**3. Simulation and Validation of the Model of Bidirectional SiC-Based Battery Chargers**

A bipolar PWM was implemented with a switching frequency *fs* = 100 kHz. The switching frequency was selected as the best compromise between efficiency and high power density due to the reduction of the passives composing the AC grid filter and the DC bus link. The modulating signal was evaluated by the voltage grid angle implementing a grid synchronization algorithm setting a unity Power Factor (PF) in G2V or a stable grid synchronization in V2G. The gate signals used to control the SiC MOSFETs were set by the current control loop.

The technical specification of the filter parameters, DC bus link and grid operating conditions considered in the following analysis are listed in Table 1. The design specifications of the DAB of the proposed BBC are listed in Table 2.


**Table 1.** Technical specification of the AFE parameters.



For this bidirectional converter, the EE core geometry was chosen with N87 material grade. This choice was related to the high switching frequency (fs = 100 kHz) and high-power density of the transformer, whose characteristics are listed in Table 3. An increment in the switching frequency enabled a reduction of passive component size and weight but at the cost of greater switching power losses that, in turn, involve reduced efficiency. Therefore, the adopted frequency was the best compromise for such an application.

**Table 3.** Technical specification of the transformer parameters.


A prototype of the converter was designed and realized using components made by STMicroelectronics to validate the proposed tool by testing the performance and efficiency of the BBC designed using the proposed modelling approach. The power devices are SiC MOSFETs SCT50N120 (Table 4).

**Table 4.** Power device description: SiC MOSFET SCT50N120.


A mixed-signal MCUs STM32G474 was used to generate the phase-shift control signal and to manage the dead-time in each power converter leg exploiting the High-Resolution Timer (HRTIM) with 184 ps resolution. The digital control signals were conditioned and applied to the power switches using high-performance gate drivers STGAP2S, a galvanically isolated 4 A single gate drivers. This made it possible to achieve more compact and robust solutions for the entire experimental system.

The modular prototype and the test-bench are shown in Figures 11 and 12.

**Figure 11.** A prototype of the bidirectional battery charger.

**Figure 12.** Prototype test-bench.

The power required from the AFE acts on the phase-shift; by varying this reference, it is possible to reverse the power flow. Some simulated and measured waveforms obtained during the G2V mode are shown below. In Figure 13, the simulated first stage waveforms that are the grid voltage *vac* and current *iac* with unitary PF, and the ripple of the DC voltage are shown. The total harmonic distortion for the AC current was close to 7%, which is in accordance with the value measured (less than 10%) using the prototype.

**Figure 13.** Voltage and current of the AC grid and ripple on the DC side.

The voltage and current on the primary side of the transformer are shown in Figure 14. The simulated waveforms were in good agreement with the measured ones. The main difference was the lack of oscillations in the simulated voltage. These oscillations were due to the coupling between the parasitic capacitance of the devices and the parasitic inductances in the power loop that were neglected in the model. The current waveform depends on the phase-shift between the two transformer-ends voltages. The secondary side quantities were pretty similar, as a turn ratio *n* equal to one was chosen. The leakage inductance, *Llk*, affected the power delivered in the DAB converter. Therefore, the voltage *vL* waveform was strictly related to the power direction. The DC output waveforms are shown in Figure 15, where the ripple of the voltage *Vo* and current *Io* are highlighted.

**Figure 14.** *Cont*.

**Figure 14.** Voltage on both sides of the transformer, voltage and current of the inductance. (**a**) Simulated waveforms; (**b**) measured waveforms.

**Figure 15.** Ripple on the DC output Voltage and Current.

In V2G mode, the power flows from the battery to the grid to satisfy the power demand. In this case, the reference power is modified and acts on the phase-shift value as described above. The transition from G2V to V2G mode at the instant t\* requires current inversion, as illustrated in Figure 16. In this case, the PF has been maintained, meaning that no reactive power was requested by the converter thanks to the proper control. The main simulation results are summarized in Table 5, while in Figure 17, the efficiency of the whole converter is shown.

**Table 5.** V2G operation—Simulation quantities and results.


**Figure 16.** AC voltage and current from G2V to V2G mode.

**Figure 17.** Efficiency vs. (**a**) power output and vs. (**b**) phase shift.

Some other comparisons are reported in Figure 18, confirming the consistency of the proposed modelling approach.

**Figure 18.** DAB: voltage and current waveforms. (**a**) Simulated (**b**) Measured.

#### **4. Conclusions**

This paper dealt with SiC MOSFET-based BBC with galvanic isolation. A promising topology was studied as the best choice in terms of efficiency, bidirectional power flow management and complexity. The development of an accurate tool accounting for the model of the converter in computer simulator and which was able to exploit FPGA was proposed. It has been shown that this is a suitable approach to design and test the performance of the complex control algorithm, both in G2V with PFC capability and V2G operation modes. The control strategy of the AC/DC converter is composed of a cascade control. One is able to regulate the power flow with the grid, while control of the DC/DC stage consists of the management of the battery charge/discharge. The design and the proposed approach were validated by comparing the simulation results with some experimental tests, confirming the consistency of the proposed method.

**Author Contributions:** Writing-Review & Editing: G.A., M.C., F.G., S.A.R., G.S. (Giuseppe Scarcella), G.S. (Giacomo Scelba). All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Nomenclature**


