The rapid development of electric vehicles (EVs) has necessitated advanced charging infrastructure that supports bidirectional power flow, enabling vehicle-to-grid (V2G) capabilities. This paper presents a single-stage resonant three-port bidirectional converter for EV charging stations, which integrates V2G, grid-to-vehicle (G2V), and high-to-low voltage (H2L) power transfer modes. The proposed topology enhances power density and efficiency while maintaining high grid-side power factor correction (PFC). An optimized modulation strategy is developed to minimize current stress and achieve wide-range soft switching, improving the overall performance of EV charging stations. The equivalent model of the converter is established, and mathematical expressions for output power are derived. Simulation results validate the effectiveness of the proposed strategy under various operating conditions.
The topology of the proposed isolated three-port bidirectional AC/DC converter is illustrated in the figure. It consists of a full-bridge circuit formed by switches S1 to S8, a bidirectional half-bridge circuit with switches S9 to S12 and capacitors Cf1 and Cf2, a three-winding transformer T, power inductors L1, L2, and L3, leakage inductance Lk3, and a resonant capacitor Cr. The ports are connected to the high-voltage traction battery, low-voltage battery, and grid, respectively. The converter operates in three modes: G2V (power from grid to vehicles), V2G (power from vehicle to grid), and H2L (power from high-voltage to low-voltage battery). This structure eliminates the need for a two-stage PFC+DC/DC configuration, reducing component count and enhancing reliability for EV charging stations.
To analyze the power transfer characteristics, an equivalent model of the converter is developed. In V2G or G2V modes, the Y-model impedances are given by:
$$ L’_1 = \frac{L_1}{(N_1 / N_3)^2} $$
$$ L’_2 = \frac{L_2}{(N_2 / N_3)^2} $$
$$ L_{3eq} = L_{k3} + L_3 – \frac{1}{\omega_s^2 C_r} $$
where $N_1$, $N_2$, and $N_3$ are the transformer turns ratios, and $\omega_s$ is the switching angular frequency. The delta-model impedances are derived as:
$$ L^*_{13} = L’_1 + L_{3eq} + \frac{L_{3eq} L’_1}{L’_2} $$
$$ L^*_{23} = L’_2 + L_{3eq} + \frac{L_{3eq} L’_2}{L’_1} $$
$$ L^*_{12} = L’_1 + L’_2 + \frac{L’_1 L’_2}{L_{3eq}} $$
For H2L mode, where the grid port is inactive, the equivalent inductance referred to port 2 is:
$$ L”_1 = \frac{L_1}{(N_1 / N_2)^2} $$
The power transfer between ports is modeled using phase-shift modulation. The control variables include intra-bridge phase shifts $D_{11}$ (for port 1) and $D_{22}$ (for port 2), and inter-port phase shifts $D_{13}$ (between port 1 and port 3) and $D_{23}$ (between port 2 and port 3). The power expressions are:
$$ P_{12} = \frac{n_{21} U_{dc1} U_{dc2}}{4 f_s L^*_{12}} \left[ 2(1 – D_{22})(D_{13} – D_{23}) – (D_{13} – D_{23})^2 – D_{11} + D_{22} + D_{11} D_{22} – D_{22}^2 \right] $$
$$ P_{13} = \frac{2 n_{31} U_{dc1} u_{ac3}}{\pi^3 f_s L^*_{13}} \sin\left( \frac{1 – D_{11}}{2} \pi \right) \sin\left( D_{13} \pi – \frac{D_{11}}{2} \pi \right) $$
$$ P_{23} = \frac{2 n_{32} U_{dc2} u_{ac3}}{\pi^3 f_s L^*_{23}} \sin\left( \frac{1 – D_{22}}{2} \pi \right) \sin\left( D_{23} \pi – \frac{D_{22}}{2} \pi \right) $$
where $P_{xy}$ denotes power from port $x$ to port $y$, $f_s$ is the switching frequency, $n_{xy}$ is the turns ratio between ports $x$ and $y$, and $U_{dc1}$, $U_{dc2}$, $u_{ac3}$ are the port voltages. The total port powers satisfy:
$$ P_1 = P_{12} + P_{13} $$
$$ P_2 = P_{23} – P_{12} $$
$$ P_3 = -P_{13} – P_{23} $$
The optimized modulation strategy aims to achieve low current stress and high power factor for EV charging stations. In V2G mode, power flows from the traction battery to the grid. The voltage ratio $M_{13}$ is defined as:
$$ M_{13} = \frac{N_1 u_{ac3}}{2 N_3 U_{dc1}} $$
To achieve unity power factor, the phase shift $D_{13}$ must satisfy:
$$ D_{13} = \frac{1}{\pi} \arccos\left( \frac{M_{13}}{\cos\left( \frac{D_{11} \pi}{2} \right)} \right) $$
