Hybrid electric vehicles powered by Li-ion batteries and supercapacitors have recently been considered a promising solution to enhance battery lifetime and driving range. Integrating Bi-directional DC/DC converters is vital to interface various electrical sources with the DC-bus level. Therefore, ensuring a stable DC-bus voltage under uncertainties and fluctuations of electrical power loads is essential for the reliable operation of the on-boarded electrical sources. This paper proposes a super-twisting sliding mode control technique to regulate the DC-bus voltage and control the electrical currents of battery and supercapacitor sources. An Energetic Macroscopic Representation (EMR) and an Inversion-Based Control (IBC) are adopted as a graphical formalism to represent the system under study and the proposed controller. Furthermore, a stability analysis based on the Lyapunov theorem is conducted to ensure the stability of the proposed controller under the Artemis Urban driving cycle.