Power Converter with Inbuilt Charging technology using WBG Devices

Building onboard battery charging into the traction drive systems may offer to cut costs and boost power density. While some implementations sacrifice galvanic isolation, a recent approach incorporates a frond converter formed by traction drive inverters, motors, and a 14 V accessory DC-DC converter, ensuring safety compliance. This solution not only reduces component count but also aligns with the segmented traction drive system, effectively minimizing DC bus ripple current.

Moreover, bidirectional onboard chargers can improve PEV functionalities, providing grid support, emergency mobile power generation, and enabling collective control of PEV batteries for future smart grid integration and intermittent renewable energy support. The ongoing shift from silicon to wideband-gap devices may further advancements in power density, efficiency, and cost reduction for galvanically isolated integrated onboard chargers.

Specifications of the Charger and its Features

Architecture: Key components include a High Voltage (HV) battery, a segmented traction drive system, three sets of contact switches (CS1, CS2, and CS3), and a battery charging converter. The traction drive system divides the inverter (INV) switch dies and motor stator windings into two groups, forming two drive units. This design achieves over a 60% reduction in the DC bus capacitor by using interleaved switching between the drive units. The neutral points of the stator winding group (N1 and N2) connect to a charging port through contact switch CS3. Notably, the motor serves as the filter inductor in the integrated charger, eliminating the need for an external filter inductor.

The battery charging converter utilizes a phase-shifted dual active full-bridge converter with two H-bridges (HB1 and HB2) and a buck converter connected through a high-frequency transformer (Tr). This configuration provides galvanic isolation during battery charging, and HB2, the transformer, and the buck converter collectively form a 14 V accessory converter, delivering around 2 kW in Plug-in Electric Vehicles (PEVs) for charging the 14 V battery and powering vehicle low-voltage accessories.

Figure 1: Circuit diagram representing the segmented traction drive system of a PEV

The electric drive system operates in three modes: (1) Propulsion mode powers the vehicle, and charges the 14V battery, with interleaved switching for motor control. (2) Charging mode disconnects the HV battery from drive units, using a front active converter for power factor correction and charging the HV battery. (3) Sourcing mode reverses power flow, using a single-phase inverter to supply external loads or the grid, with the option to activate the buck converter for 14V battery charging. Each mode involves specific configurations of contact switches and converter components to achieve the desired functionality in vehicle propulsion, charging, or power supply.

Charging Control: Figure 2 presents a control block diagram with three loops. The DC bus voltage control loop, using a PI regulator, maintains a constant level. A feed-forward compensation, determined by the battery charging power, (P*_bat) , and a feed-forward gain (kff) contribute to the current command (݅i*_s). The charger converter controller regulates battery terminal voltage (V_bat) or charging current (I_bat) using a PI controller.  This converter can maintain a unity power factor or provide reactive power for grid support. The grid current (is) divides into three equal parts, flowing through the branches of two motor ZS networks, without generating torque. The charger converter controls the V_bat in constant voltage mode or the I_bat in constant current mode. A band-pass filter (BPF) prevents significant fluctuations in DC bus voltage (V_dc), ensuring a stable battery charging current. The PWM block controls duty cycles and, if necessary, phase shifts between the two H-bridges.

Figure 2: Circuit diagram of the control block of the PEV. 

Prototype Developed and Results

A prototype with an electrical drive system and charger converter was constructed for charging tests. The traction drive inverter, with six 1200V/120A SiC MOSFET modules, was flexible for dual 3-phase operation at 45 kW each or as a segmented 3-phase inverter at 90 kW. The 6.8 kW charger converter utilized four SiC phase leg modules and a planar transformer, achieving efficiencies of 99.0% at 100 kHz, 97.5% at 200 kHz, and 97.3% at 250 kHz.

Figure 3: Prototype of the traction drive developed and used in the integrated charger tests.  

The segmented inverter, charger converter, and an off-the-shelf induction motor (14.9 kW) were integrated and tested as an onboard charger (OBC). The OBC successfully operated with resistive load banks at both 120 V and 240 V grid voltages. Figure 4 shows the waveforms of performance at various charging powers (1.0 kW, 1.7 kW, 2.7 kW, and 6.8 kW) and grid voltages. Notably, the motor's high resistance (315 mΩ) impacted charger system efficiency compared to production PEV motors. The integrated OBC demonstrated functionality, showcasing the adaptability of the inverter and the efficiency of the charger converter in diverse operating conditions.

Conclusion

An experiment was conducted, and a prototype was developed for an integrated onboard charger, featuring a segmented traction drive system and a battery charging converter. The system's architecture, utilizing interleaved switching and a unique motor-as-filter inductor design, demonstrated over 60% reduction in DC bus capacitor size.

The charging control system, with three operational modes and precise regulation through feedback loops, showcased versatility in vehicle propulsion, charging, and power supply. The final prototype, utilised SiC MOSFET modules and achieved high efficiencies at varying frequencies, successfully operated as an onboard charger with resistive load. Despite motor resistance impacting efficiency, the integrated system displayed adaptability and efficiency in diverse conditions, affirming its potential for Plug-in Electric Vehicles (PEVs).