On-board chargers (OBCs) for electric vehicles can take many forms depending on the power level and function, with charging power ranging from 2kW in micro EV applications to 22kW in high-end EVs.
Usually on-board chargers are unidirectional, but in recent years, bidirectional charging has attracted more and more attention, and this article will discuss the role of silicon carbide (SiC) in medium power 6Advantages in 6kw and high power 11 and 22kw bidirectional on-board chargers.
As the market share of pure electric vehicles continues to increase, the installed capacity of power batteries is also increasing, and consumers are also demanding faster charging times for high-capacity batteries, which has also led to an increase in the operating voltage of batteries from 400V to 800V.
Electric vehicles with sufficient battery capacity will have the potential to act as an energy storage system to realize various power supply scenarios from vehicles to other electrical devices, such as vehicle-to-home, vehicle-to-grid, and vehicle-to-vehicle charging, etc., so OBC is changing from a one-way topology to a two-way topology, and the adoption of two-way OBC in electric vehicles in the future is a common trend.
EV on-board charger designs require high power density and high conversion efficiency to make the most of the limited available vehicle space and minimize volume and weight. The bidirectional OBC front end consists of a bidirectional AC-DC converter, typically a power factor correction PFC circuit or active front-end AFE circuit, and an isolated bidirectional DC-DC converter on the back end.
01.PFC or AFE module
At the input, the traditional PFC boost converter is the most widely used unidirectional topology, but it does not support bidirectional operation, whereas totem-pole PFC not only supports bidirectional operation, but also improves efficiency by eliminating the bridge rectifier stage, reducing the number of semiconductor devices in the conduction path from three to two.
The totem pole PFC contains two or more half-bridges operating at different frequencies, the high-frequency bridge arm is boosted and rectified to improve frequency switching, and the low-frequency bridge arm mainly rectifies the input voltage and switches at a frequency of 50-60Hz.
02.DC DC converter module
The DC converter in a one-way on-board charger is typically an LLC resonant converter, but this is a one-way topology where the voltage gain of the converter is limited in reverse operation mode, which reduces its performance. As a result, the bidirectional CLLC resonant conversion car in Figure 3 is better suited to the DC DC stage of the bidirectional OBC, which has high efficiency and a wide voltage range in both charge and discharge modes.
In EV on-board charger applications, CLLC resonant converters use soft switching to improve efficiency, with zero-voltage turn-on (ZVS) on the primary side and a combination of ZVS+ZCS on the secondary side.
03.Advantages of SIC:
SiC SiC is the device of choice for high-power OBC due to its unique combination of high critical electric field, high electron drift velocity, high temperature and high thermal conductivity, and at the transistor level, the SIC has low on-resistance and low switching losses, making it ideal for high-current and high-voltage applications.
In addition to the SiC, there are two other options for active components in high-power designs, including silicon SI MOSFETs and IGBTs, and for high-power applications in totem-pole PFCs, the reverse recovery of the SI MOSFET diode results in high power losses in continuous conduction mode, so its use is limited to discontinuous mode operation and low-power applications.
In contrast, SiC MOSFETs allow totem-pole PFCs to operate in continuous conduction mode to achieve high efficiency, low EMI, and higher power density.
04.Medium power 66KW bidirectional OBC architecture
The mid-power bidirectional OBC typically operates with a single-phase 120V or 240V input and a 400VDC bus, with a topology pre-amp being a single-phase totem-pole PFC and a CLLC DC DC converter on the post-amp, as shown in Figure 4.
For 6The table below summarizes the device selection for a bidirectional OBC design with two 60m MOSFETs in parallel or a 25m MOSFET in PFC and a 60m or one 45m MOSFET in DCDC.
05.High power 11kW or 22kW bidirectional OBC design
At higher power levels such as 11kW or 22kW, the battery voltage can be 400V or 800V, and the market is currently moving towards 800V high voltage platforms, Figure 5 shows a system block diagram of a high-power three-phase bidirectional OBC, which is designed to be compatible with 400V or 800V batteries.
The 11kW bidirectional OBC design can use 75M 1200V MOSFETs for the primary side of PFC and CLLC converters, on the secondary side, 800V battery applications use the same 75M MOSFETs as primary, 40M 1200V MOSFETs can be used for high performance applications, and for 400V battery applications, four 650V 25V MOSFETs can be selected as the secondary side.
The 22kW design is similar to the 11kW bidirectional OBC design, but the higher power output requires a lower RDS(ON) device, and a 32m 1200V MOSFET can be used for the primary side of the PFC and DCDC, and the same secondary side can be used for 800V bus applications. Table 2 summarizes the device selection for a three-phase, high-power OBC design.
Three-phase power is readily available in many European homes, but typical American, Asian, and South American homes only have standard single-phase 240V, in which case the high-power 22kW bidirectional OBC needs to be compatible with both single-phase and three-phase inputs, designers can use interleaved technology for single-phase inputs, adding a fourth arm to a traditional three-phase PFC.
Figure 6 shows a staggered totem-pole PFC with three high-frequency arms and a fourth low-frequency bridge, each of which is delivered by a 32m 1200V SiC MOSFETWith 6kW of power, the LF arm can be reduced by using two SI or IGBTs, and when three phases are available, the circuit can be automatically reconfigured to operate with three phases, leaving the fourth arm in the air.
In bidirectional OBC, SiC-based solutions outperform SI-based solutions in terms of cost, size, weight, power density, and efficiency.
For example, a 22kW bidirectional OBC SiC-based solution requires 14 power devices and 14 gate drivers, and an SI-based design requires 22 power devices and 22 gate drivers. When comparing performance, the SiC design achieves an efficiency of 97% and a power density of 3kW L, while the SI design achieves an efficiency of 95% and a power density of 2kWl.
On-board charger is a necessary installed component of electric vehicles, Dillon New Energy Technology Hebei *** through the use of SiC devices with low on-resistance, low output capacitance and low source inductance, the perfect integration of low switching loss and low conduction loss, improve the power density and conversion efficiency of the company's OBC products, have a higher switching frequency, reduce the number of components, and reduce the size of components such as inductors, capacitors, filters and transformers, and reduce the cost of OBC.