With the global shift towards eco-friendly transportation, electric cars have emerged as a viable answer to curbing greenhouse gas emissions and reducing reliance on non-renewable energy sources. At the core of an electric vehicle lies its electrical system, where a crucial element called the Direct Current link plays a significant role in managing frequency switching, usually within the 5-10 kHz range. However, the conventional DC link used is very inefficient and outdated, with significant parasitic and inductive losses housed by a bulky filter. Therefore, it is vital for industries to innovate new technology to enhance the potency of DC links, by exploiting new semiconductor materials like gallium nitride and silicon carbide.

The first improvement was implemented on electrolytic capacitors, which were quite famous for having high energy density and low cost, however, it was also prominent for having frequent catastrophic failures and poor current ripple handling in long-term operations. These issues paved the way for designers to use film capacitors, which were made out of typical materials like polyethylene terephthalate, polypropylene, and polyethylene naphthalate due to their self-healing properties. Moreover, film capacitors had lower cost, high consistent manufacturing, and low resistance making them more power denser and improving peak current.

On the other hand, current industries research a lot on keeping overall manufacturing costs low as newer designs for DC links require circuits that can sustain temperatures up to 125℃ and should support switching more than 100kHz. Therefore, it was concluded that the obsolete bulky DC bus link can be replaced with PCB-mounted multi-layer ceramic capacitors or MLCCs to reduce impedance and maximize heat dissipation by stacking layers alternatively with interdigitated metal electrodes. These electrodes are made out of precious metals like silver and palladium alloy which helps in supporting high resonant frequency, long-term reliability and high volumetric current densities.

Ⅰ. Exploring Current Control Strategies and DC Link Circuit Model for Enhanced Performance in EV Converter-Inverter Systems

The electrical power unit in any system plays a vital role in providing essential electrical power to numerous components and systems within the system. It powers the ignition system, enables the starting of the engine through the starter motor, and supplies electricity to the lights, audio system, climate control, and other accessories. Furthermore, it facilitates the operation of crucial

instruments and controls, allowing the controller to monitor the performance of the system. Overall, the electrical power unit is essential for the proper functioning of various electrical systems, enhancing safety, convenience, and comfort for the user.

The current electrical power unit of a common vehicle as shown in Figure 1 has a 27 kW bi-directional DC-DC converter, which is used for boosting battery voltage. On the other hand, the high voltage (HV) DC link bus plays a pivotal role in hybrid electric vehicles by facilitating efficient power transfer between different components. Acting as a connection point, it links the high side of the DC-DC converter, the generator inverter (MG1), and the motor inverter (MG2). This configuration allows for seamless energy flow between the battery and a combination of the internal combustion engine (ICE), MG1, and MG2. The HV DC link bus enables bidirectional power transfers, optimizing energy efficiency by minimizing conversions and reducing energy losses. Moreover, it enhances vehicle performance by providing flexibility in power distribution, resulting in improved responsiveness, acceleration, and overall driving experience.

Figure 1: Conventional circuit diagram for the electrical power unit of a Prius.

In order to understand the model correctly, an equivalent circuit model is shown in Figure 2, where an interconnect to the DC-DC converter is represented on the left side, with equivalent resistances, and parasitic inductances. Moreover, capacitors 1 and 6 represent the first filter capacitor and capacitors 3-5 are the smoothing capacitors. On the other hand, when examining the total impedance of the DC link, taking into account the presence of capacitors, a distinct behaviour emerges as frequency varies. At lower frequencies, the impedance portrays a capacitive characteristic until it reaches the first resonance. Beyond this critical point, the impedance ascends, assuming an inductive nature. As frequencies continue to increase, additional resonances come into play, with the effects of parasitic inductances and capacitances. This showcase of impedance characteristics throughout the frequency spectrum highlights the dynamics of the DC link and its relationship with capacitors.

Figure 2: Resistance and equivalent series inductance of DC link capacitors with DC link equivalent circuit.

Ⅱ. Next-Generation Electric Mobility: Breakthrough DC Bus Link Design Paves the Way

The new DC bus link aims to reduce the overall size and improve the efficiency of the system, also supporting the new UWBG and WBG-based power systems while maintaining excellent thermal conditions. The new DC bus link design houses a flat capacitor and two metal plates with a PC board and surface mount capacitors as shown in Figure 3. In order to reduce costs, the manufacturing and assembly are done using soldering methods with high volume reflow on SMD devices.

Figure 3: New DC bus link design

The DC link capacitance required to attain a specified voltage ripple is given by equation 1, where it is inversely proportional to the switching frequency and directly proportional to the current.

Equation 1: New and improved equation for finding required DC link capacitance with respect to the voltage ripple.

Apart from the higher density and reduced power thermals, this DC bus link design managed to require only 296 capacitors to reach the 44.4 µF threshold at 0.15 µF each. Moreover, impedance was reduced in the PCB layout made of 2 oz. copper by introducing several via in pads per component.

Ⅲ. Conclusion

The current rate of increase in the need for better electric vehicles demands the development of a high-frequency/low-impedance DC link bus design for power inverters in WBG-based power systems. The new design of the DC link indicates that the bus can accommodate WBG devices operating at 100-130 kHz, suggesting its suitability for SiC-based inverter systems. Further improvements in design could potentially raise the resonant frequency, enabling the use of GaN and even UWBG devices. It should be noted that the measured impedance at the resonant frequency was approximately 25 mΩ, with a significant portion likely attributed to solder connections and other imperfections in the prototype assembly. However, it is worth mentioning that the prototype was constructed using components costing less than 700 dollars, including the printed circuit board. The cost is expected to decrease further with larger-scale production and better efficiency can be introduced with the help of an HF bus integrated into the inverter system.