Filter load transient from power rail
I considered that, although I only have access to the 12.3V coming in from the 220V AC transformer. I was considering a buck-boost converter, although that seemed overkill, and would also probably be expensive, add more noise on the 'unfiltered' rail, and might cause other unforeseen problems


15V AC-DC SMPS is commonly cheap nowadays, adding buck-boost would complicate even more. Depending on your current demands, for example take a look at Aliexpress here, the price can be as low as below 3 USD for 15V 2A supply. While it might be fun for you learning about the filter, I suggest that you shouldn't over-complicate yourself, I don't know if you have cost or time limits in your project.




  The 12.3V rail supplies a 3A 4V SMPS


Does the 4V (or 5V?) supply that you have can handle 15V input voltage? If it does, then you can use the 15V SMPS, put a filter between before the second stage supply and then parallel it with LM7812.

I don't know that in the end you'll be using SMPS or linear regulator for the +5V rail.

It's all is depends on your needs.







simulate this circuit &ndash; Schematic created using CircuitLab