Battery charging solutions for enhancing efficiency of on-board chargers

Contributed by Arrow Electronics

The development of the electric vehicle (EV) market has been rapid, but the charging speed and efficiency of vehicle batteries remain significant obstacles to the widespread adoption of electric vehicles. Enhancing the charging speed and efficiency of automotive batteries is a crucial factor in determining whether electric vehicles can fully replace internal combustion engine vehicles. This article will introduce the development trends in the architecture of on-board chargers (OBCs) and the product features of the new SiC MOSFETs introduced by onsemi.

The charging efficiency of OBC is crucial for the development of electric vehicles

Ever since electric vehicles (EVs) found their firm position on the automotive market, EV manufacturers have pushed towards higher power drivetrains, larger battery capacity and faster charging. To meet the demands of customers and extend driving range, EV manufacturers have been increasing energy capacity of a vehicle’s batteries. Larger batteries, however, mean longer charging time.

The most common charging methods are to charge from home overnight or at the workplace during the day. Both scenarios present different levels of power available to the EV. Drivers may not be able to fully charge their EVs overnight at home with a residential power outlet. At the workplace, a medium power AC charging station is likely available, but time at the charging station may become an issue if the car is equipped with a lower power OBC. Increasing the OBCs power capacity provides more reasonable charging time but also increases system complexity and design challenges. While high power DC charging stations can rapidly refresh a battery to 80% of its capacity, this form of charging is not the norm.

To address both charge time and performance issues, many EV platforms are migrating from the current 400V battery pack to an 800V battery pack. When the vehicle is in drive mode, the higher available voltage can be utilized to increase electric motor power output or improve system efficiency while maintaining the same power level. In the charge mode, the higher battery voltage reduces the current required to charge the battery and can reduce charge time. Two crucial factors that impact OBC design are voltage and switching frequency. By increasing voltage and switching frequency, OBC capacities can be improved significantly. The system architecture must account for higher voltages which is why 1200 V devices become preferred thanks to their higher blocking voltage capability.

In addition to the trend toward 800V main battery packs, there is a parallel trend of increasing the power capability of the OBC. Units in the 6.6kW range were common in the past. Now many designs are 11kW (split phase mains) and 22kW (three phase mains). While this power level is often not supportable in the home, it is at the more than 126,000 AC charging stations currently available in the US. A higher power OBC can allow faster charge times while at work or in many public spaces, negating the need to establish a full charge while at home. As OBC power levels increase, the advantages of silicon carbide (SiC) MOSFETs also increases.

SiC-based components have proven advantageous in comparison to IGBT components when it comes to higher switching frequency applications. SiC technology continues to provide design benefits in the transition towards 800V batteries. OBC systems can be downsized and increase the overall “wall to wheel” efficiency.

SiC MOSFETs can enhance charging efficiency and increase power density

Power converters based on Silicon Carbide (SiC) are becoming increasingly popular in the field of power electronics due to their high efficiency and power density, which are critical factors considering the environment and energy costs. SiC power devices have been rapidly adopted in energy infrastructure applications, including solar energy, UPS systems, energy storage, and electric vehicle charging systems, to improve efficiency or increase power density.

The goal of design engineers is to minimize power losses in converters and devices to achieve higher levels of efficiency and power density. SiC devices are favored in today’s world of power electronics because they can meet the requirements of design engineers. SiC devices have higher dielectric breakdown strength, energy bandgap, and thermal conductivity compared to silicon, enabling the creation of more efficient and compact power converters with low R_DS(on) and body diode reverse recovery charge, which are key parameters in minimizing switching and conduction losses.

Compared to Silicon (Si) MOSFETs or IGBTs, the material characteristics of SiC allow design engineers to achieve lower switching and conduction losses. SiC devices have faster switching speeds and higher operating frequencies, resulting in space savings, reduced heat dissipation, improved efficiency, and lighter power converters. Lower switching losses can be achieved by operating at higher efficiency with less cooling, or by increasing the switching frequency through downsizing and cost reduction of passive components. These advantages can justify the higher cost of SiC power devices.

Power losses can be divided into conduction losses and switching losses, with switching losses occurring when the current or voltage does not rise or fall instantaneously when other variables are non-zero. For power MOSFETs, the time required for the current or voltage to rise or fall depends on the rate of charging or discharging of parasitic capacitances. In addition to parasitic capacitance, the reverse recovery charge (Q_rr) of the body diode introduces additional switching losses. Conversely, when the device is conducting current, losses occur due to conduction. Switching losses depend on the dynamic parameters of the device, while static parameters contribute to conduction losses.

