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I took a WEBENCH design for a 10-14 Vin, 12 V / 8 A out buck/boost converter controlled by the LM5175. The output is extremely unstable without adding significantly more capacitance. The design specifies 9 x 22 uF (198 uF) capacitance. I can somewhat stabilize the output by adding another 1000 uF
Here is a snapshot of the schematic from the tool:
Here is my re-drawn schematic used for my PCB design:
Here are scope shots showing different loads & capacitance. Yellow = input to PCB. Blue = Output from PSU:
I'm curious why essentially 6x'ing the output capacitance solves the problem...
Thanks in advance.
Hi Garrett,
Pleasure to meet you, and thanks for the advice.
I played around the quick start calculator you linked to, a nice tool for sure. I have a couple questions:
Given my I/O parameters, the minimum output capacitance is given as 33.3 uF. The design from the WEBENCH tool that I use gives 198 uF; roughly 6x which should already be over-provisioned.
However above you mention that the WEBENCH tool may be under-provisioned. That agrees with my hands-on debugging, where adding another 1000uF stabilized the output.Now admittedly that may have been overkill, I can try smaller values and see what gives stability.
I am curious what you saw that made you feel the 200 uF to be under-provisioned.
I keep trying to plug in my compensation component values but it keeps crashing Excel, haha. Maybe because I am using Excel for Mac?