UCC28951-Q1: Simulation PSIM of a PSFB converter

Hi,

We are currently working on the design of a PSFB converter with :VIN = 400 VDC  / VOUT = 850 VDC, limited to 1A on the output current. We use this converter to charge a capacitive load of several Farad with supercapacitor technology. Afterwards, we will put as many as necessary in parallel to respect the required charging time.

We decided to work with the UCC28951 to avoid the development of a SOFT. To simulate the behavior of this component, we started from an example developed by PowerSim (powersimtech.com/.../) that we modified for our needs. We assumed that PowerSim based itself on the UCC28951 from Texas Instrument to develop this PSIM model (same parameter notation as the Texas datasheet, etc.).

We are currently experiencing problems with our simulation results and would like to ask several questions.

Question n°1 :

 Is it normal that the output current decreases during the load while the setpoint is fixed?

Screenshot of our simulation:

Question n°2 :

The cycle-by-cycle current limit VCS_lim is presented as the threshold of the pin CS, with a value of [1.94 ;2.06]V. Is it the same as the limit value Vp in the calculation of the current sensor resistance value Rcs (p46 of the UCC28951-Q1 datasheet) ?

Thank you for your help.