Abstract—This paper highlights the design aspects of the universal converter transformer as part of an effort done to use the universal converter for onboard charging applications for electric vehicles. Today's requirements of onboard charging ask for bidirectional follow capabilities and an ability to interface with various power grid types (single-phase, three-phase three-wires, and three-phase four-wire, etc.). Such requirements increase the difficulty of designing the transformer due to the high input to output voltage gain range of the DC-DC converter. The universal converter uses a dual active bridge that maintains consistent performance across all the operating regions. This paper provides the necessary steps of building the transformer and shows the experimental results to assess the proposed design method.
Keywords—dual active bridge, transformer design, onboard charging, electric vehicle
I. INTRODUCTION Single-phase and three-phase utility interfaced, isolated
ac-dc converters with power factor correction (PFC) cover a wide range of applications such as chargers for plug-in hybrid electrical vehicles and battery electric vehicles [1]. Bidirectional power flow is increasingly required for its ability to not only receive power, but also to trade active/reactive power [2], achieve voltage regulation [3] and active filtering [4]. The traditional electricity grid is evolving from a rather passive to a smart interactive system that allows for greater integration of new technologies to improve grid efficiency. A smart grid can monitor the usage and control operation during peak consumption. This allows for a much greater integration of sporadic renewable energy sources such as solar, wind, fuel cells, and other sources [5]. Connection to the electric power grid allows opportunities such as the ability to store surplus electricity generated from these intermittent sources in electric vehicle (EV) batteries during non- peak periods and feed power back to the grid when needed. EVs are usually parked for 90–95% of their total lifetime and with a bi-directional power flow, EV batteries can be used as an energy reservoir with improved efficiency over other power storage devices and faster response time than traditional power plants [6]. Figure 1 shows the schematic of the single-stage, bidirectional and isolated universal onboard charger topology, consisting of three-phase four wire power factor correction ac-dc converter followed by a dual active bridge dc-dc converter. The four wire PFC is connected to the grid. It allows a three-phase connection (three-wires and four-wires), one single-phase connection, or two single-phase connections for improved
current sharing. Following the PFC converter, the dual active bridge provides the isolation and voltage regulation to interface with the battery safely. Having an active secondary makes the converter bidirectional to realize vehicle-to-grid (V2G) and vehicle-to-live (V2L) applications. A dual active bridge dc-dc converter allows superior performance over the conventional LLC topologies when it comes to a bidirectional wide range on input to output voltage. On both the primary side and the secondary side, there is an electrolytic capacitor in parallel with a thin film capacitor to filter out the low frequency and the high frequency ripple components, respectively.
Figure 1: The universal converter topology.
II. DAB TRANSFORMER LEAKAGE INDUCTANCE REQUIREMENTS
The isolated dual active bridge (DAB) manages the power flow between the battery and the inverter with isolation that provide safety, a necessary feature. The topology consists of two full-bridges connected through a high frequency transformer. The transformer’s leakage inductance, , serves as an instantaneous energy storage device. The primary voltage of the transformer can be ± depending on the configuration of switches. Note that it is possible to create a zero voltage in the primary winding by utilizing the zero state. This additional control freedom allows achieving zero-current switching.
For a given switching frequency , leakage inductance , input voltage , and turn ratio n, the load voltage is proportional to the load resistance and is a function of the phase shift D. Furthermore, for a given load resistance, the maximum energy that can be transferred is by setting D to 0.5. We can also find the input-output voltage relationship:
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= (1 − )2
(1)
The detailed analysis is omitted here due to the size constrains. It shows that the DAB can be modeled as a gyrator and the dynamic between the primary and the secondary can be described as follows:
= 00 0
= (1 − )2
(2)
The transmitted power can also be expressed by the gyration ratio g as = . According to this analysis the required leakage inductance of the transformer can be expressed as:
= (1 − )2
(3)
Therefore, the maximum required value of can be calculated using , as the maximum PFC voltage (three phase case), the maximum load resistance of the battery equivalent circuit, the minimum operating power of the onboard charger, and the minimum operating switching frequency of the DAB.
III. TRANSFORMER DESIGN The Transformer design is based around obtaining the
figure merit , by which the window area and the cross-sectional area of the core are decided. The designer however, must take in consideration the relationship between the window size and the number of windings and . This relationship can be viewed by simulating the effect of these values on the cost, size, and efficiency of the transformer. The current density J and the wire cross sectional area must be designed such that the Joule heating thermal effect would not exceed the insulation limit or demagnetize the ferrite core over time. Those relationships are represented in a simplified graph shown in Fig 2.
The leakage inductance is designed using a finite element tool (ANSYS Maxwell shown in Figure 4) as follows. First the geometric parameters and their relationship with the leakage inductance are highlighted. In this case, the leakage inductance is positively proportional with the number of windings, the length of the one turn of the winding, the sum of the section layer heights (windings), and the sum of the insulation distances. Furthermore, the leakage inductance is negatively proportional to the number of insulation layers. The parameter sweep results are shown in Fig. 5.
A parametric simulation is conducted to make the leakage inductance equal to the required value, which in our case is 30μH. Leakage inductance in the simulation can be calculated from the coupling coefficient k. This coupling coefficient between the primary and secondary and is generated
automatically by Maxwell using an output matrix feature. The leakage inductance must be referred to either the primary or the secondary. The equation for calculating it is as follows:
= (1 − ) (4)
This method guarantees the accuracy of estimating the leakage inductance while maintaining the power transfer and thermal limits requirements.
Figure 2: Simplified diagram showing the relationship between transformer geometry and electric properties.
Table 1: Transformer design parameters
Figure 3: Transformer short and open circuit tests.
Figure 4: Finite element field intensity plot of minimum (left) and maximum (right) peak magnetic flux based on sweeping coil distance from core (top) and coil height
(bottom).
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Figure 5 : The Leakage inductance can be fine-tuned using
numerical parameter sweep in finite element.
IV. EXPERIMENTAL RESULTS The experimental results were conducted on the designed
transformer with the following parameters shown in Table 1. The leakage and the magnetizing inductance are obtained by conducting the short circuit and open circuit test as shown in in Fig. 3. The slope , leakage inductance is 30μH which matches the simulation.
Finally the transformer was used in a 11 kW universal converter shown in Fig. 6. The phase shift control algorithm is implemented using FPGA and both the DAB and the PFC stage uses SiC MOSFET BSM120D12P2C005. Fig. 7 shows the experimental result for the DAB.
Figure 6: The universal converter hardware with DAB transformer.
Figure 7: Dual active bridge test (phase shift control).
V. CONCLUSION The universal converter allows easy design of the galvanic isolation part, even with large variation of the PFC voltage. Using parametric design, the leakage inductance can be designed without effecting the power transfer and thermal limits requirements. An analytic formula for the required leakage inductance is derived based on modeling the dual active bridge as a gyrator. The transformer was experimentally tested by building the universal converter that includes the PFC stage and DAB stage. The experimental results presented in this paper verified the practical feasibility of the transformer design.
REFERENCES
[1] M. Yilmaz and P. T. Krein, Review of Battery Charger Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles," in IEEE Transactions on Power Electronics, vol. 28, no. 5, pp. 2151-2169, May 2013.
[2] https://www.tesla.com/support/autobidder [3] Mithat Can Kisacikoglu. “Vehicle-to-grid (V2G) Reactive Power
Operation Analysis of the EV/PHEV Bidirectional Battery Charger”(2013).
[4] Bojrup, Martin. “Advanced Control of Active Filters in a Battery Charger Application.” (1999).
[5] Markovic, Dragan S., et al. “Smart Power Grid and Cloud Computing.” Renewable and Sustainable Energy Reviews, Pergamon, 2 May 2013, www.sciencedirect.com/science/article/abs/pii/S136403211300227X?casa_token=pEZcco3JhcAAAAA%3ApQPtveL_zfjofLsRen6Ozk7OzJuv1-5wDhpusHh9q873ojhPh3U30prlsK9DCsqAC4Xb2sCe4qs
[6] Solanke, Tirupati Uttamrao, et al. “A Review of Strategic Charging–Discharging Control of Grid-Connected Electric Vehicles.” Journal of Energy Storage, Elsevier, 8 Jan. 2020, www.sciencedirect.com/science/article/pii/S2352152X19311302?casa_token=6hNfCDvj-sMAAAAA%3AlXtAQURXlLs01gT3l-zchkgDePqw9gFK40iE4wpodXq63TIxKJgsG8JMsB1i5ovxESvXsfITeQ8.
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