The 800-V onboard chargers (OBCs) are steadily gaining momentum for fast and ultra-fast charging of electric vehicles (EVs). However, moving from existing 400 to 800-V OBCs directly affects the choice of semiconductor devices, diode bridges, microcontroller units (MCUs), and transformers present in OBCs.
This article discusses these four parameters, offering insights to engineers moving to 800-V OBCs in EVs.
Semiconductor devices
The 400-V EV OBCs allow the use of matured Si-based MOSFETs, which have a typical standard voltage rating of 650 V. However, this MOSFET voltage rating has to increase for an 800-V EV OBC, for which a 1200-V blocking voltage capability is desired.
Wideband gap (WBG) devices, such as SiC-based MOSFETs, are emerging as successful alternatives with their 1200-V voltage ratings. Some advantages of SiC MOSFETs over Si MOSFETs are higher breakdown voltages, lower on-state resistance, better thermal dissipation, and higher switching frequencies.
Onsemi has developed a dedicated half-bridge SiC module, NXH010P120MNF1, featuring 1200-V/10 mΩ specification. This module has been tested in a 25 kW fast dc charger to support the 400 and 800-V EV powertrains, as shown in Figure 1. The 96% all-time efficiency, galvanic-isolated high-current driver, and auxiliary power solutions are some of the key features.
Figure 1. Onsemi’s 25-kW fast dc EV charger consisting of 1200-V rating SiC half-bridge modules. (Image: onsemi)
However, GaN, with its superior switching frequency, is also quickly competing with SiC technology. One example is using GaN transistors in an 11-kW/800-V OBC by Infineon Technologies, shown in Figure 2, which was announced at APEC 2023. The company has claimed that the OBC is better than SiC, with a 36% higher power density.
Figure 2. Infineon’s (formerly GaN Systems) 11 kW/800-V OBC implementing GaN transistors on display at the APEC 2023. (Image: Infineon Technologies)
Diode bridges
The diode bridge in the ac/dc converter stage must be rated to handle the increased voltage when an 800-V OBC is connected to a 120 or 240-V grid. The required voltage rating of the diode bridge is doubled compared to a 400-V OBC. However, even though the desired voltage rating of the diode bridge is increased, it does not mean that the losses increase.
In 800-V OBCs, the transformer turn ratio is typically doubled to compensate for the higher battery voltage. This adjustment results in half the current in the secondary windings of the transformer and reduces the overall transformer copper losses by a factor of two. This reduction in current through the secondary windings also reduces the power losses in the diode bridge.
A diode bridge suitable for 800-V OBC could possess a lower internal resistance than those used in the 400-V OBC, reducing the conduction losses. One such example to compare is the STPSC2006CW (600 V) and STPSC15H12 (1200 V) SiC diodes from STMicroelectronics, where the latter has a lower internal resistance of 66 mΩ compared to the former, rated at 84 mΩ.
Figure 3. This chart shows the differences in diode and MOSFET conduction loss for various combinations of inverter and motor rated between 400 and 800 V. (Image: IEEE)
Figure 3 summarizes the diode and MOSFET conduction losses for 800-V inverter/400-V motor, 400-V inverter/400-V motor, and 800-V inverter/800-V motor combinations in a 25-kW OBC.
While the diode specs are mentioned above, the SiC MOSFETs used are 650-V SCT3017ALHRC11 and 1200-V SCT3022KLGC11 from ROHM Semiconductor. It can be observed that the diode and conduction losses in an 800-V OBC/800-V inverter combination are less than the other OBC-inverter combinations.
Control unit
The MCU is an often overlooked component of an OBC when opting for a higher voltage EV powertrain. Though it may appear that the MCU used for a 400-V OBC EV powertrain can be used for an 800-V OBC EV powertrain, a higher resolution ADC of the MCU is necessary.
But even before we talk about MCUs for EVs, it’s clear that the conventional MCUs used in ICE vehicles cannot be used for EVs, as evidenced by Figure 4. With the advent of WBG devices hungry for higher switching frequencies, there’s a clear demand for sophisticated MCUs for use in EVs.
Figure 4. The higher switching frequencies of WBG devices in EVs ask for MCUs with the ability to generate high frequency PWM signals. (Image: embedded.com)
Figure 5 shows that in an OBC, the MCU’s role is prominent in controlling the PFC and LLC sections. In both these sections, a voltage of 800 V must be handled at either the input or the output of the converters.
Therefore, when moving from 400 to 800 V, the ADC must have a higher resolution to control the PWM fed to the semiconductor switches efficiently. In addition, a higher resolution ADC of the MCU, such as the 12-bit resolution of Stellar E1, also enables efficient voltage and current sensing.
Figure 5. The MCU in an EV’s OBC, which controls the PFC and LLC sections. (Image: STMicroelectronics)
Transformers
In 800-V OBCs, the transformer turn ratio (n=N2/N1) is typically doubled compared to 400-V OBCs. This adjustment is necessary to compensate for the higher battery voltage and maintain similar voltage and current waveforms on the primary side.
The amplitude of the secondary current is halved, allowing for the use of thinner secondary windings. This reduction in the cross-sectional area helps fit the windings into the core window and reduces conduction losses. However, as per a research study, the transformer’s iron losses are not significantly affected as the magnetic flux density remains the same.
Summary
The fast and ultra-fast charging nature of EVs encourages EV manufacturers to accommodate higher power into OBCs. While increasing the power density of the existing 400-V OBCs is an ongoing research, 800-V OBCs open new opportunities to support faster EV charging.
Nevertheless, care should be given to the ratings of the WBG semiconductor switches, diodes, and transformers. The MCU has to have a better ADC resolution to support the wide voltage swings in 800-V OBCs.
References
- GaN Systems Unveils Revolutionary New GaN-Based 800V On-Board Charger (OBC) Reference Design, Infineon Technologies
- Power Up with 800-V Systems: The benefits of upgrading voltage power for battery-electric passenger vehicles, IEEE
- An Overview of 800 V Passenger Electric Vehicle Onboard Chargers: Challenges, Topologies, and Control, IEEE
- 800-V Electric Vehicle Powertrains: Review and Analysis of Benefits, Challenges, and Future Trends, IEEE
- STPSC15H12 – 1200 V, 15 A High Surge Silicon Carbide Power Schottky Diode, STMicroelectronics
- STPSC2006CW – 600 V power Schottky silicon-carbide diode, STMicroelectronics
- SR5E1E3 – SR5 E1 line of Stellar electrification MCUs, STMicroelectronics
- On-board charging (OBC) for electric vehicles, Infineon Technologies
- SEC-25KW-SIC-PIM-GEVK Evaluation Kit, onsemi
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