The SMPS pre-regulator is assembled and tested. I had some trouble with stability when tracking is added but thanks to LTC's support and Dave's video (EEVblog) #329 I got nice results.It looks like this:

This design will be used on the new power board presented in one of previous log message. The corrected schematic is shown on the picture below.

For testing an existing power supply is used. I just remove old pre-regulator and plug a new one:

Measurement is performed on two points: switching mosfet output and post-regulator output. Standard 10x Rigol probe is connected via 1K resistor to the first point. For second one a hand-made 1x AC probe is used (see picture below).No load

Output shorted, Iout=3.5 A

Iout=1 A, Load=8R2
Iout=2 A, Load=8R2
Iout=3.5 A, Load=8R2

Output voltage ripple and noise figure is pretty good: ~1.5 mVrms and ~10 mVpp. I didn't tested yet this configuration with 100% duty cycle switch. Also sync with another channel and on-board LM5574 has to be done.