajjad

 

[motion]

Your circuit as first presented will have difficulty regulating at low voltages because of high power dissipation at high load and transistor leakage currents at low load.

You can insert a switch-mode regulator (buck regulator) between C1 and L1 of your schematic. This will output from 5V up to 30V. This will serve as preregulator for the next stage linear-mode regulator. The SMPS preregulator will output a voltage slightly above the minimum for the linear regulator stage. That is 5V for 0V and 30V for 25V final output. This way, the total dissipation of the linear stage will be 5Vx5A=25W.