In recent years, the escalating concerns over environmental pollution and energy scarcity have propelled significant advancements in battery electric car technology. As a key enabler for widespread adoption, charging infrastructure and onboard systems have evolved, with integrated on-board chargers (IOBCs) emerging as a pivotal innovation. In this review, we delve into the topological structures of IOBCs, offering a detailed analysis from a first-person perspective. We aim to provide an exhaustive overview that spans over 8000 tokens, incorporating tables and mathematical formulations to summarize key findings. The term “battery electric car” will be frequently emphasized to underscore its relevance in the context of sustainable transportation.
The proliferation of battery electric cars is often hindered by “charging inconvenience” and “range anxiety,” which stem from the limitations of traditional onboard chargers. These conventional systems, as depicted in earlier designs, add considerable weight, volume, and cost to battery electric cars, contradicting the trend toward lightweight and efficient vehicles. To address this, IOBCs integrate the motor and power converters from the propulsion system, enabling a drive-charge unification that drastically reduces the ancillary components. We classify IOBCs into three primary categories based on the type of integrated components within the drive system: integrated DC/DC converters, onboard chargers integrated with a single motor, and onboard chargers integrated with multiple motors. Each category presents unique topologies, operational principles, and challenges, which we will explore in depth.
First, let us consider the category of integrated DC/DC converters. In these configurations, the DC/DC stage of the onboard charger is merged with the DC/DC stage of the drive system, typically using bidirectional converters to serve both functions. This approach reduces the number of components but often requires additional rectification circuits for AC/DC conversion during charging. For instance, a common topology involves a boost converter derived from the drive circuit, where switches are reconfigured to facilitate charging. The power flow during charging can be expressed as:
$$P_{charge} = V_{grid} \times I_{grid} \times \text{PF}$$
where \(P_{charge}\) is the charging power, \(V_{grid}\) and \(I_{grid}\) are the grid voltage and current, and PF is the power factor. The efficiency \(\eta\) of such systems is critical and can be modeled as:
$$\eta = \frac{P_{battery}}{P_{grid}} = \frac{V_{battery} \times I_{battery}}{V_{grid} \times I_{grid}} \times 100\%$$
However, these topologies often suffer from higher losses due to the extra rectification stage. Another variant incorporates Z-source networks, which provide voltage boosting capabilities. The Z-source impedance network, with inductors \(L\) and capacitors \(C\), allows for wide voltage range charging but imposes high voltage stress on switches, as given by:
$$V_{switch} = \frac{V_{battery}}{1 – D}$$
where \(D\) is the duty cycle. While these integrated DC/DC converters offer component savings, their integration level remains relatively low compared to other categories, limiting their appeal for modern battery electric cars.
Moving to the second category, onboard chargers integrated with a single motor leverage the motor windings and drive circuit to achieve charging functionality. This represents a more advanced integration, as the stator windings of motors, such as permanent magnet synchronous motors (PMSMs) or switched reluctance motors (SRMs), are repurposed as filter inductors during charging. We can subdivide this into topologies where the motor windings and drive circuit realize DC/DC functionality and those where they achieve AC/DC functionality directly. For the former, a typical setup involves a two-stage charger: an AC/DC rectifier followed by a DC/DC converter formed by the motor windings and inverter bridges. The current distribution in the windings must be controlled to prevent torque generation during charging. For a three-phase PMSM, the condition for zero torque can be derived from the torque equation:
$$T_e = \frac{3}{2} p \left( \lambda_d i_q – \lambda_q i_d \right)$$
where \(T_e\) is the electromagnetic torque, \(p\) is the pole pairs, \(\lambda_d\) and \(\lambda_q\) are flux linkages, and \(i_d\) and \(i_q\) are dq-axis currents. By setting \(i_q = 0\) and aligning the rotor d-axis with the stator a-axis, torque can be minimized. In practice, currents are controlled such that \(i_a = -2i_b = -2i_c\), ensuring negligible net torque. The input current quality is vital, with total harmonic distortion (THD) defined as:
$$\text{THD} = \sqrt{\frac{\sum_{n=2}^{\infty} I_n^2}{I_1^2}} \times 100\%$$
where \(I_n\) are harmonic currents. Many topologies in this category report THD values around 3.5–4.5%, with efficiencies up to 95.5%. For AC/DC functionality topologies, no additional rectifier is needed, enhancing integration. A single-phase example connects the grid directly to motor winding midpoints and inverter legs, forming a rectifier. The control strategies involve adaptive proportional-derivative controllers to mitigate harmonic issues. These systems are particularly relevant for battery electric cars, as they reduce weight and cost while maintaining acceptable performance.