The optimal control variables for minimum current stress are derived as:
$$ D_{11} = \frac{\pi}{2} – \frac{2}{\pi} \arcsin\left( \sqrt{ M_{13}^2 + \frac{P_{13}}{P_{N13}} } \right) $$
$$ D_{13} = -\frac{D_{11}}{2} + \frac{1}{\pi} \arctan\left( \frac{P_{13}}{M_{13} P_{N13}} \right) $$
where $P_{N13} = \frac{2 U_{dc1} u_{ac3}}{n_{13} \pi^3 f_s L^*_{13}}$ is the base power. Similarly, for G2V mode, the equations are:
$$ D_{22} = \frac{2}{\pi} \arcsin\left( \sqrt{ M_{23}^2 + \frac{P_{23}}{P_{N23}} } \right) – \frac{\pi}{2} $$
$$ D_{23} = \frac{D_{22}}{2} – \frac{1}{\pi} \arctan\left( \frac{P_{23}}{M_{23} P_{N23}} \right) $$
with $M_{23} = \frac{N_2 u_{ac3}}{2 N_3 U_{dc2}}$ and $P_{N23} = \frac{2 N_2 U_{dc2} u_{ac3}}{N_3 \pi^3 f_s L^*_{13}}$. In H2L mode, the Lagrangian method minimizes the peak inductor current $I_{Lmax}$ subject to power constraint $P_{12} = P_{ref12}$. The optimal solutions are:
$$ D_{13} = \frac{(M_{12} – 1) P_{12}}{2 P_{N12}} $$
$$ D_{23} = 0 $$
$$ D_{11} = 1 – \frac{P_{12}}{2 (M_{12} – 1) P_{N12}} $$
$$ D_{22} = 1 – M_{12} \frac{P_{12}}{2 (M_{12} – 1) P_{N12}} $$
where $P_{N12} = \frac{n_{21} U_{dc1} U_{dc2}}{8 f_s (L”_1 + L_2 + n_{23} L_{k3})}$ and $M_{12} = \frac{N_1 U_{dc2}}{2 N_2 U_{dc1}}$.
Simulations are conducted in PLECS to validate the proposed strategy for EV charging stations. The parameters are summarized in Table 1.
| Parameter | Value |
|---|---|
| Grid Voltage (RMS) | 220 V |
| High-Voltage Battery Voltage | 240 V |
| Low-Voltage Battery Voltage | 12 V |
| V2G/G2V Rated Power | 2500 W |
| H2L Rated Power | 500 W |
| Turns Ratio N1:N2:N3 | 10:1:7 |
| Inductance L3 | 1 μH |
| Leakage Inductance Lk3 | 7.4 μH |
| Resonant Capacitor Cr | 460 nF |
| Half-Bridge Capacitors Cf1, Cf2 | 3.3 μF |
In V2G mode, the grid current and voltage are in phase, demonstrating high power factor. The resonant cavity currents align with the grid-side square-wave voltage, confirming PFC performance. The current ripple on port 1 is 0.3 A, and on port 2 is 3.5 A. In G2V mode, the grid supplies power to both batteries while maintaining unity power factor. The inductor current $i_{L3}$ is anti-phase with the port 3 voltage, ensuring efficient power transfer. For H2L mode, the optimized modulation achieves zero-current switching (ZCS) for port 2 switches and zero-voltage switching (ZVS) for port 1 switches, with current ripples of 0.2 A and 0.8 A, respectively. The inductor current forms a plateau at zero current, validating the low-stress design.
The proposed single-stage three-port converter offers significant advantages for EV charging stations, including reduced component count, higher power density, and improved efficiency. The optimized modulation strategy ensures low current stress, wide soft-switching range, and automatic PFC in G2V and V2G modes. This enhances the utilization of electrical energy in power supply lines and supports the integration of EVs into the grid. Future work will focus on experimental validation and scalability for higher-power EV charging stations.
The mathematical models and control strategies developed in this paper provide a foundation for advanced EV charging stations. The power expressions and impedance models enable precise control of power flow, while the optimization techniques minimize losses and stress. The simulation results confirm that the converter maintains high performance across all operating modes, making it suitable for modern EV charging infrastructure. The integration of V2G capabilities further supports grid stability and renewable energy adoption, highlighting the importance of efficient power conversion in EV charging stations.
In conclusion, the bidirectional resonant converter with optimized modulation presents a compelling solution for next-generation EV charging stations. Its ability to handle multiple power transfer scenarios with high efficiency and power factor makes it ideal for widespread deployment. As EV adoption grows, such technologies will play a crucial role in enabling sustainable transportation and smart grid functionalities.