Review of High Power Isolated Bi-directional DC-DC Converters for PHEV/EV DC Charging Infrastructure Yu Du Srdjan Lukic Boris Jacobson, Alex Huang (Student Member, IEEE) (Member, IEEE) (Senior Member, IEEE)(Fellow, IEEE)FREEDM Systems Center FREEDM Systems Center Raytheon Company FREEDM Systems CenterNorth Carolina State University North Carolina State University Sudbury, MA, USA North Carolina State University Raleigh, NC, USA Raleigh, NC, USA boris_s_jacobson Raleigh, NC, USAydu4@ smlukic@ @ aqhuang@Abstract—PHEV/EV DC charging infrastructure attracts more and more attention recently. High power isolated bi-directional DC-DC converters provide galvanic isolation, V2G capability and reduce the cost and footprint of the system. Maintaining high power efficiency in wide vehicle battery pack voltage range is required. Three full bridge based high power bi-directional DC-DC converters are conceptually designed for this application and their advantages and disadvantages are addressed. Experimental test bench is built and efficiency evaluation for bi-directional operation is reported.I.I NTRODUCTIONElectric power and transportation industries are two major sources for primary energy consumption on Earth. For example, according to the United States Energy Information Administration, the primary energy flow by source and sector in 2009 is shown in Fig. 1. The total energy consumption of United States in 2009 is 27.7 trillion kilowatt hours (kWh), with about 11.2 trillion kWh or 40% in electric power sector and about 7.9 trillion kWh or 29% in transportation sector. For transportation sector, 94% of the supply source is from petroleum and only 3% is from renewable energy. 72% of petroleum is used for transportation sector and about 63% of crude oil relies on import for United States in 2009 [1]. The majority of the energy sources such as petroleum, natural gas, coal are nonrenewable or not environment-friendly. The associated energy shortage, green house gas emission and energy security issues are well known.Plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) are considered as one of the potential solutions for these issues. Based on EPRI’s prediction, 60% of U.S. fleet will be PHEVs by 2050. With increased penetration of PHEVs and EVs, and increased capacity of the on-board battery energy storage system for extended EV range, DC charging infrastructure (Level III according to SAE J1772) becomes more and more important. Galvanic isolation is required by regulation and safety requirement which is either in low frequency (50/60Hz) or high frequency side [2]. High frequency isolation brings higher power density, lower system cost and smaller footprint, although non-isolated DC-DC converters are simpler and cheaper than the isolated DC-DC converters [2-3]. For potential V2G (vehicle-to-grid) operation which uses mass PHEV batteries as energy storage devices for grid support, bi-directional power flow is required. A few bi-directionalDC-DC converters reported in the literatures are reviewed and categorized. Three high-power isolated bi-directional DC-DC converters are studied in detail. Conceptual design is conducted and their advantages and disadvantages are analyzed for the application in DC charging infrastructure. The input is 750V and output ranges from 300V-600V with 35kW rated power. Scale down lab test bench is built and test results are reported.II.R EVIEW OF THE B I-DIRECTIONAL T OPOLGOIESA.Isolated Bi-directional Topology CategorizationIn general, the bi-directional DC-DC converter topologies that can be potentially used for high power Fig. 1: The primary energy flow by source and sector of USA in 2009 according to the U.S. Energy Information Administration [1]This work made use of ERC shared facilities supported by the National Science Foundation under Award Number EEC-0812121.applications can be categorized to current-fed, voltage-fed and the combinations of them, as shown in Fig. 2. Generally bi-directional DC-DC converters consist of a high frequency inverter, a high frequency transformer with leakage inductance which may be either preferred or not preferred in different topologies, and a high frequency rectifier. Bi-directional DC-DC converter can be directly connected to the voltage source or the DC capacitor bank, in a voltage-fedmanner, or with additional DC inductor in between, in a current-fed manner.The major high frequency inverter and the correspondinghigh frequency rectifier topologies which can be used for bi-directional operation are shown in Fig. 3. These topologies include full-bridge