Analysis of High-Frequency Isolated DC-DC Topologies for Medium-High Power SSTs: DAB and LLC Resonant Converters

The 'Heart' of SST - High-Frequency Isolated DC-DC Power Conversion Topology.This is the core link for SST to achieve high efficiency, high power density, and electrical isolation.

Core Challenges and Design Objectives

In SST, this stage needs to address:

1. High Voltage/High Power: Typically connected to the medium-voltage grid side (thousands of volts).
   
2. High-Frequency Isolation: By increasing the switching frequency (from the traditional power frequency of 50Hz to the range of 10kHz~1MHz), the transformer volume is significantly reduced.
   
3. Bidirectional Power Flow: Adapts to the bidirectional power flow in future smart grids (such as V2G, energy storage integration).
   
4. High Efficiency and High Reliability: Must resolve switching losses and electromagnetic interference issues at high frequencies.
   

Below, we will focus on analyzing the two most representative topologies in SST: the Dual Active Bridge (DAB) and the LLC Resonant Converter.

1. Dual Active Bridge (DAB)

DAB is one of the most mainstream and favored topologies in current SST research, particularly suitable for applications requiring bidirectional power flow and wide voltage ranges.

Topology Structure

• Symmetry: Consists of two identical 'H-bridges' (composed of four active switches), located on the primary and secondary sides of the transformer, respectively.
  
• Key Components: The two H-bridges are connected via a high-frequency isolation transformer and a series inductance (L_s). This inductance can be the transformer's leakage inductance or an external inductor; it is the key to achieving power transfer and control.
  

Working Principle and Core: Phase-Shift Modulation

DAB's power transfer does not rely on duty cycle adjustment but is achieved by controlling the phase difference φ between the output voltages (typically square waves) of the two H-bridges.

• Single Phase-Shift (SPS) Control: The simplest method. Controls power by adjusting the phase shift angle φ between the primary square wave V_p and the secondary square wave V_s.
  
• Idealized Power Transfer Formula: P = (n V_p V_s φ (1 - |φ|/π)) / (2π f_s L_s), where *n* is the turns ratio, and f_s is the switching frequency.
  
• Advantage: Simple control.
  
• Disadvantage: When input and output voltages are mismatched (i.e., the voltage conversion ratio deviates from the designed optimum), significant reactive circulating power is generated, leading to increased RMS current, higher conduction and switching losses, and reduced efficiency.
  

Soft-Switching Technology

DAB inherently possesses the potential to achieve soft switching, which is key to its high efficiency.

• Zero-Voltage Switching (ZVS): Achieved by utilizing the current in the series inductor L_s not being zero before the switching action, to charge and discharge the switch's junction capacitance, thereby enabling the switch to turn on when the voltage is zero, eliminating turn-on losses.
  
• Implementation Conditions: The ZVS range depends on the phase-shift angle, load current, and voltage conversion ratio. Full-load-range ZVS can typically be achieved near rated load and the designed voltage ratio.
  

Application and Optimization in SST

In SST, the voltage/power rating of a single DAB module is limited. Therefore, 'modular' or 'cascaded' structures are often adopted:

• Input-Series Output-Parallel (ISOP): The input sides of multiple DAB modules are connected in series to withstand high voltage, and the output sides are connected in parallel to provide high current. This requires precise voltage/current sharing control.
  
• Advanced Modulation Strategies: To overcome the shortcomings of SPS, extensively researched in SST:
  
• Dual Phase-Shift (DPS) / Triple Phase-Shift (TPS): In addition to inter-bridge phase shift, phase shift between diagonal switches within a single H-bridge (i.e., 'inner phase shift') is introduced. Through multi-degree-of-freedom optimization, the ZVS range can be extended, reactive power and current stress reduced over a wide voltage range, thereby maximizing efficiency. This is a current research hotspot for DAB control algorithms.
  

Summary of DAB Advantages in SST:

• Naturally bidirectional, symmetrical structure.
  
• Easy modular expansion, suitable for medium/high voltage applications.
  
• Multiple control degrees of freedom, allowing performance optimization via advanced modulation strategies.
  
