Design Philosophies of High-Frequency Magnetic Components for Four Major Topologies: A Comparative Analysis of PSFB, Full-Bridge LLC, CLLC, and DAB

I. Phase-Shifted Full-Bridge (PSFB)

Topology Characteristics

• An improved version of the traditional hard-switching full-bridge, utilizing inductors and transformer leakage inductance to achieve Zero-Voltage Switching (ZVS) of switching devices.
• Exhibits duty cycle loss, imposing special requirements on transformer design.

Key Design Points of Magnetic Components

Transformer Design

• Precise control of leakage inductance is required to collaborate with external inductors for ZVS implementation; however, excessive leakage inductance exacerbates duty cycle loss.
• Sandwich winding or adjusted winding spacing is often adopted to obtain repeatable leakage inductance values.
• Operating frequency typically ranges from 100kHz to 500kHz, necessitating the use of low-loss magnetic cores (e.g., PC95, ferrite).

Output Filter Inductor

• Must handle large current ripples, requiring a balance between core loss and copper loss.
• Inductance stability must be maintained under wide input voltage ranges.

Application Scenarios: Medium-to-high power (500W-3kW) industrial power supplies, communication power supplies, and cost-sensitive applications requiring high efficiency.

II. Full-Bridge LLC

Topology Characteristics

• Achieves soft switching through resonant inductor (Lr), resonant capacitor (Cr), and transformer magnetizing inductance (Lm).
• Enables ZVS of primary switching devices and Zero-Current Switching (ZCS) of secondary rectifier diodes across the entire load range.

Core Design of Magnetic Components

Integrated Design of Transformer and Resonant Inductor

• The resonant inductor can be integrated into the transformer by utilizing the transformer's leakage inductance as the resonant inductor, improving power density.
• Precise control of leakage inductance is required, with consideration given to coupling coefficient and process consistency during design.

Optimization of Magnetizing Inductance (Lm)

• The value of Lm affects the resonant cavity gain characteristics and circulating current.
• A trade-off must be struck between providing sufficient magnetizing current for ZVS implementation and reducing conduction losses.

Magnetic Core Selection

• Due to operation with sinusoidal resonant current, core loss is relatively low, allowing for higher operating frequencies (200kHz-1MHz).
• Low-loss ferrite or planar magnetic core structures are commonly used.

Application Scenarios: High-end server power supplies, LCD TV power supplies, electric vehicle On-Board Chargers (OBC), and other applications requiring high efficiency and high power density.

III. CLLC

Topology Characteristics

• A symmetric bidirectional derivative topology of LLC, featuring symmetric resonant cavities on both the primary and secondary sides.
• Inherently supports bidirectional power flow with symmetric bidirectional gain characteristics.

Special Design Considerations for Magnetic Components

Fully Symmetric Transformer Design

• The primary and secondary winding structures, turns, and resonant parameters must be highly symmetric.
• Consistency of magnetizing inductance and leakage inductance parameters must be ensured during bidirectional operation.

Feasibility of Magnetic Integration for Dual Resonant Cavities

• Attempts can be made to integrate part of the primary and secondary resonant inductors into the same magnetic core, but attention must be paid to parameter deviations caused by coupling.
• Finite Element Analysis (FEA) is required during design to verify the actual parameters after magnetic integration.

Higher Requirements for High-Frequency Operation and Low Loss

• Commonly used in new energy fields (e.g., energy storage systems, Vehicle-to-Grid (V2G)), with operating frequencies up to 500kHz-2MHz.
• Stringent requirements for magnetic core materials (e.g., magnetic materials compatible with Gallium Nitride (GaN)) and winding AC losses (utilizing Litz wire or flat wire).

Application Scenarios: Applications requiring efficient bidirectional power flow, such as energy storage converters, bidirectional on-board chargers, and DC microgrid interconnection units.

IV. Dual Active Bridge (DAB)

Topology Characteristics

• Composed of two full-bridges connected via a high-frequency transformer, with power transmission controlled through phase shifting.
• Features wide voltage range regulation capability and inherent bidirectional power flow capability.

Core Design Challenges of Magnetic Components

Transformer Design for Minimum Leakage Inductance

• In contrast to LLC, DAB requires the transformer leakage inductance to be as small as possible, as leakage inductance acts as an equivalent series inductor for power transmission, increasing reactive circulating current.
• Tight coupling windings (e.g., parallel winding, interleaved winding) are often adopted.

Design of External Integrated Inductors

• If an inductor is needed to form current ripples or assist in soft switching, an independent external inductor is typically used.
• This inductor must handle high-frequency (usually 100-300kHz) square-wave voltage, requiring attention to the magnetic core's dv/dt withstand capability and low-loss design.

High-Frequency and High Power Density Design

• To reduce volume, operating frequencies are moving toward the MHz range, requiring the use of planar transformers or PCB windings.
• Magnetic core materials commonly include high-frequency ferrite or metal powder cores.

Application Scenarios: DC Transformers (DCX), electric vehicle chargers, renewable energy generation systems, solid-state transformers, and other medium-to-high power bidirectional applications.

Conclusion