Application of plastic optical fiber in solid-state transformers (SST)

In solid-state transformers (SSTs), Plastic optical fibers primarily handle short-distance digital signal transmission under high-voltage electrical isolation. Their core function is to handle control command issuance and status feedback in high dv/dt environments, rather than power transmission.

Core Application Scenarios:

Isolation Drive Signal Transmission: Connecting the central controller (DSP/FPGA) to the isolation driver of the high-voltage side power module (such as a SiC/GaN H-bridge), transmitting PWM trigger signals and protection signals (such as DESAT fault feedback).

Cascaded Module Synchronization Communication: In multi-level topologies (such as CHB, MMC), utilizing the low-latency characteristics of POF to transmit clock synchronization signals, ensuring phase consistency of switching actions of dozens of series-connected sub-modules and suppressing harmonic distortion.

Internal Status Monitoring Data Feedback: Transmitting temperature, voltage, and current sampling data from the high-voltage side back to the low-voltage side processing unit via optical fiber, achieving fully digital monitoring. Technical Advantages and Selection Logic

**Extreme Electromagnetic Interference Resistance:** SST internal SiC device switching generates extremely strong high-frequency noise (>100kV/μs common-mode transient), and copper wires are prone to coupling interference leading to false triggering. POF, based on optical transmission, is naturally immune to electromagnetic interference (EMI).

**High Voltage Withstand and Safety:** POF has high dielectric insulation strength, easily achieving electrical isolation above 10kV, meeting the safe creepage distance requirements for medium-voltage direct-connected SSTs, and eliminating the risk of arcing.

**Low Cost and Easy Installation:** Compared to quartz glass fiber, POF has a larger core diameter (typically 0.5-1mm), larger mating tolerance, and requires no precision fusion splicing; inexpensive injection-molded connectors can be used, significantly reducing the modular assembly cost of SSTs.

**Sufficient and Moderate Bandwidth:** SST control signal frequencies are typically <10MHz, while POF transmission bandwidth (reaching tens of MHz to GHz levels) fully covers the requirements, and the delay is fixed (approximately 100ns/10m), facilitating timing compensation. Limitations and Replacement Trends

Limited Transmission Distance: Due to the attenuation of plastic materials, the effective transmission distance is typically <50 meters, suitable only for communication within the SST chassis or between adjacent modules. Long-distance communication still requires quartz fiber optics.

High-Temperature Stability Challenges: PMMA materials typically withstand temperatures <85℃, while fluoroplastics are slightly better but more expensive. SSTs have high power density and large temperature rise, requiring strict thermal management or the selection of special high-temperature resistant POF.

Emerging Technology Competition: Some new SST designs are beginning to use millimeter-wave wireless isolation or high-performance capacitive isolation chips to replace optical fibers, further reducing size, simplifying wiring, and improving integration. However, in industrial-grade SSTs with extremely high reliability requirements, POF remains the mainstream mature solution.

In summary, plastic optical fiber is a key low-cost medium for achieving "safe isolation between high-voltage and low-voltage" in SSTs, especially suitable for control links in short-to-medium distance, high-noise environments. However, its application is limited by temperature and distance, requiring comprehensive selection based on the specific SST topology and heat dissipation design.