Bench Talk

(Source: Mouser Electronics)

Onboard chargers (OBCs) solve a vital problem in the operation of electric vehicles (EVs). They convert AC grid power to DC power compatible with the battery, enabling EV charging. With more designs, architectures, and sizes of EVs entering the market each year, OBC implementations become increasingly complex. In addition, as the industry trends toward handling higher battery voltages for faster charging, and bidirectional charging becomes more common, system designers face critical decisions regarding the topologies and materials to use in their OBC offerings. This blog provides an overview of OBCs and compares material choices for their construction.

OBC Overview

Global CO₂ emissions standards continue to tighten. As a result, demand for charging capacity is outpacing the availability of DC fast chargers (level 3), creating the market pull for OBCs. Onboard chargers consist of several primary components, outlined in Figure 1 below:

Figure 1: Block diagram of an onboard charging (OBC) system. (Source: onsemi)

AC power from the electrical grid flows through an electromagnetic interference (EMI) filter to mitigate “noise” from external sources and prevent the OBC from emitting noise back into the grid. From there, the power enters the first of two main stages in the OBC, which is known as the power factor correction (PFC) stage. The PFC stage converts the AC line power to DC while significantly reducing phase distortion of the input voltage and current waveforms. This step delivers a power factor greater than 0.9 to minimize reactive power injection into the grid. The current then enters an isolated DC-DC converter to match the output voltage and current to the battery's state of charge, providing galvanic isolation between the input and output.

PFC Topologies and Materials

OBCs can use a multitude of PFC topologies, depending on the number of input AC phases and grid output power delivered to the OBC. Single-phase AC input typically uses traditional boost or totem pole configuration. For bidirectional designs, the PFC would employ the totem pole arrangement. Engineers could configure the totem pole PFC for single- or three-phase operation, which can operate in one or two directions.

Traditional Boost PFC

The traditional boost PFC is easy to implement, allows low EMI noise, and provides scalable power by interleaving the phases. The use of diodes reduces complexity but hinders efficiency. Traditional PFC is best for single-phase AC input OBC and is unidirectional. Ideal device options for this topology are super junction (SJ) MOSFET, insulated gate bipolar transistor (IGBT), and silicon carbide (SiC) diode.

Bridgeless Boost PFC

Bridgeless boost PFC is also appropriate for single-phase OBC and does not suffer bridge losses like the traditional boost. However, the diode of the inactive MOSFET reduces power correction effectiveness, limiting its practicality for OBCs.

Totem Pole PFC

As the traditional boost PFC is cost-effective but less efficient, the totem pole bridgeless PFC delivers the highest efficiency of commercial options but at a cost premium. Efficiency is highest with wide bandgap (WBG) devices on the fast legs, particularly in continuous conduction mode (CCM) and triangular conduction mode (TCM). It supports bidirectional power flow but is complex to implement. Device choices for totem pole bridgeless PFC include SiC MOSFET (fast legs) and IGBT (slow legs) for CCM and Si MOSFET for TCM modes.

SiC versus IGBT Use Cases

The variable power requirements with new EV charging systems create an opportunity for engineers to leverage semiconductor device options to improve the efficiency or cost of their system. Below is a review of the PFC material choices in onboard chargers.

SiC MOSFETs

A robust material choice, SiC MOSFETs are adept at all power tiers and topologies and are ideal for high-efficiency OBCs in luxury or high-performance EVs. These applications, and others requiring high switching frequencies with low losses, can achieve fast charging with better thermal management. SiC MOSFETs are recommended for use in PFC, primary side DC-DC, and secondary side rectification (bidirectional) in 800V battery systems due to their high efficiency and power density compared to IGBTs or Si SJ MOSFETs.

IGBTs

IGBTs are also suitable for most 400V PFC topologies and DC-DC stage, although the higher losses at 11kW and 22kW reduce their performance versus SiC. IGBTs work well for cost-sensitive applications in mid-range EVs and lower switching frequency applications in which cost-effectiveness is a higher priority.

Si SJ MOSFETs

These devices apply to a narrower use case: boost and bridgeless boost below 7.2kW power levels. Adding a Vienna design for 11kW and 22kW power tiers can boost performance in those applications. SiC SJ MOSFETs are suitable for use in 400V battery systems for PFC and DC-DC stage.

Generally, the two best options are SiC MOSFETs and IGBT for performance and system design flexibility.

SiC versus IGBT Comparative Analysis

SiC MOSFETs offer better efficiency at high voltages and frequencies, making them ideal for applications requiring high efficiency and compact designs due to their lower power loss. In addition, these devices deliver- premium performance that will enable 800V EVs in demanding applications requiring high power and high efficiency.

However, IGBT has created an emerging opportunity for applications where cost-effectiveness is more critical than maximizing efficiency. This scenario also offers a cost benefit to system manufacturers, as IGBT delivers sufficient secondary-side performance for 400V EVs.

Conclusion

OBCs convert AC grid voltage to DC voltage that is suitable for battery charging, which represents the majority of EV charging. Choosing the proper devices and topologies for OBCs is crucial for optimizing performance and efficiency in EV charging. Different topologies and devices have tradeoffs, so designers must choose the best application implementation. SiC MOSFETs are essential for high-efficiency, high-voltage applications, while IGBTs offer a cost-effective alternative for lower-voltage systems. By understanding the tradeoffs and use cases for different components, designers can make informed decisions that enhance the overall performance of their EV charging solutions.

From industry-leading SiC MOSFETs to circuit protection, onsemi’s onboard charging solutions include the components you need for a reliable and robust OBC design.

Author

Adam Kimmel has nearly 20 years as a practicing engineer, R&D manager, and engineering content writer. He creates white papers, website copy, case studies, and blog posts in vertical markets including automotive, industrial/manufacturing, technology, and electronics. Adam has degrees in chemical and mechanical engineering and is the founder and principal at ASK Consulting Solutions, LLC, an engineering and technology content writing firm.