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.