Help needed to solve SEPIC converter

[ronsimpson]

The 6A4 diode is a general purpose diode for 60hz operations. It is not made to run at 300khz. What happened to the SB540 in your schematic?

 

[ronsimpson]

Hi ronsimpson,

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

No. I now think the inductors are OK.

Are you using a slow "general purpose diode"? You need a Schottky diode! Do you have any Schottky diodes?

Go back to post #36 and do trouble shooting.

 

[samson.bunty1986]

ronsimpson,

I din't get that diode in market so i placed this diode...

In SB540 datasheet where they have mentioned it is used at 300Khz???Sorry am not familiar with these are all..Is it compulsory to change with that diode??

 

[samson.bunty1986]

Ronsimpson,

as you suggested i will do all that stuff...give some time..

 

[ronsimpson]

Schottky diodes are very fast. I went to 4 different companies that make the SB540 and they do not state the speed. The diode will respond in less than 50nS and probably less than 20nS.

The 6A4 diode is 2uS to 3uS in speed.

A Schottky diode is 100x faster.

 

[samson.bunty1986]

Hi Ronsimpson,

One good News. I changed my diode to 1N5822(3A) Schottky Diode. Now with load my output is dropping from 22V-20V.

If my input is varying, my output also varying little.. I dnt know why it is happening..

Here am attaching the output waveforms.

 

[samson.bunty1986]

How to attach these files?? am sending all to rinov..

Rinov please attach all my files here..

 

[samson.bunty1986]

I have checked the feedback voltage..

At No-Load it is 1.55V...

At Load condiation it is dropping to 1.430V..

Output current is 500mA now..My load is 44Ohm 20W resistor. It is good now..Only the voltage drop & ouput voltage varying we need to rectify..

 

[ronv]

Attachments

Here are the files.

I'm thinking the problem is now with the input supply.

Samson, If you go into spice and change the spice analysis menu to start external supplies at 0 volts soft start will work and the 8 amp problem goes away.

 

[samson.bunty1986]

Hi ronv,

Thanks for the posts..

What does that 8 Amp problem..Sorry i dint understand what you are saying..is it inductor current you are talking??

 

[samson.bunty1986]

I have observed in LT spice that at soft start pin there is around 23V..but on my PCB am getting 2.4V..Can anyone knows about it??

 

[ronsimpson]

I have observed in LT spice that at soft start pin there is around 23V..but on my PCB am getting 2.4V..Can anyone knows about it??

Spice models are not very good. Little details are not in the models. I think the real parts equal models when SS is 0 to 2.0 volts. The real parts have a clamp at 2.4V so the voltage is limited to 2.4V. This clamp is not in the model. This is not a big problem.

 

[ronsimpson]

Here are the files.

I'm thinking the problem is now with the input supply.

Samson, If you go into spice and change the spice analysis menu to start external supplies at 0 volts soft start will work and the 8 amp problem goes away.

Samson please increase the capacitance on the SS pin. This will make the power start up slower.

 

[ronsimpson]

Samson please re-post you schematic. You have made some changes. (RC on current limit pin)

Please post the wave form across the 0.01 ohm resistor.

 

[samson.bunty1986]

Dear Ronsimpson,

How much capacitance need to put??2.2uf is ok???How to make my ouput pure DC???you have seen my output right..it is having nearly 4V peak - peak..Why my ouput is not pure DC without pulsating..

samson

 

[ronsimpson]

A 2.2uF capacitor is very small for the input. What is the input ripple voltage?

The output ripple is a function of:
1. Capacitance.
2. ESR of the capacitors.
3. ESL of the capacitors.
4. Inductor.
5. Printed circuit board layout.

4 volts p-p is very high. What current are the output capacitors rated for? What is their ESR?

Now you have a LC filter between the Diode and load. Adding a second LC filter will reduce the ripple.

 

[ronv]

I have observed in LT spice that at soft start pin there is around 23V..but on my PCB am getting 2.4V..Can anyone knows about it??

That is caused because you haven't changed the Spice analysis command to "start external supplies at 0 volts".

I have one more theory then I think you must take the measurements Ron S. has asked for.

Here is the theory: The .01 ohm sense resistor is very small. The traces on your PCB could easily be .05 ohms. So check this:

The resistor going to the sense pin should be connected at the top of the .01 ohm resistor. When I say at the top I mean right to it not an inch or cm away.
The other end of the .01 ohm resistor should have a big short trace (or wire) directly to the ground of the IC.

 

[ronsimpson]

I have one more theory then I think you must take the measurements Ron S. has asked for.

Here is the theory: The .01 ohm sense resistor is very small. The traces on your PCB could easily be .05 ohms. So check this:

The resistor going to the sense pin should be connected at the top of the .01 ohm resistor. When I say at the top I mean right to it not an inch or cm away.
The other end of the .01 ohm resistor should have a big short trace (or wire) directly to the ground of the IC.

We need to see the PCB layout!

 

[samson.bunty1986]

Hi Ronsimpson,

My capacitance value is 102uf(Two 56uF in parallel)
Each capacitor ESR is 29mOhm.
Don't know about ESL..The part number of the capacitor is PLV1V560MCL1TD..Kindly go through it..

I din't kept any LC filter between Diode and Load..About which one your are talking??What are the values of inductor and capacitor should use for LC filter??

 

[samson.bunty1986]

Hi Ronv,

Thanks for your reply

I will send the PCB layout just check it out..