With the development of electric vehicle (EV) charging systems, energy storage systems (ESS), data center power supplies, and solid-state transformer (SST) systems, the fixed-ratio isolated DC-DC converter, namely the DC transformer (DCX), has gained significant popularity. Similar to the passive AC transformer, DCX can bidirectionally convey DC power with very high efficiency. Due to zero-voltage switching (ZVS) and a small root mean square (RMS) current, the open-loop CLLC resonant converter operating at the resonant frequency is a promising candidate for DCX with a constant voltage transfer ratio.
In Chapter 2, to solve unsmooth bidirectional power flow and current distortion in the traditional CLLC-DCX with synchronization rectification (SR) modulation, a dual-active-synchronization (DAS) modulation is adopted with identical driving signals on both sides. First, the switching transition of this modulation is thoroughly analyzed considering the large switch's output capacitances. After comparing different transitions, a so-called sync-ZVS transition is more desirable with ZVS, has no deadtime conduction loss, and almost has load-independent voltage gain. An axis and center symmetric (ACS) method is proposed to achieve this switching transition. Based on this method, an overall design procedure of CLLC-DCX with DAS modulation is also proposed.
However, designing a high-power and high-frequency transformer for CLLC-DCX presents significant challenges due to the trade-off between thermal management, leakage inductance minimization, and insulation requirements. To overcome this trade-off between power rating and operation frequency, a scalable electronic-embedded transformer (EET) with a low-voltage bridge integrated into the transformer windings is proposed in Chapter 3. The EET addresses the challenge through simple open-loop control and natural current sharing, enabling easy parallel connection and scaling to different power ratings. Based on this concept, a bidirectional, EET-based DC transformer (EET-DCX) is proposed to solve the transformer-level paralleling and resonant point shift issues in traditional LLC-DCX designs. By employing the embedded full bridge, the EET-DCX effectively cancels out the impedance of the leakage inductance, ensuring optimal operation at any frequency. Additionally, the EET-DCX retains the inherent advantages of the LLC-DCX, such as load-independent voltage gain, simple open-loop control, full-load range ZVS, and low circulating current. Leveraging these advantages, the proposed EET-DCX solution has the potential to push the boundaries of transformer performance to the MHz operation frequency range with hundreds of kilowatts of power capability.
Moreover, to address the significant RMS current problem of the CLLC-DCX, a trapezoidal current modulation is also proposed in Chapter 3. Compared to the CLLC-DCX with a sinusoidal current, an EET-DCX with a trapezoidal current can reduce the total conduction loss by up to 23%. This total conduction loss includes semiconductor loss on both high-voltage and low-voltage bridges and transformer winding loss.
In light of this EET concept, another resonant commutation (RC) EET-DCX is proposed to streamline the circuit. First, it replaces the embedded full bridge with a low-voltage bidirectional AC switch. Second, it introduces a resonant current commutation to realize a quasi-trapezoidal transformer current with a smaller RMS value. Compared to the triangular current produced by the original EET-DCX, the RMS current can be decreased by 15%. By incorporating only one embedded bidirectional AC switch, the high-frequency transformer leakage inductance impedance is fully neutralized. As a result, the rated power of the proposed RC EET-DCX can be readily scaled up through transformer-level parallelism. Furthermore, the RC EET-DCX maintains the benefits of a typical LLC/CLLC-DCX, including load-independent voltage gain, full load range ZVS, and low circulating current.
However, either in EET-DCX or RC EET-DCX, the trapezoidal current modulation will increase the voltage stress on the low-voltage full bridge or bidirectional AC switch, especially when the leakage inductance is large and variable, such as in the high-power wireless charging application. To address this trade-off between RMS current and voltage stress, this paper proposes the concept of a hybrid resonant-type EET-DCX with a series resonant capacitor. Following this concept, two specific topologies, hybrid EET-DCX and hybrid RC EET-DCX, are proposed. The main difference between these topologies is that the former adopts a full bridge. In a hybrid RC EET-DCX, a resonant current commutation scheme is developed. Among these topologies, since the passive capacitor can mainly cancel the leakage inductance impedance, the full bridge or AC switch only needs to handle the remaining impedance. Thus, the voltage stress on active components can be dramatically decreased. Additionally, these two proposed topologies can retain all the advantages of previous EET-DCX designs, including natural current sharing, load-independent voltage gain, simple open-loop control, and full-load range ZVS. The comparison between these two topologies is thoroughly studied. Finally, a 12-kW DCX testbench is built to verify all the analysis and performance in Chapter 3.
If output voltage regulation is required, DCX can cooperate with other voltage regulators to realize high conversion efficiency and power density. In Chapter 4, two DCX applications are implemented: an 18-kW 98.8% peak efficiency EV battery charger with partial power processing and a 50-kW symmetric 3-level buck-boost converter with common-mode (CM) noise reduction.
In the first battery charger, a large portion of the power is handled by an 18 kW CLLC-DCX, and the remaining partial power goes through a 3-phase interleaved buck converter. The proposed switching transition optimization in Chapter 2 is adopted in the 18-kW CLLC-DCX to realize 98.8% peak efficiency.
To handle the step-up and step-down cases at the same time, a symmetric 3-level buck-boost converter with coupled inductors is also studied as a post regulator. With symmetric topology and quadrangle current control, the converter can achieve a CM noise reduction and full load range ZVS with a small RMS current. To further optimize the performance and simplify the control, a mid-point bridging with a better CM noise reduction and a split capacitor voltage auto-balance is implemented. A 50-kW prototype is built to verify the above analysis.
To summarize, Chapter 2 first proposes a switching transition optimization for CLLC-DCX. Later, to address the intrinsic trade-off between transformer rating power and frequency, an EET concept and its corresponding soft-switching DCX family are found in Chapter 3. Finally, to handle voltage regulation, two examples for practical applications are studied in Chapter 4 —one is an 18-kW partial power converter, and the other is a 50-kW 3-L buck-boost converter. Finally, Chapter 5 will draw conclusions and illustrate future work. / Doctor of Philosophy / With the development of electric vehicle (EV) charging systems, energy storage systems (ESS), data center power supply, and solid-state transformer (SST) systems, the fixed-ratio isolated dc-dc converter, namely dc transformer (DCX), has gained significant popularity. However, designing a high-performance DCX still has many challenges, such as large dead time loss, poor current sharing, and sensitivity to parameter tolerance.
Firstly, the state-of-the-art resonant CLLC-DCX is optimized in Chapter 2. With an optimal switching frequency and dead time, both the primary and secondary sides of zero voltage switching (ZVS) can begin and finish simultaneously, which means dead time loss caused by current through the body diode can be eliminated. Therefore, the efficiency of CLLC-DCX can be improved. However, designing a high-power and high-frequency CLLC-DCX transformer still presents significant challenges due to the trade-off between thermal management, leakage inductance minimization, and insulation requirements.
To overcome this trade-off, in Chapter 3, a scalable electronic-embedded transformer (EET) concept with a low-voltage bridge integrated into the transformer windings is proposed. The EET addresses the challenge through its simple open-loop control and natural current sharing, enabling easy parallel connection and scaling to different power ratings. In light of this EET concept, a new family of soft-switching DCXs is proposed for different applications, such as high-power wireless charging systems. All these EET-based DCXs retain the merits of typical CLLC-DCX, such as small circulating current ringing, small turn-off current, full load range ZVS, and load-independent gain.
After realizing a desirable design for DCX, Chapter 4 presents two DCX applications with voltage regulation. Firstly, an 18 kW 98.8% peak efficiency battery charger is designed with partial power processing. Most of the power will go through an optimized DCX, and the remaining small portion of power will go through a 3-phase interleaved buck converter. On the other hand, DCX can also be adopted as a front-end or rear-end converter in a typical two-state DC-DC converter. As for another stage, a non-isolated DC-DC converter with a large output range can be used to handle voltage regulation. Following this structure, a 50-kW symmetric 3-L buck-boost converter with coupled inductors and reduced common emission is proposed.
To summarize, the state-of-the-art CLLC-DCX is optimized in Chapter 2. Afterward, a new concept of EET-DCX and its corresponding DCX family is proposed in Chapter 3. After obtaining an optimized DCX, two practical applications with DCX are implemented in Chapter 4. Finally, Chapter 5 will draw conclusions and illustrate future work.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/117331 |
| Date | 09 January 2024 |
| Creators | Cao, Yuliang |
| Contributors | Electrical Engineering, Dong, Dong, Southward, Steve C., Burgos, Rolando, Ngo, Khai D., De La Reelopez, Jaime |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | Creative Commons Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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