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Core message: Electric propulsion technology requires the integration of a completely new architecture for the power transmission system in vehicles. This newly added component necessitates in-depth multidisciplinary research into corresponding system components.
“Save our planet, keep it free from pollution!” This is the unanimous call from scientists and informed individuals worldwide for reducing greenhouse gas emissions. Cars powered by fossil fuel engines are the main culprits. Although there are many alternative technologies to propel vehicles, the only feasible solution currently is electricity.
Electric propulsion technology needs to integrate a completely new architecture for the power transmission system in vehicles. This newly added component requires in-depth multidisciplinary research into corresponding system components. The electric vehicle system consists of electric motors, power converters, and energy storage devices such as lithium-ion batteries. This new architecture must be optimized to maximize system efficiency, allowing the vehicle to achieve the longest driving distance on a single charge. The development of electronic technology provides significant impetus for reducing gas emissions from transportation.
Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs)
Electric vehicles run on batteries, while hybrid vehicles also use a fossil fuel ignition engine as an auxiliary. For the technologies powering these vehicles to succeed and have a bright future, energy efficiency is key. Therefore, intelligent power management mechanisms are needed to maximize the efficiency of converting battery energy into mechanical drive power for the wheels, thereby increasing the driving distance on a single charge without increasing carbon emissions, ideally significantly reducing them.
Silicon Carbide (SiC) Power in Electric Vehicles
The weight, volume, and cost of electric vehicles, as well as the driving distance per charge, are directly related to the efficiency of the power conversion system. SiC power components are particularly suitable for operation in the high-temperature environments commonly found in vehicles. Let’s take a closer look at how SiC power components can improve system efficiency.
Lighter weight means extended mileage. One typical way to reduce the weight, cost, and size of the power conversion system is to increase the switching frequency of the switching regulator. We all know that at higher frequency points, the size and weight of active components such as inductors, capacitors, and transformers can be reduced. Therefore, it’s time to adopt SiC solutions.
Although silicon (Si) power components can also operate at high frequencies, SiC has the advantage of handling much higher voltages than Si. SiC is a wide band gap (WBG) semiconductor component, and a wider band gap means a higher critical electric field (the blocking voltage in the off state). The high voltage capability of wide band gap SiC components allows them to have lower conduction resistance, thus achieving faster switching speeds and unipolar operating states. Part of the principle is that its carrier frequency needs to be accelerated to higher speeds (higher kinetic energy) to overcome the wider band gap.
While Gallium Arsenide (GaAs) and Gallium Nitride (GaN) also have high critical electric fields and are improved components targeting high-power solutions, SiC has other advantages, such as a higher maximum operating temperature, very high Debye temperature, and high thermal conductivity (in polycrystalline SiC), achieving rapid switching and low resistivity in the electric field due to high carrier saturation speed, facilitating lower production costs from the generation of silicon dioxide (SiO2), and higher threshold energy leading to stronger radiation hardening.
SiC components have many critical applications in electric vehicles. Existing electric traction drive systems can convert 85% of electrical energy into mechanical energy to drive the wheels, which is quite high efficiency, but SiC can also assist in improving efficiency. The energy converter benefits from efficiency improvements because it can transfer battery energy to the engine and can be used in battery charger circuits and any necessary auxiliary power sources (Figure 1).
▲Figure 1 SiC power components have many uses in electric vehicles
A SiC power supply converting 750V to 27V for low-voltage electric vehicles is a great example of how SiC power components improve electric vehicle efficiency. This architecture increased efficiency from 88% to an astonishing 96%, reduced size and weight by 25%, and did not require fans to cool excess heat compared to Si solutions. Table 1 shows some important applications of SiC power components in electric vehicles.
Table 1 Some SiC applications in the electronic architecture of electric vehicles. (PCU refers to Power Control Unit; APS refers to Auxiliary Power Supply) (Table source: 2015 Tenth International Conference on Ecological Vehicles and Renewable Energies)
GaN Power in Electric Vehicles
GaN also plays a significant role in improving power for electric vehicles. Insulated Gate Bipolar Transistors (IGBTs), widely used in motor drives and DC/DC control, have traditionally been based on Si products. The switching times of these designs are typically on the order of 10k~100kHz, while GaN components can achieve switching times at the nanosecond (ns) level and can easily operate in automotive environments at 200℃.
Like SiC, GaN components can also reduce the size of inductors, capacitors, and transformers in the power architecture due to their higher switching speeds, thus reducing overall volume and weight due to the smaller size of passive components.