inverter and rectifier, half-bridge inverter and rectifier, push-pull inverter and center-taped rectifier, L-type half-bridge inverter and current-doubler rectifier. The voltage stress of switches in full bridges are same as that inhalf bridges, but the current stress of switches is only half of that in half bridges. The rest two inverter/rectifier topologies are more suited for low voltage high current applicationsdue to the low current stress and high voltage stress of switches.B. Review of Literature Reported Isolated Bi-directional DC-DC Converters One of the widely used topology is the current-fed and voltage-fed full-bridges and its derivations [4-11], as shownin Fig. 4-9. Fig. 4 shows a typical structure of this type of converter [4]: one full bridge is connected to voltage source or DC capacitor, which is called voltage-fed, and the other full bridge is connected to current source or DC inductor, which is called current-fed. This topology was proposed to interface between the low voltage battery and high voltage DC-link for the motor drive in a hybrid vehicle. A 1.5kW prototype was built to evaluate the converter performance. However, this topology is not practical when used in high power applications, e.g. 35kW in this study. The reason isthat the transformer is not ideal and there is always leakage inductance and leakage energy stored in the transformer, which has to find some path to discharge and causes high voltage spike on switches in current-fed side during switching. When used in higher power level, this topology has been improved by several ways. Fig. 5 shows that a RCD snubber circuit is added into the current-fed side full bridge to control the switch voltage stress [5], compared to the topology in Fig. 4. In reference [6], similar approachFig. 2: Topology categorization for bi-directional DC-DC convertersbased on voltage-fed and current-fed input/output(a) Major topologies for high frequency inverter(b) The counterpart topologies for high frequency rectifierFig. 3: Major high frequency inverter and counterpart rectifier topologieswhich are used for bi-directional operationFig. 4: Bi-directional DC-DC converter based on a voltage-fed full bridgeand a current-fed full bridgewas employed to reduce the voltage stress of switches in current-fed side but lossless snubber was used instead, as shown in Fig. 6. In reference [7-9], still based on the same idea, an active clamp snubber circuit S c and C c were added into the current-fed side full bridge, as shown in Fig. 7. The addition of active clamp provides several advantages: ZVS is achieved for all the switches in current-fed side; and ZVS or ZCS is achieved for all the switches in voltage-fed side; and there is no circulating current which helps to increase the efficiency compared with conventional phase-shift controlled full bridge converter. One major disadvantage of this converter is that the voltage stress of current-fed side switches is higher than the source voltage V ds, which indicates that the topology in Fig. 7 is better suited for applications with lower source voltage in current-fed side. Fig. 8 shows one derivation of this type of topology [10]. In this topology, a half bridge was employed in voltage-fed side instead of a full bridge. In Fig. 9 a voltage-fed and current-fed half-bridge converter was reported in [11]. This topology doesn’t need snubber because S1 can act as an active clamp switch and there is no need to add additional snubbers to reduce the voltage stress of S3 and S4.Another type of topology that is widely used in industry for bi-directional operation is dual active bridges (DAB) [12-17], as shown in Fig. 10. In Fig. 10, dual active bridges consist of two voltage-fed full bridges, which are directly to interface with two DC voltage sources. Example applications of this topology can be found in energy storage systems and motor drives [13-15]. The control of dual active bridges is very flexible. For instance, either, one bridge is Fig. 10: Bi-directional DC-DC converter based on two voltage-fed fullbridgesFig. 11: Bi-directional DC-DC converter based on a voltage-fed halfbridge and a voltage-fed full bridgeFig. 12: Bi-directional DC-DC converter based on two voltage-fed halfbridgesFig. 5: Bi-directional DC-DC converter based on a voltage-fed full bridge and a current-fed full bridge with RCD snubber Fig. 6: Bi-directional DC-DC converter based on a voltage-fed full bridge and a current-fed full bridge with lossless snubberFig. 