Original question:
Hi Team,
UCC28951-Q1, how to set the protection point for the output current? When the current transformer of current sense CS pin is pulled down to 0V, the output power still can't be increased, which is limited.
That is to say, I want to know how the overcurrent protection is triggered and how to remove it.
Beat Regards,
Tom Liu
Hello,
The CS is sensed on the primary while energy is being delivered to the secondary. This current will be the reflected output current ((Iout) + Delta*ILout )*(Ns/Np))
When the PSFB is freewheeling you are correct that the CS signal will be zero. Lout and Cout will be providing energy during the free wheeling period to the output during this free wheeling period. The energy should be decreasing based on the output power.
The UCC28951-Q1 has cycle by cycle current limiting and hiccup mode to protect against overload conditions. Please note that is triggered by the CS pin reaching VCS_LIM = 2 V every switching cycle. Section 7.3.14 of the data sheet goes into detail describes in detail how the UCC28951-Q1 cycle by cycle current limiting and hiccup mode works. 
To protect your design I would recommended that you select your components to handle a 125 to 150% of the nominal output power and set the 2V CS limit to less than that. For example if your components could handle a load that was equal to 135% you could set the CS limit to cycle by cycle current limit at 110% pf the output power.
Regards,