We will analyze their effectiveness based on the chemical composition of electric vehicle batteries, such as lithium-based chemistries and high energy density Nickel-Metal Hydride (NiMH) batteries. As mentioned in the SiC section, to achieve longer distances on a single charge, the efficiency of the power conversion architecture also needs to be improved.
The switching speed and minimum conduction resistance of Si components have reached their maximum limits, and GaN seems to be a viable solution to exceed these limits. Experiments have shown that if the switching frequency can be increased fivefold, the size of inductors and capacitors can be reduced to one-fifth. Today’s GaN technology supports very high speeds.
GaN power components perform excellently in four key areas: high-temperature operation, higher breakdown voltage, low conduction resistance, and nanosecond switching speeds suitable for higher operating frequencies. These advantages are similar to those of GaN and SiC, with two differences: LEDs and RF transistors have always used GaN; many Si processes are compatible with GaN processes, which reduces wafer and process costs compared to the higher substrate costs of SiC.
Since reliability issues were resolved as early as 2003, today’s technology has successfully produced the first GaN High Electron Mobility Transistor (HEMT) components. These are normally-on components, meaning that 0V gate voltage will form a conducting state, while any voltage less than 0V will turn off the component. Early versions used SiC substrates; once Si substrates can perfectly integrate with GaN, production costs can be significantly reduced. In 2014, a new cascaded architecture was achieved that transformed normally-on components into normally-off components.
Since then, driving technology has made great progress, with increasing integration and significant improvements in power inverters. GaN components also perform remarkably in electric vehicle battery chargers, which consist of AC/DC converters and DC/DC converters. This combination is a power factor controller (PFC) (Figure 2).
▲Figure 2 A typical power architecture for electric vehicles
Using GaN, along with the higher switching speeds of GaN HEMT, allows for smaller passive components. The increased frequency through smaller inductors leads the power architecture to lower ripple currents, thus improving the power factor and resulting in smaller and lower-cost capacitors. Lower ripple currents also reduce stress on capacitors, thereby enhancing their reliability and lifespan.
In recent years, GaN’s reliability has been raised to a high standard, which is key to its use in vehicles.
Reducing Greenhouse Gas Emissions Using Hybrid Vehicle Drive System Efficiency
Currently, approximately 72% of traffic emissions are generated by vehicles on the road. Improving the design of hybrid vehicle drive systems to enhance their efficiency is a primary means of reducing emissions. One approach is to enhance the efficiency of the DC-link voltage control architecture, which means first improving the efficiency of the power converters in series hybrid vehicle drive systems.
The DC-link typically connects three drive systems: a primary power source composed of three-phase rectifiers; a secondary power source composed of a Dual Active Bridge (DAB) DC/DC converter; and a propulsion load made up of three-phase inverters (Figure 3), which are associated with series hybrid vehicles.
▲Figure 3 Block diagram of the drive system of hybrid vehicles
In designs where the DC-link and battery voltages are not equal, an intermediate solution in the DC/DC converter is necessary. An IEEE paper titled “Voltage Control for Enhanced Power Electronic Efficiency in Series Hybrid Electric Vehicles” describes many methods to study different architectures and control schemes for various DC-link voltages and DC/DC converter controls.
The following will discuss the proportional control law, which is used to control the dynamic DC-link voltage to achieve a phase shift between the gate switching waveforms of the DAB DC/DC converter. This converter is located between the DC-link and the battery in the series hybrid vehicle drive system, as shown in Figure 4. In this case, the controller reduces both the energy loss of the DC/DC converter and the losses of the entire drive system.
▲Figure 4 Interconnection diagram of the hybrid vehicle drive system in the control principle diagram
The engine (ICE), Continuously Variable Transmission (CVT), Permanent Magnet Synchronous Motor (PMSG), or the primary power source of the hybrid vehicle, and the Permanent Magnet Synchronous Motor (PMSM) or propulsion load of the hybrid vehicle are all key components of the system shown in the figure.
In this model, the diesel engine is the primary power source of the hybrid vehicle, while the DC battery serves as the secondary power source. The Management Control System (SCS) controls the power ratio provided by these two power sources based on the state of charge (SOC) of the battery and the motor load.
In fact, in this series hybrid vehicle, the DC-link voltage will impose conditions on the ideal operating area of the PMSM and PMSG corresponding to the unit modulation index, thereby avoiding over-modulation that leads to signal distortion and reduced system efficiency. Keeping the modulation index close to 1 can improve the overall efficiency of the power circuit in the drive system, maximizing the efficiency of the inverter and rectifier, where the switching process is the main factor for efficiency loss. Therefore, reducing the switching voltage can enhance efficiency.
