In order to solve the problem of building two-stage DC transformer impedance model

we propose a unified impedance modeling method for cascaded DC transformers based on input-series-output-parallel connection. Firstly

the topology of the Boost + LLC cascaded DC transformer is introduced. This topology has the following advantages: it not only makes up for the non-isolation characteristics of the Boost converter

but also compensates for the narrow-range voltage regulation characteristics of the LLC resonant converter. Secondly

combined with the constant gain voltage clamping function of LLC resonant converter under the operating mode of approximate resonance point

the two-stage coordinated control strategy of Boost + LLC cascaded DC transformers is merged into the single-stage unified control strategy; based on this

combined with the reduced-order small-signal model of the LLC resonant converter under the fundamental wave approximation method

the system model of the Boost + LLC two-stage DC transformer is established

and the low-voltage side impedance model is derived. Finally

a simulation model of Boost + LLC is established in MATLAB/Simulink. Simulation results have verified the correctness of the modeling method

providing a theoretical basis for the stability analysis of the DC network.