Designing a SiCMOSFET Based 66kW BiDirectional EV OnBoard ChargerSeptember 11, 2019 by Chen Wei
This article discusses how to design a SiC-MOSFET Based 6.6kW Bi-Directional EV On-Board Charger.
In accordance with the world moving towards cleaner fuel alternatives, the EV transportation segment is experiencing rapid growth. Further, EVs equipped with a sufficient battery capacity can potentially be used to support standalone loads (V2L), and to supplement grid power (V2G). Thus, the design trend of the EV’s OBC is a transition to bidirectional operation capacity.
In order to optimize EV space and weight, an OBC design requires high power density and maximized efficiency. The bi-directional OBC consists of a bidirectional AC-DC converter followed by an isolated bi-directional DC-DC converter. The conventional LLC resonant converter was originally proposed as a solution to improve the efficiency of the DC-DC converter. However, given its unidirectional design, in reverse operation mode voltage gain of the converter was limited, thus the intended advantages of the converter could not be realized  - . Subsequently, a bidirectional CLLC resonant converter  -  was selected for the DC-DC stage, as it was found to provide high efficiency and a wide output voltage range in both charging and discharging modes.
The most popular single-phase PFC topology is a conventional PFC boost converter. Unfortunately, conduction losses of the diode bridge rectifier are not efficient, nor does it support bi-directional operation . Next, the totem pole bridgeless PFC boost converter was considered in order to reduce diode numbers and increase efficiency , . However, reverse recovery of the body diode of silicon MOSFETs resulted in high power losses in continuous conduction mode (CCM) making them impractical for high power applications. Following, lGBTs paralleled with SiC Schottky diodes were considered to replace silicon MOSFETs in CCM totem pole PFC and CLLC converters . Sadly, the practical switching frequency is limited due to the high switching loss of IGBT. Further, the objective of a lighter OBC with higher power-density is negatively affected by the weight and size of the magnetics and resonant tank, as well from the additional anti-parallel SiC diodes.
Owing to the favourable reverse recovery performance of the body diode of SiC MOSFETs, interleaved CCM totem pole PFC is enabled as the front-end stage of a 3.3 kW OBC . For high-power density and simple control, a single-phase single choke CCM totem pole PFC solution was selected for this design.
For thermal management, MOSFETs in TO-247 package are normally reverse-assembled on the PCB in OBC applications. They are then mounted on a flat cooling baseplate. However, when the MOSFETs are bent down PCB area is increased, thus negatively impacting the overall power density of the system.
The proposed method is utilization of a tooled heat sink which accommodates both semiconductors and magnetics. Power semiconductors are mounted on the outer side of the heat sink, allowing for vertical MOSFET assembly, thus reducing PCB footprint. Magnetics are then potted using a thermal compound inside the slots of the heat sink. Thermal resistance from the tooled aluminum heat sink to the system cooling baseplate is low. As an example, a SiC MOSFETbased 6.6 kW bi-directional OBC was designed. The experimental results for the converter operating in the charging mode and discharging mode manifest both high efficiency and high power density.
Specifications and Architecture of a Bi-Directional OBC Bidirectional OBC Specifications
Major design specifications of the 6.6kW bi-directional on-board charger
- AC input/output Voltage 90 – 265AC
- DC input/output Voltage 250 – 450VDC
- Rated Power 6.6kW charging; 3.3kW discharging
- Peak Efficiency > 96.5% charging and discharging
- Baseplate temperature 65°C
- PCBA dimensions 220x180x50mm
Block Diagram, DC-link Voltage and Switching Frequency Selection
Figure 1 shows the system block diagram of the bidirectional OBC.
An OBC design with a 500-840V variable DC-link for 250-450V battery voltage based on 1200V SiC MOSFET was demonstrated . Overall efficiency of the OBC is optimized, however, 1200V SiC MOSFET cost is high. Power loss of PFC MOSFET and PFC choke is also elevated with high DC-link voltage. Two 500V or 450V rated E-caps in serials are required for the 840V DC link design. The size of the DC link capacitors and PFC choke are larger.
Totem Pole PFC DC Link 385-425V Bi-Directional DC/DC Battery 250-450
450 V Ecap, which is commonly used in the industry, is optimized with smaller size and low cost. When using 450V Ecaps not in serials, the DC-link voltage is maxed at 425V. A 385V DC link is the minimum voltage to maintain adequate power for an AC input, up to 265VAC. In this design, as shown in Fig. 2, the DC-link voltage of the OBC is variable from 385 to 425V, to allow the CLLC converter a smaller required gain range and better efficiency over the 250V-450V battery voltage range compared to a fixed 400V DC-link in charging mode.