7: Bi-directional DC-DC converter based on a voltage-fed full bridge and a current-fed full bridge with active clamp Fig. 8: Bi-directional DC-DC converter based on a voltage-fed half bridge and a current-fed full bridge with active clampFig. 9: Bi-directional DC-DC converter based on a voltage-fed half bridge and a current-fed half bridgephase-shift controlled and the other is uncontrolled (only anti-parallel diodes conduct), or, both bridges output a square voltage waveform and the phase between two voltage square waveform can be controlled. DAB topology has several advantages: ZVS can be achieved for switches in both bridges; the number of switches is lower compared with voltage-fed and current-fed full bridges such as Fig. 7; low voltage stress of switches compared to current-fed full bridges; high efficiency can be achieved. One major disadvantage for this topology is that at light load the ratio of reactive power to active power is high and so it is difficult to achieve high efficiency in this scenario [14]. The other disadvantage is that the voltage range for optimal operation is narrow. Dual active bridges also have some derivations, such as the combination of a voltage-fed full bridge and a voltage-fed half bridge as shown in Fig. 11 [16], or the combination of two voltage-fed half bridges as shown in Fig. 12 [17].Other topologies under survey include the improved full-bridge converter with auxiliary resonant network to facilitate soft switching [18] and L-type full bridges with two inductor rectifier which is more suitable for low voltage high current applications [19, 20]. The bi-directional series resonant DC-DC converter does not fall into above categories but it is worth studying.For high power (35kW for instance) DC charging infrastructure, since the current stress of device is smaller and it is easier to achieve soft switching, three full bridge based ZVS topologies are selected for further study: the bi-directional series resonant DC-DC converter with clamped capacitor voltage, dual active full bridges (DAFB) as shown in Fig. 10, and voltage/current-fed full bridge with active clamp (VCFFB) as shown in Fig. 7.III. V OLTAGE AND C URRENT F ED F ULL -BRIDGE (VCFFB)C ONVERTER The voltage and current fed full-bridge bi-directional DC-DC converter is shown in Fig. 7. It consists of one voltage-fed full bridge S1-S4 in high voltage side, one current-fed full bridge S5-S8 with inductor L and active clamp circuit S9/C3 in low voltage side, and one high frequency transformer with the leakage inductance of L lk .The selection of the transformer turns ratio N and the leakage inductance L lk should meet the wide voltage range requirement of V s . It can be designed based on either the forward mode or reversed mode operation and then check if the design meets all the specifications. In this paper, the design is based on reversed mode operation.Based on the voltage-second balance of the inductor L, the voltage across clamp capacitor C3 is,s c V DV ⋅−=11(1)where D is defined as the ratio of simultaneous conduction time of S5-S8 to the half switching cycle. Based on the current-second balance of the clamp capacitor C3, neglect the ripple current of inductor L, the peak current through the leakage L lk is,L pk Llk I I ⋅=2_ (2)The fall time of the leakage inductor current ∆t 0 is,2)1(2)1()1(0s d s s d c TD V V N T D V V N t ⋅+−⋅=⋅−⋅−⋅=Δ (3)Where V c is the clamp capacitor voltage and T s is theswitch cycle of S5-S8. The low voltage side input current is,lks d s d L s L f ND V V V I I −−⋅==4)1( (4)where f s is the switching frequency of S5-S8.The power transferred from low voltage side to high voltage side in reversed mode is,lks c d s lk s s d s lk s d s d s L f NV V V L f N V D V V L f NDV V V V P o ⋅⋅−=⋅⋅−−=⋅⋅−−=4)1(4])1(1[4)1(22 (5)Therefore, the transformer turns ratio is,lks o s ds L f P V V V D N ⋅⋅⋅−⋅⋅−=4)1(2 (6)In order to decrease the current in leakage inductance during S5-S8 simultaneous conduction period such that ZCS can be achieved for S5-S8, based on (3),22)1(0ss dsT D T D V V N t ⋅<⋅+−⋅=Δ (7)25.1600750max_==<s d V V N (8) From (5) in order to reverse the power flow,Fig. 13: The required region of N and L lk (shaded area))1(N or 0)1(1D V V N V D V sd s d −>>−− (9)Practically there should be a minimal D to charge theinductor L, for example, D min =0.2. Based on (6), the transformer turns ratio can be calculated with different D and leakage inductance L lk . To limit the voltage stress of the switches in current fed side, the maximal clamp capacitor voltage has to be limited, for example, to 1000V if 1200V IGBTs are employed. From (1) D max is 0.4 if V s =600V and D max