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I have designed a SEPIC and Buck converter using the TI power designer tool, which I have individually tested, and proved to be working correctly. They are both of the non-isolated type, but when I use them after one another on the same ground the voltage drops fast very low to around 1V and steadily decrements to around 0.5V.

The specs are:

SEPIC: 12-22v IN, 13.8V OUT, 2A, 400kHz based on LM3488 (which I found is 14.5V OUT in reality so I have used a LM7812CV to drop the voltage)

Buck: 12V-14.5V IN, 5V Out, 2A, 698kHz based on a TPS563231DRLR

I have used my multi-meter to test the output, I have used a small LED as a starting point for my load.

From my basic understanding there should not be a problem using a SEPIC and Buck this way, when using the correct filtering for the high frequency switching noise coming out of the first. Which I have created using a Ferrite Bead. What could be the problem for the invalid voltage output? Perhaps I am missing something elementary as I am not lectured in EE.

SEPIC

Buck

Overview

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    \$\begingroup\$ can you post a schematic of what you have built? \$\endgroup\$ – Aaron Aug 19 '20 at 15:26
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    \$\begingroup\$ You have not mentioned current and loads. \$\endgroup\$ – Andy aka Aug 19 '20 at 15:28
  • \$\begingroup\$ Show us schematics. The LM3488 is an adjustable regulator, so you ought to be able to set it for 12V output without too much trouble. \$\endgroup\$ – TimWescott Aug 19 '20 at 15:55
  • \$\begingroup\$ Chances are high that you're drawing too much current with the second regulator. Either just plain too much current, or too much inrush current. Either way -- schematics. The devil is in the details here. \$\endgroup\$ – TimWescott Aug 19 '20 at 15:57
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    \$\begingroup\$ How did you do this: I have used my multi-meter as load and to test the output? \$\endgroup\$ – Andy aka Aug 19 '20 at 17:32

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