To attain a balance between power density, efficiency, thermal performance, and conducted EMI, 67kHz is selected for the switching frequency of the high frequency half bridge Q1 and Q3 of the totem pole PFC.
To achieve high power density and efficiency, 200kHz is selected for the CLLC converter resonant frequency, and 150 to 300kHz for the frequency range. It is a trade-off between power density, efficiency and thermal performance. For a light load at low output voltage, combined PFM and phase shift control are applied in this design.
Power MOSFET Selection
A fast reverse recovery body diode is required for efficiency and reliability for both the CCM totem pole PFC and bidirectional CLLC resonant converter. A smaller Coss is preferred for high frequency hard-switching operations for the totem pole PFC, and it is also critical for achieving zero voltage switching (ZVS) with a lower magnetizing current and a shorter blanking time for the CLLC resonant converter. With reduced magnetizing current, the conduction loss and turn-off switching loss of the MOSFET can be minimized. That is important for optimizing the efficiency of the CLLC converter, particularly at high frequencies. The maximum DC-link voltage is 425V and the battery is 450V. In consideration of voltage de-rating reliability requirements, a 650V SiC MOSFET is preferred in the OBC application.
In order to deliver 6.6kW output power, C3M0060065D 650V 60mohm SiC MOSFET in TO-247 package, two parts in parallel are selected for the high frequency half bridge of CCM totem pole PFC. A single C3M0060065D is selected for the low frequency half bridge of PFC and both the DC-link side and the battery side of the CLLC resonant converter.
Digital Controller Selection
Digital controller TMS320F28377D was chosen to implement the flexible control of both the totem pole PFC and CLLC converter of the OBC in charging and discharging modes. As shown in Figure 1, TMS320F28377D provides 12 independent PWMs G1-G12 to MOSFETs Q1-Q12 for the totem pole PFC and CLLC converter. The digital controller also handles the real-time CAN communication, start-up sequence, OCP, OTP, UVP and OVP.
Magnetics and Key Parameters
The PFC choke is designed to keep the totem pole PFC current ripple under 40%. The maximum current ripple occurs at low line, high battery voltage and full load. The minimum required inductance is 75µH, which is calculated in the following equation.
Considering the permeability degrade with DC bias, 230µH is selected for the PFC choke without DC bias. To get a balance between core loss and DC bias capability, the choke is fabricated with 2 stacks of KAM185-060A cores. The winding consists of 36 turns of 2-strand AWG-13 magnet wires.
The main transformer of the CLLC converter is designed to meet the requirements of both 450V/14.67A and 366V/18A output. The maximum flux density and core loss are designed for and verified at 425V DC-link and 450V/14.67A output. The winding wire size is designed for maximum current conditions with a 366V/18A output. With a bobbinless design, the window area of the core can be fully utilized. Next, a PQ5040 core using 3C97 material is selected for the 6.6kW CLLC converter. To meet the gain range requirements for 250V450V battery in both charging and discharging mode, a 15:14 turn ratio is selected. A 60µH magnetizing inductance is selected to ensure the ZVS of CLLC MOSFETs.
In this article, a 6.6kW bi-directional OBC on SiC MOSFET is designed and evaluated. The DC-link voltage range is optimized to 385 to 425V per a common battery voltage range of 250 to 450V for an OBC. A prototype based on engineering samples of 650V 60mohm SiC MOSFET C3M0060065D is built to verify the performance and thermal integrity of the design. 54 W/in3 power density and above 96.5% peak efficiency in both charging and discharging mode are demonstrated by the prototype with 67 kHz for the CCM totem pole PFC converter and 150-300 kHz for the CLLC resonant converter.
By integrating the power semiconductors and power magnetics on the same tooled heat sink, high power density and high efficiency can be achieved in bi-directional high power conversion applications such as OBC for EV, owing to the low power loss of the 650V SiC MOSFET.
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About the Author:
Chen Wei is working at Wolfspeed China, a Cree Company, in its Power Applications Division.
Dongfeng Zhu is working at Wolfspeed China, a Cree Company, in its Power Applications Division.
Haitao Xie is working at Wolfspeed China, a Cree Company, in its Power Applications Division.
Jianwen Shao is an experienced Power Applications Manager at Wolfspeed US, A Cree Company. He has 20 years of experience in the design and application of power electronics and embedded systems.