is 0.7 if V s =300V. The calculated transformer turns ratio N is calculated with P o =35kW and selected D for 300V and 600V battery pack voltage. The values for transformer turns ratio N and the leakage inductance L lk in the shaded area in Fig. 13 can output 35kW power in 300V-600V wide battery voltage range. Therefore, the transformer turns ratio N can be selected as 1 for simplicity and the leakage inductance should be blow 3uH, for example, L lk =2uH which is a practical value for the transformer design.The voltage stress of the current-fed side switches are shown in Fig. 14, indicating high voltage stress of S5-S9.In forward mode operation, S5-S8 are turned off with their anti-parallel diodes as rectifier. S1-S4 are modulated in thephase-shift manner to control the output power. The active-clamp circuit S9 and C3 absorb the energy in leakage inductance and limits the voltage stress of D5-D9.IV. D UAL A CTIVE F ULL -BRIDGE (DAFB) C ONVERTER The dual active bridge bi-directional DC-DC converter is shown in Fig. 10. It consists of two voltage-fed full bridges S1-S4 and S5-S8. The power transferred by the dual active full bridges is,1(2πφφπ−⋅⋅⋅⋅⋅⋅⋅=lk s sd s L f V N V P (10)where Ф is the phase shift angle in radian between the squarewave voltage outputs of the two full bridges. Define,lks dbase L f V P ⋅⋅⋅=π22 (11)sd V N V d ⋅=(12)So the normalized output power is,1(1πφφ−⋅⋅=d P pu (13)If the reactive power is defined as the product of rmscurrent and rms voltage of the leakage inductance, the ratio of reactive power to the real power is,)(6])1(4[)]2()()1()23()1[(322222πφφπφπφπφ−⋅⋅⋅−+⋅+−−+−+=d d d d d PQ (14)V s is in the wide voltage range, for example, 300V-600V. When the output power is 35kW, based on (12) the phase shift angle for 300V output is larger than 600V output. And when a maximal angle is selected for 300V V s , there exists one maximal angle for 600V V s , to regulate the power up to 35kW. For 2:1 V s range, the relationship between these two angles is,2)(22L L Hφπφππφ−−−=(15) Three pairs of maximal phase shift angles for 300V output and 600V output are selected, in the form of [phi_L,Fig.14: Clamped Capacitor Voltage in Reversed Mode OperationFig. 16: The ZVS region of the DAFB converterFig. 15: The selection of the transformer turns Ratio Nphi_H] such as [0.60, 0.27], [0.90, 0.36] and [1.20, 0.43]. And the ratio of the reactive to real power Q/P is described in Fig. 15 in terms of different transformer turns ratio N. N is selected such at the reactive power in the circuit is same at 300V and 600V V s at 35kW condition. It can be found that the transformer turns ratio N should be around 2. It can be also found that the selection of N to minimize the reactive power for either 600V or 300V V s will greatly increase the reactive power at the other output voltage. Therefore, the required leakage inductance is,)(3.261(2uH P f V N V L s s d lk =−⋅⋅⋅⋅⋅⋅⋅=φφ (16)The ZVS boundary for S1-S4 can be determined by,04)2(0=⋅⋅⋅⋅⋅−⋅−⋅=lks dL f d d V I πφππ (17) The ZVS boundary for S5-S8 can be determined by,04)2(1=⋅⋅⋅⋅⋅−⋅⋅+⋅=lks dL f d d d V I ππφπ (18) Based on (11) and (13), P base is 68kW and the normalized power rating P (pu) is 0.515. When V s is 600V, primary side switches S1-S4 loses the ZVS condition in the full load range and there is no problem with S5-S8. When V s is 300V, secondary side switches S5-S8 loses ZVS below half load and there is no problem with S1-S4. The zero voltage switching (ZVS) operation of the dual active full bridge converter based on the charging specifications is shown in Fig. 16. It can be found that ZVS condition is lost for high V s voltage and light load range.The transformer primary and secondary winding current stress is shown in Fig. 17, which is an indication of the current stress of components in the circuit. The current stress is similar at heavy load for 300V and 600V V s . However, at light load condition, there is a lot of circulating current in the circuit, which will dramatically decrease the light load efficiency. Also, the circulating current at 600V V s is much higher than that of 300V. It should be noted that the output V-I characteristic of dual active full bridges is a current source.V.B I -DIRECTIONAL S ERIES R ESONANTC ONVERTERWITH C LAMPED C APACITOR V OLTAGEFig. 