The new generation of SiC MOSFETs exhibits lower switching losses

After the successful introduction of first generation 1200 V EliteSiC M1 MOSFETs, onsemi recently released its second generation 1200 V EliteSiC M3 MOSFETs that focus on optimizing the switching performance. M3S products are lined up 13/22/30/40/70 mΩ for discrete packages of TO247−4L and D2PAK−7L. NVH4L022N120M3S is auto-qualified MOSFET with the lowest R_DS(ON) 22 mΩ at 1200 V.

The onsemi team has done extensive tests on key characteristics of M3S against M1. The M3S (NTH4L022N120M3S) requires less total gate charge Q_G(TOT) than M1 (NTH4L020N120SC1) which significantly reduces the amount of sinking and sourcing current from gate drivers. The M3S has 135 nC at its recommended V_GS(OP) = +18V and 44% reduced FOM (Figure of Merit) factor in R_DS(ON)*Q_G(TOT) than its older M1 counterpart, meaning it needs only 56% of the gate charge for switching in the same R_DS(ON) device.

The M3S also features better efficiency at lighter loads by storing less energy E_OSS in its parasitic capacitances C_OSS than the M1. Because the E_OSS is dependent on the Drain−Source Voltage, not current, it becomes a critical loss for the efficiency at light loads.

Switching losses are critical parameter in system efficiency. Where M3S achieved much improved switching performance at the given conditions, 40% lower in E_OFF, and 20−30% lower in E_ON, 34% lower in the total switching loss than M1. In high switching frequency applications, it will cancel the disadvantage of the higher R_DS(ON) temperature coefficient.

Increasing switching frequency helps designers to reduce the size of energy storage components such as inductors, transformers and capacitors, resulting in smaller volume of the system. More compact size and higher power density enable smaller package size for the OBC system, which gives engineers further options to budget additional weight elsewhere in the vehicle. Furthermore, operating at a higher voltage also reduces the current required throughout the vehicle, leading to lower cable costs between the power system, battery, and OBC.

SiC MOSFETs offer excellent switching performance and higher reliability

onsemi’s EliteSiC MOSFET uses a completely new technology that provide superior switching performance and higher reliability compared to Silicon. In addition, the low ON resistance and compact chip size ensure low capacitance and gate charge.Consequently, system benefits include highest efficiency, faster operation frequency, increased power density, reduced EMI, and reduced system size.

The M3S (NTH4L022N120M3S) from onsemi is a 22 mΩ, 1200 V EliteSiC MOSFET in TO247-4L packaging. It has a maximum R_DS(on) of 30 mΩ at V_gs = 18V and Id = 60A. The device is compliant with the AEC-Q101 automotive standard, supports gate drive voltage from 15V to 18V, is Pb -free and RoHS compliant. With the innovative M3S technology, it offers low R_DS(on), E_ON, and E_OFF losses.

The M3S series focuses not only on reducing the RSP resistance (defined as the R_DS(on) area) but also on improving the switching performance compared to the first-generation 1200 V SiC MOSFET. Optimized for high-power applications in industrial power systems, such as solar inverters, ESS, UPS, and off−board electric vehicle chargers, it enables designers to increase the switching frequency while improving system efficiency.

onsemi has previously introduced the first-generation 1200 V SiC MOSFET, known as the SC1. Although the SC1 significantly improved performance compared to traditional IGBT solutions in 1200 V switching for industrial power systems, it was a general-purpose device without specific trade-off parameters. Hence, some designers sought more targeted products for their systems.

The second-generation 1200 V SiC MOSFET from onsemi is divided into two core technologies: the T-design and the S-design. The T-design focuses on lower R_DS(on) and improved short-circuit capability rather than faster switching speeds for traction inverters. The S-design, on the other hand, is optimized for high switching performance, resulting in lower Q_G(TOT) and higher di/dt and dv/dt, thereby reducing switching losses. The M3S series is suitable for applications like OBC and DC/DC converters in electric and hybrid electric vehicles (EV/HEV).

Conclusion

With the maturity and rapid development of SiC MOSFET technology, it offers effective reduction in switching losses, superior switching performance, and higher reliability. As a result, it can provide higher efficiency, faster switching frequencies, higher power density, lower EMI, and smaller system footprint, making it an excellent solution for electric vehicle applications. onsemi’s next-generation M3S EliteSiC MOSFETs exhibit outstanding performance, enabling improved performance of OBC and DC/DC converters, making them an ideal choice for relevant applications.