A 6.
6kW High Power Density Bi-directional
EV On-Board Charger Based on SiC MOSFETs
Chen Wei1, Dongfeng Zhu1, Haitao Xie1, Jianwen Shao2
1
Wolfspeed, a Cree Company, China
2
Wolfspeed, a Cree Company, United States
Abstract
This paper presents a SiC MOSFET-based 6.6 kW bi-directional Electric Vehicle (EV) on-board
charger (OBC), with high efficiency and high power density. A digital controlled prototype with
a switching frequency of 67kHz for CCM totem pole PFC, and 150kHz-300kHz for a CLLC resonant
converter is demonstrated with 54W/in3 power density exceeding 96.5% in peak efficiency. In order
to achieve a high power density, all SiC MOSFETs are mounted on a tooled AL heat sink, with all
magnetics potted internally. MOSFETs and magnetics meet thermal requirements on a 65°C
cooling plate.
Introduction
As the world moves towards cleaner fuel alternatives, the EV transportation segment is experiencing
rapid growth. Further, EVs equipped with a sufficient battery capacity have potential application in
supporting standalone loads (V2L), and supplement grid power (V2G). Thus, the design trend of the
EV’s OBC is a transition to bi-directional operation capacity.
In order to optimize EV space and weight, OBC design requires high power density and maximized
efficiency. The bi-directional OBC consists of a bi-directional 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 [1]. 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 [2] – [3]. Subsequently, a bi-directional CLLC
resonant converter [3] – [4] was selected in this paper 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.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
A conventional PFC boost converter is the most popular single-phase PFC topology.
Unfortunately, conduction losses of the diode bridge rectifier are not efficient, nor does it support
bi-directional operation [5]. Next, the totem pole bridgeless PFC boost converter was considered in
order to reduce diode numbers and increase efficiency [6], [7]. 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, insulated gate bipolar transistors
(lGBT) paralleled with silicon carbide (SiC) Schottky diodes were considered to replace silicon
MOSFETs in CCM totem pole PFC and CLLC converters [8]. Sadly, the practical switching frequency
is limited due to the high switching loss of IGBT. Further, the objective of a lighter, more power-dense
OBC 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 favorable 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 [9]. With its ability to achieve
high-power density and simple control, a single-phase single choke CCM totem pole PFC solution is
selected for this paper.
For thermal management, MOSFETs in a 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 MOSFET-based 6.6 kW bi-directional
OBC is designed. The experimental results for the converter operating in the charging mode and
discharging mode manifest both high efficiency and high power density.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
The Specifications and Architecture of a
Bi-Directional OBC
Bidirectional OBC Specifications
Table 1 provides the major design specifications of the 6.6kW bi-directional on-board charger.
Table 1: Specifications of a 6.6kW bidirectional OBC
Block Diagram, DC-link Voltage and Switching Frequency Selection
Figure 1 shows the system block diagram of the bidirectional OBC.
Figure 1: System block diagram of a bi-directional OBC
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
An OBC design with a 500V-840V variable DC-link for 250V-450V battery voltage based on 1200V
SiC MOSFET is demonstrated [10]. 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. The size of the DC link capacitors and PFC choke are larger for the 840V design. 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 385V 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.
Figure 2: DC-link voltage vs. battery voltage
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 150kHz 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.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
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. A smaller Coss 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 Fig. 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 75uH, which is calculated in Eq. (1).
Considering the permeability degrade with DC bias, 230uH 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 [11]. The winding consists of 36 turns of 2-strand AWG-13 magnet wires.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
Figure 3: Distributed air gaps at center leg of the transformer.
The main transformer of the CLLC converter is designed to meet the requirements of both
450V/14.67A and 366V/18A output. The max 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 bobbin-less 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 250V-450V battery in both charging and
discharging mode, a 1:1 turn ratio is selected. As shown in Fig.3, to minimize the power loss caused
by eddy current, the distributed air gaps are designed at center leg of the core. 14 turns triple isolated
400-strand AWG-38 liz wire is designed as both the primary winding and the battery side winding.
A 60uH magnetizing inductance is selected to ensure the ZVS of CLLC MOSFETs.
Key design parameters are shown in Table 2.
Table 2: Key design parameters
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
Hardware Implementation and Test Results
As shown in Fig. 4, (including the input EMI filter, totem pole PFC, CLLC resonant converter, control
board, and auxiliary power board), a 6.6kW bidirectional OBC prototype is built to verify the design.
The PCBA dimensions are 220mm × 180mm × 50mm.
Figure 5 shows the integrated heatsink for both the semiconductors and magnetics. The power
semiconductors are mounted on the outer side of the heatsink. The magnetics are potted by thermal
compound inside the slots of the same heatsink. C3M0060065D engineering samples are used in
the prototype.
Figure 4: 6.6 kW bidirectional OBC prototype based on TO-247 650 V SiC MOSFET.
Figure 5: Photo of magnetics and power semiconductors sub-assembly.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
Key Waveforms
The testing waveforms of totem pole PFC and CLLC stage in charging mode under 380V battery
voltage, 230Vac input and full load are shown in Fig. 6 and Fig. 7 respectively.
Figure 6: PFC Waveform 6.6kW in charging mode Figure 7: DC/DC Waveform 6.6kW in charging mode.
The testing waveforms of DC/AC and DC/DC stage in discharging mode under 380V battery voltage
input and 220Vac output are shown in Fig. 8 and Fig.9 respectively.