• Good soft-switching characteristics.
  
• 
  

2. LLC Resonant Converter

LLC is renowned for achieving extremely high efficiency in unidirectional, fixed voltage ratio, or narrow-range applications. In SST, it is often used as a specific voltage conversion stage or in scenarios with extremely high unidirectional efficiency requirements.

Topology Structure

• Resonant Network: Consists of two inductors (a resonant inductor L_r and a magnetizing inductor L_m) and a resonant capacitor C_r, forming the 'LLC' network.
  
• Half-Bridge/Full-Bridge: The primary side is typically a half-bridge or full-bridge structure, and the secondary side is a full-wave rectifier (diodes or synchronous rectification MOSFETs).
  

Working Principle and Core: Frequency Control

LLC's power transfer and voltage regulation are primarily achieved by varying the switching frequency f_s.

• Two Resonant Frequencies:
  
• Series resonant frequency f_r = 1 / (2π√(L_r C_r)): Determined by L_r and C_r.
  
• Equivalent resonant frequency f_m = 1 / (2π√((L_r + L_m) C_r)): When L_m participates in resonance
  
• Operating Regions:
  
• f_s > f_r: This is the normal operating region. Here, the rectifier-side current is continuous, the primary-side switches can achieve ZVS, and the secondary-side rectifier diodes can achieve Zero-Current Switching (ZCS). This is the 'golden region' where LLC can reach peak efficiency (often > 98%). Output voltage decreases as frequency increases.
  
• f_s < f_r: This region should be avoided. ZCS is lost here, and efficiency drops sharply
  

Soft-Switching Technology

LLC achieves 'full soft switching':

• Primary-side ZVS: Aided by the current in the magnetizing inductor L_m.
  
• Secondary-side ZCS: The resonant current naturally turns off the rectifier diode at the zero-crossing point, eliminating reverse recovery loss. This is a significant advantage of LLC compared to other topologies.
  

Application and Challenges in SST

• Advantages: Unparalleled efficiency near the optimal design point. Good electromagnetic interference (EMI) characteristics.
  
• Challenges:
  
• Unidirectionality: Traditional LLC is inherently unidirectional. Achieving bidirectionality requires complex modifications (e.g., symmetrical LLC or combining with active rectification), often sacrificing some performance.
  
• Limited Wide-Range Regulation Capability: When input/output voltage variation is large, the required frequency variation range may become excessively wide, leading to difficulties in magnetic component design and reduced light-load efficiency.
  
• Magnetic Integration Design: For high power density, L_r, L_m, and the transformer are often integrated into one magnetic core, making the design complex.
  

Summary of LLC's Position in SST:

• More suitable for SST subsystems with extremely stringent efficiency requirements, limited voltage variation range, and fixed power flow direction.
  
• Often used in combination with topologies like DAB to leverage strengths and compensate for weaknesses.
  

Comparison and Selection Considerations

Conclusion and Trends

In SST design, DAB and its derivative topologies (e.g., using DPS/TPS modulation) have become the de facto first choice for the isolated DC-DC stage in medium/high power SST due to their inherent bidirectionality, excellent modular scalability, and controllability over wide ranges.

However, technology is not static. Current research trends include:

• Hybrid Topologies: Combining the advantages of DAB and LLC, such as introducing resonant elements into DAB to improve soft-switching conditions.
  
• Deep Application of Wide-Bandgap Devices (SiC/GaN): Leveraging their high-speed switching characteristics to push switching frequencies to hundreds of kHz or even MHz, thereby further reducing magnetic component volume. This presents new challenges in driving, layout, and EMC for both topologies.
  
• Artificial Intelligence Optimization: Using machine learning algorithms to online optimize modulation parameters (such as optimal phase-shift angle combinations) to respond to grid fluctuations in real-time and achieve global optimal efficiency.
  

Therefore, mastering these topologies not only means understanding their circuit principles but also means mastering their dynamic characteristics under wide-bandgap devices, the implementation of advanced digital control algorithms, and the co-design capability of multi-physics fields (electrical-magnetic-thermal) under high-frequency and high-power-density conditions. This is precisely the jewel in the crown of SST technology.