This continuous zero-voltage switching (ZVS) mechanism, which minimizes power losses, is most suitable for vehicles with a high hybrid factor (HF), especially in urban environments. The hybrid factor refers to the ratio of the installed power from the power source to the total installed power. This hybrid factor affects fuel consumption in hybrid vehicles.
Automotive Inverter
The main power inverter controls the motor in the power transmission system and is an essential device in hybrid/electric vehicles. The power inverter, like the engine management system (EMS) in an engine vehicle, determines driving behavior. This inverter is suitable for any motor, such as synchronous, asynchronous, or brushless motors, controlled by an integrated electronic PCB. This PCB is specially designed by automotive manufacturers to minimize switching losses and maximize thermal efficiency. Another function of the inverter is to capture energy released from regenerative braking and feed it back to the battery. The driving distance of hybrid/electric vehicles is directly related to the efficiency of the main inverter (Figure 5).
▲Figure 5 Framework diagram of the Infineon main inverter in hybrid/electric vehicles. (Image source: Infineon)
Dual Voltage Battery System
Managing batteries in hybrid and electric vehicles requires the use of high-voltage technology. The dual voltage system, which combines 12V and 48V batteries, requires bidirectional DC/DC conversion, as shown in Figure 6, to protect circuits and support structured functions.
▲Figure 6 Bidirectional DC/DC converter from 48V to 12V
Additionally, automotive architecture designs typically include an onboard charger module (OBCM) of 3.5kW or 7kW to charge electric vehicles or plug-in hybrid electric vehicles (PHEVs) from the grid. Conversely, electric vehicles and plug-in hybrid electric vehicles can serve as energy sources and can also be integrated into smart grids that utilize renewable energy as storage devices. Smart grids consider intelligent charging and discharging for electric vehicles and plug-in hybrid electric vehicles, which is why OBCM must be a bidirectional DC/DC charger.
The optimal architecture for this design is a boost series resonant bidirectional topology, as shown in Figure 7. It operates above the resonant frequency, with zero-voltage switching capability, achieving maximum power transfer performance at the minimum switching frequency point. Compared to unidirectional power flow converters, this technology replaces diode rectifiers with MOSFET rectifiers. This solution also offers higher efficiency and a wider battery capacity. A major drawback of this architecture shown in Figure 7 is that the rectifier bridge has significant losses when turned off, which must be addressed in future designs.
Figure 7 Designers sometimes use modulated DAB converters to control simple high-frequency isolation. The advantage of this architecture is lower stress on components; its main drawback is that ZVS cannot be extended across the entire output range, especially under light load conditions. This figure shows that the boost series resonant bidirectional converter is a better architecture.
Delphi Integration and Wiring
Delphi has integrated all the subcomponents discussed in this article along with several other power electronic components for hybrid electric vehicles (Figure 8), which is impressive.
▲Figure 8 Delphi achieves high integration in hybrid/electric vehicles
Using the right internal connectors in hybrid/electric vehicles is also crucial (Figure 9).
▲Figure 9 Key elements in hybrid/electric vehicles are minimizing mass
Delphi has made significant innovations in low gauge cable technology, insulation materials, and lighter copper alternatives (such as aluminum or some special proprietary alloys). (Image source: Delphi)
Electric Wheel Drive System
The paper “Design and Implementation of an Electric Drive System for In-Wheel Motor Electric Vehicle Applications” recommends a motor drive system suitable for hybrid and electric vehicles. A Matlab SIMULINK model has been developed for a motor drive hybrid vehicle that provides computational performance. Two 14kW DC brushless motors have been designed and manufactured based on the literature, mounted inside the rims of the hybrid vehicle.
▲Figure 10 A brushless DC motor for one rear wheel
Additionally, two independently driven rear wheels are also mounted on a Fiat Linea vehicle. By detecting the angle of the steering wheel, electronic control technology replaces the mechanical differential device. The electric drive control system of the vehicle communicates with the electronic control unit (ECU) via the CAN bus, successfully cascading between the power-driven rear wheels and the ICE-driven front axle.
This design opted for brushless DC motors with centralized coils due to their low power-to-weight ratio and high efficiency, making them easy to control.
▲Figure 11 Breakdown of direct drive brushless DC motors in wheel rims and electric generator systems
In summary, this article presents some recent developments in power management for electric and hybrid vehicles. More development results are sure to emerge in the future, further improving these systems for the benefit of the planet. Source: EDN Taiwan