The third category involves onboard chargers integrated with multiple motors, which is increasingly relevant for high-performance battery electric cars with multi-motor drives. Here, the windings of several motors are combined with drive converters to enable high-power charging. For instance, in a four-motor system, the three-phase grid can be connected to the midpoints of windings from three motors, while the fourth motor assists in DC/DC conversion. By ensuring balanced currents in each motor, rotating magnetic fields cancel out, preventing torque production. The power capability scales with the number of motors, as shown by:
$$P_{total} = \sum_{k=1}^{N} P_{motor,k}$$
where \(N\) is the number of motors. These topologies often support vehicle-to-grid (V2G) operations, allowing battery electric cars to feed power back to the grid, but they may require additional filtering due to low equivalent inductance from windings. Control complexity increases with the number of phases, but advancements in multiphase machine theory, such as decoupling transformations for five-phase or six-phase systems, facilitate torque cancellation. For a six-phase asymmetric motor, the current decomposition into orthogonal subspaces helps manage torque, with the condition for zero torque expressed as:
$$\begin{bmatrix} i_{\alpha1} \\ i_{\beta1} \\ i_{\alpha2} \\ i_{\beta2} \end{bmatrix} = \mathbf{T} \begin{bmatrix} i_a \\ i_b \\ i_c \\ i_d \\ i_e \\ i_f \end{bmatrix}$$
where \(\mathbf{T}\) is the transformation matrix. By controlling \(i_{\beta1} = 0\) and \(i_{\alpha2} = 0\), torque components vanish. These integrated systems showcase the potential for high-power charging above 30 kW, making them suitable for fast-charging battery electric cars, though they often involve more switches and complex control algorithms.
To provide a clear comparison, we summarize the performance metrics of various IOBC topologies in the table below. This includes the integrated motor phase number, power supply phase number, number of power switches, need for additional components, charging power, maximum efficiency, and support for bidirectional power flow (V2G). The data is synthesized from numerous studies, with a focus on applications in battery electric cars.
| Topology Type | Integrated Motor Phase Number | Power Supply Phase Number | Number of Power Switches | Zero-Torque Charging | Additional Components Required | Charging Power (kW) | Maximum Efficiency (%) | Bidirectional Power Flow (V2G) |
|---|---|---|---|---|---|---|---|---|
| Integrated DC/DC Converter | – | 1 | 8 | – | Yes | 1.2 | 90.4 | No |
| Integrated Z-Source Converter | – | 1 | 4 | – | Yes | 3.3 | 94.0 | No |
| Single Motor, DC/DC Function | 3 | 1 | 10 | Yes | Yes | 3.0 | 93.1 | No |
| Single Motor, AC/DC Function | 3 | 3 | 12 | Yes | Yes | 3.3 | 92.6 | No |
| Single Motor, Single-Phase AC/DC | 3 | 1 | 6 | Yes | No | 2.0 | – | No |
| Five-Phase Motor Integrated | 5 | 3 | 12 | Yes | No | 7.32 | 95.08 | No |
| Six-Phase Motor Integrated | 6 | 3 | 14 | Yes | No | – | – | Yes |
| Nine-Phase Motor Integrated | 9 | 1/3 | 20 | Yes | No | 3.3 | 90.0 | Yes |
| Multiple Motors Integrated | 2×3 | 3 | 14 | Yes | Yes | >30 | – | No |
From the table, we observe that IOBCs integrated with motor windings generally offer higher efficiency and better integration, with charging powers ranging from 2 kW to over 30 kW. The support for V2G is more common in multiphase and multi-motor systems, aligning with the smart grid capabilities of future battery electric cars. The number of power switches correlates with control flexibility, but also with cost and complexity. For battery electric cars, the choice of topology often balances these factors against charging speed and vehicle design constraints.
