Design Challenges in SST Auxiliary Power and Cooling Systems
Two Critical and Challenging Subsystems in Solid-State Transformer (SST) Design
Auxiliary Power Supply and Thermal Management System.
Although they do not directly participate in the main power conversion, they serve as the "lifeline" and "guardian" ensuring stable and reliable operation of the main circuit.
Auxiliary Power Supply: The System's "Pacemaker"
The auxiliary power supply provides power for the "brain" and "nerves" of the entire solid-state transformer. Its reliability directly determines whether the system can operate normally.
I. Core Challenges
High Voltage Isolation: It must safely extract power from the high-voltage side to supply control and driver circuits on the primary side, requiring the power module to have extremely high electrical isolation capability.
Strong Immunity to Interference: The main power circuit’s high-frequency switching (tens to hundreds of kHz) generates large voltage transients (dv/dt) and electromagnetic interference (EMI). The auxiliary power supply must maintain stable output in this harsh environment.
Multiple, Precise Outputs:
Gate Driver Power: Supplies isolated power to the gate drivers of each power switch (e.g., SiC MOSFETs). Each output must be independent and isolated to prevent crosstalk that could cause shoot-through faults.
Control Board Power: Powers digital controllers (DSP/FPGA), sensors, and communication circuits, requiring clean, low-noise power.
II. Typical Power Extraction and Design Approaches
High-Voltage Power Extraction: Use an isolated switching power supply (e.g., flyback converter) to extract energy from the high-voltage input. This is the most technically challenging part and requires specialized design.
Multi-Output Isolated DC-DC Modules: After obtaining an initial isolated power source, multiple isolated DC-DC modules are typically used to generate additional required isolated voltages.
Redundancy Design: In ultra-high reliability applications, the auxiliary power supply may be designed with redundancy to ensure safe shutdown or seamless switchover to a backup supply in case of primary failure.
Thermal Management System: The System's "Air Conditioner"
The thermal management system directly determines the SST’s power density, output capability, and lifespan.
Why is it so critical?
Extremely High Power Density: By replacing bulky line-frequency transformers, SSTs concentrate energy into much smaller power modules, leading to a sharp increase in heat flux (heat generated per unit area).
Temperature Sensitivity of Semiconductor Devices: Although SiC/GaN power devices offer high efficiency, they have strict junction temperature limits (typically 175°C or lower). Overheating leads to performance degradation, reduced reliability, or permanent failure.
Direct Impact on Efficiency: Poor heat dissipation raises chip junction temperature, increasing on-state resistance, which in turn increases losses—creating a vicious cycle.
III. Types of Cooling Methods
| Cooling Method | Principle | Application Scenarios and Features |
| Natural Convection | Heat is dissipated through fins on the heatsink via natural air circulation. | Suitable only for low-power or very low-loss experimental setups. Cannot meet the requirements of most SST applications. |
| Forced Air Cooling | A fan is mounted on the heatsink to significantly enhance airflow. | The most common and lowest-cost solution. However, heat dissipation capacity is limited, and fans introduce noise, limited lifespan, and dust accumulation issues. Suitable for medium- to low-power density designs. |
| Liquid Cooling | Heat is removed by a liquid cooling plate and circulation pump. | The mainstream and preferred choice for high-power-density SSTs today. |
| Cold Plate Liquid Cooling | Power devices are mounted on internal metal plates with fluid channels. | Heat dissipation capability is several times that of air cooling; compact structure enables very low temperature at the heat source. |
| Immersion Cooling | The entire power module is submerged in an insulating coolant. | Highest heat dissipation efficiency; non-boiling single-phase immersion vs. boiling two-phase immersion. Capable of handling extreme power densities, but system complexity and cost are highest. |
3. Advanced Thermal Management Concepts
3.1 Predictive Thermal Control
The system monitors temperature and load in real-time, predicts future temperature rise trends, and preemptively adjusts fan speeds, pump rates, or even slightly reduces output power to prevent temperatures from reaching critical levels.
3.2 Electro-Thermal Co-Design
Thermal design is synchronized with electrical and structural design from the early stages of development. For example, simulations are used to optimize the layout of power modules, ensuring that high heat flux components are preferentially placed near the coolant inlet.
4. The Lifeline System Working in Concert
Auxiliary power supplies and thermal management systems together form the core safeguards of a solid-state transformer. Their relationship can be summarized as follows:
4.1 The Auxiliary Power Supply - Ensuring System Operability
It is the prerequisite for ensuring that the system "can operate," providing power to all control units, including those of the thermal management system (fans, water pumps).
4.2 The Thermal Management System - Ensuring System Durability
It is the cornerstone for ensuring that the system "can sustain operation," safeguarding main power devices and the auxiliary power supply itself from failure due to overheating.
A highly reliable SST is inevitably the result of a perfect integration of outstanding electrical design, thermal management, and control design.
