# Electronic Circuits and Projects Forum

## Help needed to solve SEPIC converter

1. Originally Posted by samson.bunty1986
I used R5 exact 0.01Ohm , 1W resistor.
I checked with resistive load. Two resistors 22 Ohm 10W and 5W in series. It is taking 270 - 300mA. That time my output getting drop to 13-14V. If i connect bulb it is dropping to 8V. The mosfet is getting too hot once i keep load. C3 & C4 are alluminium capacitors.
Using spice I changed the 0.01 ohm resistor to 0.1 ohm and I get the 300mA in and 9 volts out.

2. Hi ronv,

Thanks for your reply.

The C2,C8,C3 and C4 i have calculated based on rms and esr rating. i can send my simulation circuit to your mail. You post it here. Because i dont know how to attach files. Kindly go through my circuit. simulate it. Send your mail id.

3. I dint understand properly what you are saying...

"Then think about why it should take 8 amps of 24 volts to output 2 amps at 24 volts" What does it mean??

My inductor current at starting reaching to nearly 8A . And then it is stabilising at 2.2A if i connect 2A Load. So you are saying to change the inductor with sat current having 8A??

4. Dear Ronsimpson,

Am sure i hve connected 0.01 Ohm only.

As per your suggestion can i change my inductor having Isat 8A?? I have gone through the Inductor waveform. At starting my inductor current reaching to nearly 8A. So,should i need to change inductor??? But as per calculation why my inductor Peak i got 4.5A and rms current 3.7A??

5. Dear Ronsimpson,

At full load my inductor peak current is 7.5A. So i need to use inductor having Isat 10A ?? Correct me if am wrong..

One small query, Will the voltage is dropping because of lacking proper inductor Isat(Peak) value??

Now,What is the Irms have to use?? will it be same as i calculated?? Noe need to change the Irms value??

Thanks in Advance.

6. We need to do some trouble shooting.

The duty cycle is 30%. Why? It should be about 50%.
Duty cycle is set by many things.
1) What is the voltage on FBX pin? Is it at the reference voltage? It should be at the reference voltage.
2) Do you have more 0.01 ohm resistors?
Parallel two resistors. Does the duty cycle change?
or
Series two resistors. Does the duty cycle change?
If you are at current limit then the duty cycle will change. If current limit is not the problem then Parallel two resistors will not make a change.
3) Soft Start problem.
What is the voltage on SS pin. If this is too low the duty cycle will be low. Read about SS pin and soft start.

7. Along the same lines you might try adding a 100 ohm resistor between the current sense resistor and the sense input. Then add a 560 pf cap from sense to ground. This will keep the reverse recovery current from the diode from tripping the current limit and is seen in many designs. What diode are you using?

8. He is using a SB540 which should not have much reverse recovery time. You are right about the RC. I continue to think the problem is on the current sense pin. At first I thought core saturation but now I think that is OK. If I could just get a current and a voltage probe on the supply then the problem should be easy to find.

9. Hi ronv,

Thanks for your reply..

I already tried with adding 22 Ohm(between sense resistor and sense pin) and 10nf(between sense pi and ground). Now my circuit is added with these two components. Am using 6A4 MIC Diode(Rectifier) .

10. Hi ronsimpson,

As you suggested me before Should i change the inductors???

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