UCC28730: Why Vcl is related to load?

Hello Naizeng,

Thank you for your interest in the UCC28730 PSR flyback controller. 

As you say, Vcl is the output of the error amplifier (EA) which produces an error voltage proportional to the difference between the output voltage and the internal reference voltage.  The output is sampled at a specific time in the switching cycle from the Auxiliary winding voltage and scaled with a resistor divider at the VS input.

Since the EA output voltage represents a difference between the reference and the scaled output, a higher error voltage represents a lower output voltage. The output voltage sags as the output load increases.  The load can change from 0% to 100% while the output stays regulated within 10's or 100's of mV, so we can assume Vout is basically constant while the load current varies.  So load current and load power are proportional.  But the current variation also does affect Vout roughly proportionally even though it may change only by 10's of mv, so this reduction in Vout with increased load is detected at VS and generates a change in Vcl.

The feedback loop settles at an equilibrium level for a constant load and line condition, and Vcl can range from 4.85V at max-load, min-line down to 1.3V at min-load, max-line.  However, if a lower maximum switching frequency is chosen during the design phase, (such as 60kHz, for example) then the maximum Vcl will be lower than 4.85V, because the constant current regulation would take over if the load was attempted to be increased above the Iocc setting and the Vcl attempted to move higher than its value at 60kHz.  Unfortunately, Vcl is internal and cannot be observed externally.  But it does behave the way the control law curve describes it.  

Regards,

Ulrich