Other Parts Discussed in Thread: LM5088
I created two versions of a large PCB, both with a[26V down to 15V]-12V DC-DC up to >5A output current based on the same LM25088 schematic but different layouts.
Both times I had issues with reliability and getting the DC-DC to work at all.
Both times I had issues with getting a reliable output voltage:
- between ~7V to ~20V with 100Hz frequency with 13.4V RMS on V1 (Version 1) but >5A maximum output (not surprising that I am not able to filter that out with even a large C and L...)
- constant 9V output only up to a <1.5A with intended osciallting output with 400kHz on V2, but of course wrong voltage (9V instead of 12V)
or - also ~9V to ~17V with 100Hz frequency with 13.4V RMS on V2 similar to V1 behaviour but >5A maximum output
V2 of the PCB was made with a professionell Pick&Place-Machine, proper Solder Paste and industrial solder oven. Therefor I am assuming
there has to be some error in Schematic and/or Layout for both V1/V2, but I can't figure out what/were.
The design was taken out of the TI Webench Designer and used on another PCB, where it worked without any issues (a few years ago, constant exact 12V for up to 8A).
I hope to have only copied the schematic and made it a little more readable, but of course I have to have went in the wrong direction at some point.
On the V2 layout instead of the 10mOhm resistor on the bottom as in the layout picture,
the part was placed between the backside of the LM25088 and the Diode Kathode to make sure the trace resistances are low enough.
Pin 10 was pulled to ground with a 0 Ohm resistor to make sure there is no difference between different LM25088 versions.
The parts used:
- All capacitors should be rated for over the maximum voltage (35V to be sure)
- Diode: MBRD20100CT-13 (DPAK package)
- Transistor: BUK7S1R0-40H (PLFPAK88 Package)
- Inductor + Capacitor Output Filter: SRP1270-6R8M + EEEFT1V0101AP (100uF 35V)
If any oscilloscope measurements might be helpfull, I'll add them as requested.
I would appreciate any idea/any tipps where I can start "debugging" the hardware to test some part of the functionality and get towards solving the issue.
I would be glad if someone sees a Schematic/layout fault/issue, because then I will be able to fix it fast.
I have spent quite some time comparing working and non-working versions of the DC-DC on different PCBs, but could not draw any logic conclusions. Therefor starting over with fault-finding might be the best idea.
A little more info to:- constant 9V output only up to a <1.5A with intended osciallting output with 400kHz on V2, but of course wrong voltage (9V instead of 12V)
With a load that wants to draw up to 5A of current the start-up seems to work up to 1.5A, but then oscialltes between higher and lower voltage with a frequency of a few Hz.
Without any load, the voltage drops to oscillating 0V to 4V with a few Hz frequency. If anyone has had similiar issues, I would appreciate a short info about what/why.
Top View of Layout (All "Power" parts, also added 10mOhm on all gnd vias to the Diode Input):
Bottom View of Layout:
Schematic: