# Design and Evaluation of Single-Active Bridge Configurations for Optimal Megawatt Charging Systems

Narendra S Walawalkar, Shubham Gupta, Tanmay Narendra, Martin Kjaer, Florin Iov, Stig Munk-Nielsen

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Narendra S Walawalkar, Shubham Gupta, Tanmay Narendra, Martin Kjaer, Florin Iov, et al. Design and Evaluation of Single-Active Bridge Configurations for Optimal Megawatt Charging Systems. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0160. hal-05108309

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Distributed under a Creative Commons Attribution 4.0 International License
Design and Evaluation of Single-Active Bridge Configurations for
Optimal Megawatt Charging Systems

Narendra S Walawalkar, Shubham Gupta, Tanmay Narendra, Martin Kjær, Florin Iov,
and Stig Munk-Nielsen

DEPARTMENT OF ENERGY/ AALBORG UNIVERSITY

Pontoppidanstræde 111, Aalborg, Denmark

E-Mail: nwalaw23@student.aau.dk

Institute URL: https://www.en.aau.dk

***Index Terms***—DC-DC power converter, Charging in-
frastructure for EVs, Hardware design, Medium frequency
transformer, and Silicon Carbide (SiC).

***Abstract***—The increasing demand for high-
performance isolated DC-DC converters is particularly
evident in the domains of charging infrastructure and
electromobility. Among various converter topologies, the
Single-Active Bridge (SAB) stands out as a preferred
choice for applications requiring unidirectional power
flow. This preference is driven by the SAB's inherent
simplicity, robustness, and ease of control. The paper
explores multiple configurations of the SAB architecture
[1]. A prototype is developed to validate the features
and the design procedures presented in this paper. This
paper summarizes testing and analysis, to evaluate the
performance of each configuration to identify the optimal
design for an efficient and reliable high-power charging
system. The results offer valuable insights for developing
advanced MCS solutions, ensuring robust and efficient
energy transfer in high-demand scenarios. Ultimately the
SAB configuration is loaded at 88 % of the nominal load
and the results are showcased.

# I. INTRODUCTION

The evaluation of total energy supply since 2000
shows a reduction in the contributions of oil and coal,
with a rise in renewable energy sources like wind,
solar, and biofuels. However, oil and coal still domi-
nate the overall energy mix. This is primarily due to
the transportation sector, where heavy-duty vehicles are
the largest contributors, driven by recent technological
advancements [2]. According to statistical analyses from
organizations such as the IEA and IRENA, there is an

urgent need to electrify heavy-duty vehicles, including
ships and trucks. This shift requires battery sizes ex-
ceeding 400 kWh, with individual commercial electric
vehicles demanding power from 400 kW to over 1 MW
[3].

Isolated DC/DC converters have a wide range of ap-
plications in the Charging and EV infrastructure having
various beneficiary factors such as galvanic isolation,
energy efficiency, compact design, and robustness. The
Dual Active Bridge (DAB) and Single-Active Bridge
(SAB) are the two most widely adopted converters, the
converter topology shown in Fig. 1 for SAB and Fig. 2
for DAB, among all isolated DC/DC converter options
[4]. Understanding the comparison between the SAB and
DAB in paper [5], SAB is the most efficient for the uni-
directional power flow. The SAB designs are known for
their efficiency and robustness, utilizing diodes in the
output bridge. In contrast, DAB topologies offer the ad-
vantage of bidirectional power flow but require additional
switches, leading to increased losses. The SAB cascaded
topologies promise higher reliability, without increasing
the losses significantly [6], making them an attractive
solution for future unidirectional applications of Mega-
watt charging stations, for heavy-duty commercial elec-
tric vehicles (HDEV) [7].

The novelty of this paper is to identify an optimal
solution for Megawatt Charging Systems (MCS) by
evaluating key factors such as efficiency, reduced compo-
nent count, minimized losses, and cost-effectiveness. The
structure of the paper is as follows: Section II begins by
detailing the conventional Single-Active Bridge (SAB)
setup, followed by modifications and testing of different
variants using various modulation schemes. Section III
<figure><img src="image_2.png" /><figcaption>Fig. 1. Single Active Bridge Converter Topology.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2. Dual Active Bridge Converter Topology.</figcaption></figure>

presents the results and provides a comparative analysis of the tested variants. Section IV offers conclusions on the optimal design for MCS. Lastly, Section V outlines potential future improvements to further enhance the system's accuracy and performance, ensuring optimal megawatt power transfer.

## II. CONVERTER & VARIANTS DESCRIPTION

### A. System Layout

The system is designed and manufactured as a laboratory prototype for a downscale version of MCS. It resembles the 2-3 wheeler application with a power of 6-10 kW [8] and 48 V to 60 V of battery output voltage. The Grid filter and the AFE section of the charging system are assumed to be a black box (available off-the-shelf) and the system has the input of nominal DC-Link voltage 750 V.

The system configuration is illustrated in Fig. 4. It includes the first active bridge for inversion, followed by an AC filter designed to bypass harmonics and transmit a pure sine wave to the transformer. The transformer prototype was both designed and manufactured in-house. A SiC-based full bridge diode rectifier is then employed for rectification and further a DC filter comprising a DC inductor and capacitor is added to minimize ripple in the output voltage and current. The components in this system are sized to handle a nominal power rating of 6 kW and a peak power rating of 10 kVA. Detailed hardware design of the critical components is discussed in subsequent sections. The laboratory setup for the SAB,

along with the DSOs and multimeter used for recording parameters, is shown in Fig. 3 and Fig. 4. Table I is used to cross-reference the section present in the setup.

During testing, temperature sensors were attached to the cores of the AC inductor, transformer, and DC inductor, allowing for simultaneous temperature monitoring for each test. Additionally, the internal temperatures of the MOSFETs were monitored using an NTC connection provided by Wolfspeed Inc. on the module.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Front View SAB Assembly.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Top View SAB Assembly.</figcaption></figure>

1) Transformer Design: The medium-frequency transformer is designed using silicon steel EI plates, specifically EI-180 laminated plates, and is rated for a peak power of 10 kVA. The preliminary analysis using classical theory indicates that operating the transformer at higher-frequency, reduces the operating B-field, which in turn lowers core losses as governed by the Steinmetz equation (1), where $k_2$ and $n$ stands for Steinmetz constant [9]. Studying the available window gap and the current handling capacity of winding, the transformer is designed for the rated frequency of 400 Hz at 1.2T of
<table><caption>TABLE I
10 KW SAB LABORATORY SETUP</caption><thead><tr><th colspan="2">Setup Designators</th></tr><tr><th>Box Number</th><th>Item</th></tr></thead><tbody><tr><td>1</td><td>Inverter & Gate Driver</td></tr><tr><td>2</td><td>AC-Filter</td></tr><tr><td>3</td><td>10 kVA Transformer</td></tr><tr><td>4</td><td>Diode Bridge Rectifier</td></tr><tr><td>5</td><td>DC-Inductor</td></tr><tr><td>6</td><td>Output Capacitor</td></tr></tbody></table>

flux density. The operating frequency of the transformer and the whole system is increased to 1000 Hz to reduce the flux density from 1.2T to 0.48T, further reducing the core losses. Using equation (2) Volts/Turn are derived, used to calculate the design parameters which are stated in the table II

$$P_{hysterisis} = k_2 \cdot f_o \cdot B_{mp}^n \quad (1)$$

$$V = 4.44 \cdot f_o \cdot A_c \cdot B \cdot N \quad (2)$$

<table><caption>TABLE II
TRANSFORMER PARAMETERS</caption><thead><tr><th colspan="3">Design Parameters - 10 kVA Transformer</th></tr><tr><th>Parameter</th><th>Value</th><th>Unit</th></tr></thead><tbody><tr><td>Power</td><td>10</td><td>kVA</td></tr><tr><td>Primary Voltage</td><td>540</td><td>V<sub>rms</sub></td></tr><tr><td>Secondary Voltage</td><td>60</td><td>V<sub>rms</sub></td></tr><tr><td>Primary Current</td><td>21</td><td>A<sub>rms</sub></td></tr><tr><td>Secondary Current</td><td>196</td><td>A<sub>rms</sub></td></tr><tr><td>Rated Frequency</td><td>400</td><td>Hz</td></tr><tr><td>Operating Frequency</td><td>1000</td><td>Hz</td></tr><tr><td>Operating B-field</td><td>0.48</td><td>T</td></tr></tbody></table>

2) **Inverter**: Using SiC-based CBB032M12FM3T MOSFETs [10] and the compatible CGD1700HB2M-UNA gate driver from Wolfspeed Inc., a 2-Level Full Bridge inverter was designed to meet the required power and current levels. With a fundamental operating frequency of 1 kHz, the switching frequency was set to 20 kHz—20 times the fundamental frequency—to avoid operating in the audible range for DC/DC converters [11].

3) **Filters**: The AC filter was selected based on the fundamental frequency and the current through the inductor. It was designed with a cut-off frequency of 4.25 kHz to prevent resonance with the fundamental and 3rd harmonic frequencies. The AC filter inductor was

tested with a magnetic analyzer, to ensure its inductance remained within the range of 700 µH at RMS current of 21 A. Similarly, the DC filter inductor was custom-designed and manufactured in the laboratory to suit the converter's specific needs. EI150 laminated plates were used for the core, yielding an inductance between 0.85 mH and 1 mH, with a resistance of 6.4 mΩ. Due to limitations set by the laboratory equipment capabilities, testing was performed up to 75 A of DC-bias. The results are shown in Fig.5 and Fig.6 confirm that the AC inductor and DC inductor meet the system requirements.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Inductance vs DC-Bias - AC filter Test.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. Inductance vs DC-Bias - DC filter Test.</figcaption></figure>

The capacitor selected for both AC and DC filters are film capacitor. DC filter inductor and capacitor values are designed to attenuate harmonics with 40 dB/decade and
to handle the ripple current of 5% at 120 Arms. For AC filter a capacitor of 2 uF is selected, and for DC filter the capacitor selected is 400 uF.

4) Theoritical Power Loss Distribution- Fig8: The pie chart illustrates the power loss contribution of each component, based on an empirical equation of Thevenin's theorem. This paper outlines strategies to minimize these losses and enhance overall system efficiency. By analyzing the data, it becomes possible to determine the impact of removing or optimizing specific components and how such changes would influence the system's performance. This approach helps in identifying key areas for improvement and guides design modifications to achieve a higher power transfer capability.

<figure><img src="image_8.png" /><figcaption>Fig. 7. Power Loss Distribution of components.</figcaption></figure>

## B. Conventional Setup - Variant 1

The setup depicted in Fig. 8 represents the conventional SAB configuration. The AC filter ensures smooth AC power transfer by bypassing harmonics, which minimizes losses in the transformer's winding and core. The modulation technique employed is Sinusoidal Pulse Width Modulation (SPWM) with a modulation index of 0.95 to prevent overmodulation. Measurements were taken using the equipment shown in Fig. 3 and Fig. 4, with results and analysis detailed in section III.

## C. Eliminating $C_{ac}$ - Variant 2

As discussed in section III, the inductor's temperature was unexpectedly high accounting for the reactive power consumption caused by the AC filter. To mitigate

<figure><img src="image_9.png" /><figcaption>Fig. 8. Conventional SAB Converter - Variant 1.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 9. SAB Modified Variant 2.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 10. SAB Modified Variant 3.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 11. SAB Modified Variant 4.</figcaption></figure>

this issue, the 2 µF $C_{ac}$ capacitor was removed from the test setup, and testing was performed with the same modulation and parameters as the conventional setup to facilitate a benchmark comparison between the variants. While this change increased the harmonic content reaching the transformer, it led to lower power dissipation in the $L_{ac}$ inductor, as shown in the results. This adjustment not only enhances efficiency and overall power transfer but also advances the SAB toward reducing components and costs while increasing overall reliability.

## D. Removal of AC Filter - Variant 3

The transformer's leakage inductance can effectively replace the role of $L_{ac}$. Although the leakage inductance is lower than the original $L_{ac}$, the modulation scheme has been modified to square wave modulation, with the switching frequency aligned with the fundamental frequency. The results demonstrate an improvement in overall efficiency, with a 14% reduction in total loss
distribution when the *Lac* is removed as showcased in Fig. 7.

### E. Updated modulation (Square PWM) - Variant 4

In this variant, the hardware setup is identical to that of Variant 3; however, the modulation technique is replaced with Square Pulse Width Modulation (SqPWM). This modification enables better control of the output voltage by adjusting the inverter stage's duty cycle. The objective of this variant is to evaluate the system's efficiency variation under the updated modulation technique.

### F. Maximum Load testing

Post 10% loading it was deduced that variant 4 proved to be most efficient in early-stage low load testing. Hence it was decided to perform a maximum loading test on variant 4, and the test parameters are provided in table III. The setup was tested at 88% of the nominal load capacity, as the laboratory equipment constrained it. The availability of the control over the inverter's duty cycle, allowed to control of the output voltage of the SAB. Using this functionality the duty cycle was varied from 20% to 70%. The system parameters followed during the testing are provided in the table III:

<table><caption>TABLE III
MAXIMUM LOAD TEST PARAMETERS</caption><thead><tr><th colspan="3">Test setup parameters</th></tr><tr><th>Parameter</th><th>Value</th><th>Unit</th></tr></thead><tbody><tr><td>InputPower</td><td>5.25</td><td>W</td></tr><tr><td>OutputPower</td><td>4.36</td><td>W</td></tr><tr><td>VDC<sub>Link</sub></td><td>550</td><td>V</td></tr><tr><td>R<sub>Load</sub></td><td>0.3</td><td>Ω</td></tr><tr><td>OutputVoltage</td><td>36.2</td><td>V</td></tr><tr><td>InputCurrent</td><td>9.55</td><td>A</td></tr><tr><td>OutputCurrent</td><td>120.67</td><td>A</td></tr><tr><td>Efficiency</td><td>83.16</td><td>η</td></tr></tbody></table>

## III. RESULTS & ANALYSIS

The multi-line plot in Fig. 12 illustrates the efficiency variation for each SAB variant across the range of loading conditions.

Fig. 12 illustrates the efficiency improvement observed when transitioning from Variant 1 to Variant 4. The testing was conducted at 10% of the nominal power rating to assess efficiency at lower loading before advancing to higher load levels. Several factors contributed to this increase in efficiency. The observations are outlined as follows:

<figure><img src="image_13.png" /><figcaption>Fig. 12. Efficiency vs. DC Load current for Various SAB Variants</figcaption></figure>

* Removing the capacitor in Variant 1 significantly boosted efficiency, increasing it from 35% to 61.7%. This improvement can be attributed to reduced reactive power consumption, which previously caused substantial power losses and led to a temperature rise in the AC inductor from 25°C to 70°C during testing.

* Transitioning from SPWM to a 1kHz fundamental square wave modulation, further increased efficiency, increasing it from 61.7% to 63.8% as shown in table IV. In Variant 3, as the diode rectifier draws a square wave current, synchronizing the voltage and current waveforms with the square wave modulation. This alignment eliminates the need for the *Lac* (AC filter inductor), reducing power losses and lowering the overall component count.

* Upgrading to Square PWM modulation showed an efficiency improvement for the same output power, as noted in Table IV. This highlights that advancements in modulation techniques not only enhance efficiency but also provides control over the output voltage.

* The Efficiency vs duty cycle variation of variant 4 shown in Fig. 13 highlights the increase in the efficiency with the duty cycle. The efficiency rose significantly at lower duty cycles and began to plateau at higher duty cycles.

* During testing, a jittering noise was detected from the transformer, corresponding to the system's fundamental frequency. This noise was particularly
noticeable in Variant 3, which operates with a 1kHz fundamental square wave.

* Temperature sensor data proved crucial in detecting the abnormal temperature rise of the AC inductor in Variant 1. The sensors also provided important information about the transformer temperature across different variants, as illustrated in Table IV.

<figure><img src="image_14.png" /><figcaption>Fig. 13. Efficiency vs. Inverter Duty cycle variation</figcaption></figure>

## A. Model Comparison

Table IV presents a comparison of the different variants tested at 10% loading, including temperature data and efficiency variations.

<table><caption>TABLE IV
TRANSFORMER PARAMETERS</caption><thead><tr><th colspan="5">Variant Comparison at 10% of nominal load</th></tr><tr><th>Parameter</th><th>Var 1</th><th>Var 2</th><th>Var 3</th><th>Var 4</th></tr></thead><tbody><tr><td>Modulation</td><td>SPWM</td><td>SPWM</td><td>Square</td><td>Sq PWM</td></tr><tr><td>Reduce Component</td><td>0</td><td>1</td><td>2</td><td>2</td></tr><tr><td>Transformer Temp</td><td>35°C</td><td>38°C</td><td>40°C</td><td>45°C</td></tr><tr><td>η</td><td>35%</td><td>61.7%</td><td>63.8%</td><td>66.2%</td></tr></tbody></table>

From Table IV, it is evident that the increase in efficiency correlates with a reduction in the count of components. The transformer's temperature rises from Variant 1 to Variant 2 due to the removal of the $C_{ac}$ capacitor, which causes more harmonics to be transferred to the transformer. This temperature increase becomes more pronounced from Variant 2 to Variant 3 with the shift from SPWM to square wave modulation, leading

to higher transformer losses. Despite this, the overall system efficiency improves.

Finally, at maximum load, the system efficiency was observed to be 83%. The losses were attributed to the transformer core, RC snubbers, and inverter switching losses. At maximum load, the transformer temperature was noted to be around 45°C; however, the test was conducted before the transformer could reach thermal equilibrium. Consequently, when scaling the system to higher power levels, it is essential to focus on optimizing transformer design to effectively address the thermals, core losses and MOSFET switching behaviour impact on overall system efficiency.

## IV. CONCLUSION

This paper has presented multiple configurations of SAB systems, with the downscale version of hardware developed for testing. Each configuration was tested, and results were provided for every variant. Based on the final testing, variant 4 was found to be the most efficient among the compared variants. When tested at maximum load, the efficiency was observed to be 83%. A systematic analysis of the testing scenarios, including monitoring quantities like temperature and noise, was conducted.

The efficiency analysis indicates that SAB systems perform optimally with the Square PWM technique, which should be considered and tested when designing systems for megawatt charging applications to obtain robust and efficient energy transfer.

## V. FUTURE WORKS

In the short term, future work will focus on extended-duration testing to observe the settling temperatures of key components, such as MOSFETs and magnetic components. Further investigation will address constraints like volume and dimensions. Additionally, improvements in transformer design, including core selection and thermal management, will be explored.

In the long term, efforts will focus on optimizing the system by developing SAB configurations capable of operating at megawatt power levels and testing them under comparable conditions. The goal is to validate the hypothesis that SAB systems provide an optimal solution for energy transfer in megawatt charging applications.

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