In order to properly simulate/model the current mode control SMPS, it is necessary to know some details of the current sense section of the device. According to Sheenan (SNVA555 & SNVA542), Sanjaya Maniktala et al, current sense gain, ramp peak-to-peak and current compensation ramp are necessary.
From what I can derive from the UCC2897A datasheet, it appears the following is true:
- The current control ramp is 2Vpp. Source: Step 6. Current Sense and Slope Compensation
- Compensation factor is directly controlled via Rslope pin with Rslope slope multiplied by 5. Source: Source: Step 6. Current Sense and Slope Compensation
- gain from the dFB/dCS interaction is 5. Gain definition according to UC3842 datasheet SLUS224C
There is somewhat problematic level shifter in the circuit. Basically I need to add that +0.5V term to the transfer function for the sensed current. This appears to be done in order to allow using smaller current sense resistors than with UC3842.
The reason why I feel it's necessary to model the circuit at this level is that I'll be running with 24Vin, 1500Vout. That means my voltage measurement circuit will have extremely high input impedance and the voltage division is far bigger than normal. If you have 10Mohm input impedance, with 10pF pin capacitance that works out to 1.6kHz pole. Oops. It is also necessary to maintain tight regulation of the output as the voltage is biasing a sensitive sensor crystal.