new power board is assembled and so far no surprises was found. Only correction on the PCB was one capacitor in pre-regulator's tracking circuit. I used one THT and solder it over SMD component.

I borrowed one channel from existing PSU for digital control. It wasn't be an issue because new 26-pin output connector is pin compatible with old 10-pin SPI bus (see flat cable directly plugged in on picture below). On the same connector is also connected output capacitors combination that is in our case 22 uF elco in parallel with 470 nF ceramic with damping 1 R in series.

AC input comes from 40 V toroidal transformer. I did some testing trying to load it with up to 5 A. That required some cooling and I temporarily mount post-regulator's power mosfet on external heatsink and fan for blowing air over PCB for cooling pre-regulator's Q1, D5 and L1, L2 and L3.

For testing such load I had to made few changes. Referring to schematic from one of the previous post R5 is increased from 36 K to 47 K that increase switching frequency to over 300 kHz. That is required if 33 uH power inductor L1 is used. Higher output current require lower sense resistor therefore R2 is decreased from R015 (that is enough for up to 4.5 A) to R012 or could be R010.

Switching frequency synchronization between LM5574 and LTC3864 is now established. LM5574 is assigned as master and its frequency is set to about 330 kHz (Rt on pin 7 is decreased to 18K, and Cramp is now 3n9). in operation it looks like this (magenta trace is LTC3864, cyan is LM5574).

Two new functions: 100% duty cycle operation and remote programming remains untested since that requires additional code in firmware.