Figure 8: DC/AC Waveform 3.3kW 220Vac Output Figure 9: DC/DC Waveform 3.3kW in discharging mode.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
Efficiency Test Result
The efficiency of the CCM totem pole PFC, DC/DC stage and the entire unit in charging mode
at 230Vac input are shown in Figs. 10, 11 and 12 respectively. With a lower DC-link voltage, the
totem pole PFC efficiency is higher. The unit level peak efficiency in charging mode exceeds 96.5%.
C3M0060065D engineering samples are used in the prototype.
Figure 10: CCM Totem Pole PFC Efficiency. Figure 11: DC/DC Efficiency.
Figure 12: Unit Efficiency in charging mode. Figure 13: DCDC Efficiency in discharging mode.
Figure 13 shows the efficiency of DCDC in discharging mode at 380V battery voltage. Figure 14
shows the efficiency of DCAC stage in discharging mode for 220Vac output. Figure 15 shows the
unit level efficiency in discharging mode for 220Vac output. The output power in the efficiency test
is up to 3.3kW in discharging mode here. The unit level peak efficiency in discharging mode is also
above 96.5%.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
Figure 14: DCAC Efficiency in discharging mode. Figure 15: Unit Efficiency in discharging mode.
Thermal Test Result
In the thermal test of the prototype, forced air cooling is applied to the cooling base plate at bottom
side to get a 65°C cooling plate to simulate the thermal condition in an OBC application. There is
no air flow to the power MOSFETs. C3M0060065D engineering samples are used in the prototype.
MOSFETs in the CLLC resonant converter operate in the worst case thermal condition with a
366V/18A load. The totem pole PFC converter operates in the worst case thermal condition with
a 450V/14.67A load. So, the thermal test was performed at a low line 215Vac and full load 6.6kW
with 366V/18A and 450V/14.67A in charging mode. T-type thermal couplers and KEYSIGHT 34972A
acquisition unit are used to measure the case temperature of components.
Table 3: Thermal Test Results.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
The thermal test results are shown in Table 3. The junction temperature is calculated based on the
measured case temperature, the thermal resistance of the MOSFET and the calculated component
power loss. The target maximum junction temperature of C3M0060065D is 175°C. Refer to the test
result, the max junction temperature of the SiC MOSFETs is 129.4°C at worst case in the application.
So, we can get the conclusion that, based on the integrated heat sink proposal, all of the MOSFETs
meet thermal de-rating requirement in this high-power density design.
Conclusions and Future Work
In this paper, a 6.6 kW bi-directional OBC on SiC MOSFET is designed and evaluated. The DC-link
voltage range is optimized to 385V to 425V per a common battery voltage range of 250V 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 pow-
er 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 650V SiC MOSFET. The future work is to improve
thermal management for transformer to reduce its temperature rise.
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
6 References
[1] Bo Yang; F.C. Lee; A.J. Zhang; Guisong Huang: LLC resonant converter for front end
DC/DC conversion, Proc. IEEE APEC, 2002, vol. 2, pp. 1108 – 1112.
[2] Sihun Yang; Masahito Shoyama; Toshiyuki Zaitsu; Junichi Yamamoto; Seiya Abe;
Tamotsu Ninomiya: Detail operating characteristics of Bi-directional LLC resonant converter,
ICRERA, 2012, pp. 1 – 6.
[3] Wei Chen; Ping Rong; Zhengyu Lu: Snubberless Bidirectional DC–DC Converter With
New CLLC Resonant Tank Featuring Minimized Switching Loss, Industrial Electronics,
IEEE Transactions on, vol. 57, no. 9, 2010, pp. 3075-3086
[4] Zheng Lv; Xiangwu Yan; Yukang Fang; Lei Sun: Mode analysis and optimum design of
bidirectional CLLC resonant converter for high-frequency isolation of DC distribution systems,
ECCE, 2015, pp. 1513 - 1520.
[5] J. P. M. Figuerido, F. L. Tofili, and B. L. A. Silva: A review of single-phase PFC topologies based
on the boost converter, in Proc. IEEE Int. Conf. Indus. Appl., Sao Paulo, Brazil, Nov. 2010, pp. 1-6.
[6] J. W. Lim and B. H. Kwon: A power factor controller for single phase PWM rectifiers, IEEE
Trans. Ind. Electron., vol. 46, no. 5, pp. 1035–1037, Oct. 1999.
[7] B. Su, J. M. Zhang, and Z.Y. Lu: Totem-pole boost bridgeless PFC rectifier with simple
zero-current detection and full-range ZVS operating at the boundary of DCM/CCM, IEEE Trans.
on Power Electron., vol. 26, no. 2, pp. 427-435, Feb. 2011.
[8] Jih-Sheng Lai; Lanhua Zhang; Zaka Zahid; Nan-Hsiung Tseng; Chi-Seng Lee; Chin-Hone Lin:
A high-efficiency 3.3-kW bidirectional on-board charger, 2015 IEEE 2nd International Future Energy
Electronics Conference (IFEEC), pp. 1 - 5.
[9] Yichao Tang; Weisheng Ding; Alireza Khaligh: A bridgeless totem-pole interleaved PFC
converter for plug-in electric vehicles, 2016 IEEE Applied Power Electronics Conference and
Exposition (APEC), pp. 440 - 445.
[10] Bin Li; Fred C. Lee; Qiang Li; Zhengyang Liu: Bi-directional on-board charger architecture
and control for achieving ultra-high efficiency with wide battery voltage range, APEC 2017, pp.
3688 - 3694.
[11] KAM, http://kda.com.cn/cn-products/Nanodust-core
4600 Silicon Drive
Durham, NC 27703 USA
Main: (919) 313-5300
