Help needed to solve SEPIC converter

[ronsimpson]

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..

You are showing 4Vp-p of ripple. Using spice; I think you should see 0.04Vp-p of ripple. (because of ESR) There are many strange things here.

 

[samson.bunty1986]

Exactly, in spice am getting very low ripples.. but in real am getting 4V...

 

[ronsimpson]

So the PCB is the problem or your capacitors are not what you say or the P-P current in the inductors are very high. Maybe the ground on the scope is in a bad place.

I wish I could see what you see. A picture is worth many words.
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[samson.bunty1986]

Hi,

here is my circuit and PCB layout files..Kindly go through it...

For 700ma load there is no voltage drop but ripples are there...but if i go with 850mA and above the voltage drop is there..

 

[samson.bunty1986]

Here ,in the circuit i have changed only the diode..instead of SB540 i used 1N5822 schottky...

 

[ronsimpson]

I see some very big problems with the layout. I will talk about that soon. Your ground is not good!

What type of 0.01 ohm resistor. Do you have a picture or more data. I think the resistor is inductive.

 

[samson.bunty1986]

Hi ronsimpson,

This is my resistor LVR01R0100FE70 from vishay...

 

[samson.bunty1986]

It is a wirewound resistor..

 

[ronsimpson]

First I circled the two grounds. The traces are too small but maybe OK. The two grounds are connected by a very small trace! OUCH. There is resistance and inductance in this small trace!

In the second picture I showed how I would make the ground stronger. This is the minimum I would use. I use more ground area than I showed.

Then you change sides on a PCB, the VIA has resistance and inductance. If the current is large I use many VIAs on the same trace. You are using a 0.01 ohm resistor and you have traces with large current that have more resistance than the resistor.

Look at examples of other power supplies. There are examples using your IC. Use examples form Linear Tech. Do not use examples from some strange person on the internet. I included a picture of a PCB from TI. Note the large grounds. Vin is large, Vout is large. At these high speeds through hole parts are not used much. Their legs are too long to work right. Keep high current traces short.

 

[ronsimpson]

From the LT3758 data sheet. PCB design.
Note large trace from Cin-gnd to Cout-gnd.
Note the small trace length for current resistor to ground.

 

[samson.bunty1986]

Dear Ronsimpson,

Thanks you so much for your reply..

I will change the PCB layout and get back to you..

Thanks alot once again..

 

[ronsimpson]

Let us review your new layout before you make new boards.

 

[samson.bunty1986]

Hi Ronsimpson,

Here is the Layout.

 

[samson.bunty1986]

Is it Ok??can i start placing components??