Reference [8] derives LLC plant transfer function $$\left(\frac{V_{out}(s)}{FBreplica(s)}\right)$$ when it is operated under Hybrid Hysteretic Control. Simplis tool can also be used to extract these transfer functions. This method is used here.

UCC25640x EVM Power stage [3] is considered for extracting the gain plots when it is operated under different input voltage and load conditions. In Figure 1-22, we can observe that plant gain plot is close to a single pole response in the low frequency region. A Type 2 compensator (zero of the compensator needs to be located below low frequency pole of the plant) can be sufficient for secondary output voltage or current regulation. Here low frequency pole is approximately located at $$f_p=\frac{1}{\pi R_L\cdot C_{out}}$$ where R_L is load resistance and C_out is output capacitance. If the cross over frequency needs to be improved, then Type 3 compensator is recommended as at higher frequency regions plant transfer function has double poles which can degrade the phase [8].