Flyback controller chip

d52perry

New member
I am a nub is smps and just completed my first breadboard isolated smps with 24 volt input. I used the SG6848D controller from Ebay. Anyone has any suggestions for a better IC.
Just some details

Input voltage 24 volts Output 5.
Used an opto coupler for feedback.
The transformer seems to work okay. I designed it for 3 amps. The problem is when I exceed 2 amps the signal gets very jittery and it stops functioning. I was able to go to 3 amps with no feedback using a pulse generator. Even higher but no regulation.

Any helpful suggestion will be appreciated.
 

Attachments

  • IMG_0682.jpg
    IMG_0682.jpg
    1 MB · Views: 25

Silvio

Member
Can you post a schematic of the flyback converter?
Tell us also the frequency of operation and details of the trafo and windings.
 

d52perry

New member
IMG_0686.jpg
I tried to set it up for 100KHZ with the right resistor. The scope showed a much lower frequency. Then I changed it to the recommended resistor of 100K. This was suppose to yield 65 Khz. I got more like 20.
Primary was 2 #26awg 14 turns Secondary 6 #26 4 turns Aux 1#26 5 turns. I checked my formulas and compared them with excellent IT results. I believe there were some errors. First I used a max duty of 43%. Excellentit goes to 67% or so. I used ee25 pc40 from ebay.
 

Silvio

Member
I took a look at the data sheet and yes in fact you should be having around 70khz. You should try to measure frequency during load as due to the green mode operation at low power consumption the frequency will go down.

Regarding the power rating that at 2 amps you are having problems, From what I have seen from your setap you have a lot of stray capacitance and long wires going along the breadboard, be aware as EMI starts to go around your circuit it might be the case that is disturbing the chip feedback and having the odd shape of the waveform,

I suggest you try a perforated board circuit to minimize stray inductances and capacitances and see how it works. Lastly Ebay chips are all generics and sometimes they do not work well. Ask me when I first built the first smps with IR2153 I burnt a hand full of mosfets and chips. This was all due to faulty chips and mosfets bought from e-bay.
 
Top