In terms of mathematical modeling, the dynamics of IOBC systems can be described using state-space equations. For a typical IOBC with a PMSM, the charging mode equations include the grid-side dynamics and battery-side dynamics. The grid current \(i_g\) and battery voltage \(V_b\) can be modeled as:
$$\frac{di_g}{dt} = \frac{1}{L_{eq}} (v_g – R_{eq} i_g – v_{dc})$$
$$\frac{dV_b}{dt} = \frac{1}{C_b} (i_{charge} – i_{load})$$
where \(L_{eq}\) is the equivalent inductance (including motor windings), \(R_{eq}\) is the resistance, \(v_{dc}\) is the DC-link voltage, \(C_b\) is the battery capacitance, \(i_{charge}\) is the charging current, and \(i_{load}\) represents any auxiliary loads. The control objective is to regulate \(i_g\) for power factor correction and \(V_b\) for constant-voltage or constant-current charging. Proportional-integral (PI) controllers are commonly used, with transfer functions:
$$G_{PI}(s) = K_p + \frac{K_i}{s}$$
where \(K_p\) and \(K_i\) are gains tuned for stability and performance. The overall system efficiency can be optimized by minimizing losses, which include conduction losses \(P_{cond}\) and switching losses \(P_{sw}\) in the power devices:
$$P_{loss} = P_{cond} + P_{sw} = \sum_{j} (I_j^2 R_{ds(on)} + f_{sw} E_{sw})$$
where \(I_j\) is the current through device \(j\), \(R_{ds(on)}\) is the on-state resistance, \(f_{sw}\) is the switching frequency, and \(E_{sw}\) is the switching energy per cycle. For battery electric cars, reducing these losses is crucial to extend driving range and improve thermal management.
Another key aspect is the harmonic performance, which impacts grid compatibility and efficiency. The input current waveform in IOBCs can be analyzed using Fourier series. For a single-phase charger, the current \(i_g(t)\) can be expressed as:
$$i_g(t) = I_1 \sin(\omega t + \phi_1) + \sum_{n=3,5,\ldots} I_n \sin(n\omega t + \phi_n)$$
where \(I_1\) is the fundamental amplitude, and \(I_n\) are harmonic amplitudes. Standards such as IEEE 519 impose limits on THD, typically below 5% for residential applications. Advanced modulation techniques, like space vector modulation (SVM) for three-phase systems, can reduce harmonics. The modulation index \(m\) relates the output voltage to the DC-link voltage:
$$V_{out} = m \cdot \frac{V_{dc}}{2}$$
In multiphase systems, additional degrees of freedom allow for harmonic cancellation in specific subspaces, further improving current quality for battery electric cars.
Looking ahead, the development of IOBCs is closely tied to the evolution of battery electric cars. We anticipate trends toward higher power density, greater efficiency, and lower cost. Bidirectional power flow (V2G) will become increasingly important, enabling battery electric cars to act as distributed energy resources. This requires topologies that support seamless transition between charging and discharging modes, with robust control algorithms. Moreover, the integration of wide-bandgap semiconductors (e.g., SiC or GaN) can enhance switching frequencies and reduce losses, as shown by the figure of merit:
$$FOM = R_{ds(on)} \times Q_g$$
where \(Q_g\) is the gate charge. These advancements will allow for more compact and efficient IOBCs, contributing to the lightweight design of battery electric cars.
In conclusion, as battery electric cars continue to gain market share, integrated on-board chargers represent a critical technology to overcome charging limitations. We have reviewed various topological approaches, highlighting their principles, performances, and trade-offs. The integration of motor windings with drive converters offers significant benefits in terms of weight and cost reduction, making it a promising direction for future battery electric cars. With ongoing research into multiphase systems, advanced control, and new materials, IOBCs are poised to play a vital role in the electrification of transportation, supporting a sustainable energy future.