18 shows a bi-directional series resonant DC-DC converter with clamped capacitor voltage. Reference [21]-[26] describe the design of the power stage and the phase-shift modulation strategy for the reversed mode operation. For 35kW power rating, the resonant inductor L1 and L2 is 11.6uH each, which can utilize the high frequency transformer leakage inductance. The resonant capacitor is 0.9uF. The transformer turns ratio is N1:N1:N2=1:1:2.The transformer primary and secondary winding RMS current, which is an indication of the current stress of the components in the circuit, is shown in Fig. 19. Compared with the dual active full bridges, when V s is 300V, the current stress of the secondary side is similar to that of dual active full-converter in forward mode. In reverse mode, the current stress increases about 20%. The current stress in the primary side is almost two times as that in dual active bridges. However, when V s is 600V, the primary side current stress of resonant converter is similar to that in dual active bridge at full load but lower at light load. The secondary side current stress of resonant converter is much lower than that in dual active full-bridge converter. There is much smaller circulating current at light load for the bi-directional resonantFig. 17: The transformer winding current stressFig. 19: The current stress of the transformer windingsFig. 18: bi-directional series resonant DC-DC converter with clampedcapacitor voltageconverter. It should be noted that the current stress in T1 and T2 is similar in forward mode and might be different in reverse mode operation due to the modulation strategy.ZVS can be obtained in the wide V s voltage and load range, and ZVS is lost at light load conditions. The output V-I characteristic of bi-directional series resonant DC-DC converter is dual-slope shape, which is voltage source at normal load range and current source when output current is higher. This feature intrinsically limits the short circuit current in load side when there is a fault.VI.S UMMARY OF THE H IGH P OWER B I-DIRECTIONALDC-DC T OPOLOGIESAll three high-power isolated bi-directional DC-DC converters can be used in the PHEV/EV DC charging infrastructure. But they have pros and cons in terms of the number of components, voltage and current stress, light loadcirculating current, soft switching range, output faulttolerance, control complexity, etc. which should be considered for the specific application. They are compared inTable I.To evaluate the performance of the high power isolated bi-directional DC-DC converters, a 20kW scale down experimental test bench is built and shown in Fig. 20. A Rogowski coil is inserted into the IGBT setup with copper DC-bus plates to accurately measure the switching loss ofthe converter. The IGBT under test is APTGF300A120G (1200V/300A) and the tested turn-off energy at 85°C with 50nF capacitor snubber and without the snubber is plotted inFig. 21. With 50nF capacitor snubber the turn-off loss can be reduced by about 40%.The power loss model was built for each of the components in the power stage, such as IGBT conduction loss, switching loss (measured by Rogowski coil), diode conduction and turn-off loss, winding and core loss of inductors and transformer, driver loss, capacitor loss and cable loss. The accuracy of the efficiency calculation was verified by the efficiency test results of 20kW experiment prototype. The loss model is used to estimate the 35kW efficiency for DAFB and bi-directional resonant converter.Table I: Summary of the Three DC-DC ConvertersVCFFBDAFBResonant Switchingfrequency (kHz)50 50 50 ConstantfrequencyYes Yes Yes CirculatingcurrentNo Yes No Number ofIGBTs9 8 8 Number ofDiodes9 8 12 Voltage Stress ofS1-S4 (V)750 750 750 Voltage Stress ofS5-S8 (V)920 600 600 Resonant/Seriesinductor (uH)2.0 26.3 2*11.6 Resonantcapacitor (uF)0 0 0.9 Output filterinductor (uH)100 0 0Soft switching ZCS/ZVS ZVS ZVSShort-circuitcurrent limitNo Yes Yes Voltage stress High Low LowForward Output V-I characteristic VoltagesourceCurrentsourceDual V-IslopesControl complexity Moderate Simple ComplexFig. 20: Lab test bench of isolated bi-directional DC-DC ConvertersFig. 21: The measured IGBT turn-off loss with and without capacitorsnubber at 85°C by Rogowski coil for switching loss estimationFig. 22: Estimated efficiency of DAFB and resonant Converters at35kW in wide voltage rangeThe evaluated efficiency at 35kW full load of both forward and reverse mode operation for DAFB and resonant DC-DC converters in both forward and reverse mode operation is shown in Fig. 22. 94.5%-97.5% efficiency can be obainted in 300V-600V battery side voltage range at full load condition.A CKNOWLEDGMENTThis work is sponsored by Raytheon Company and the authors wish to acknowledge Raytheon for the financial support. 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