Active Snubber Technology for High-Efficiency DC Circuit Breakers

Background

In modern direct current (DC) power distribution systems, solid-state circuit breakers (SSCBs) play a critical role by providing ultrafast fault current interruption. These devices are increasingly essential due to their high modularity and superior power density compared to traditional mechanical alternatives. When a DC fault is interrupted, the circuit breaker must manage substantial inductive energy and protect internal semiconductor components from destructive overvoltages. Consequently, there is a fundamental need for effective snubber circuits capable of clamping voltage spikes and safely absorbing transient surge energy during the fault clearing process.

Currently, conventional SSCB designs rely heavily on metal-oxide varistors (MOVs) for energy absorption, which introduces severe reliability and efficiency challenges. Because MOVs remain continuously exposed to system voltages, repeated surge currents cause progressive degradation, leading to increased leakage currents, thermal runaway, and eventual device failure. Existing mitigation strategies, such as thermal fuses or mechanical disconnects, are either too slow to prevent damage or severely delay system reclosing. Furthermore, to minimize leakage, the MOV voltage rating must exceed the DC bus voltage, while the main switch rating must exceed the MOV clamping voltage. This results in a switch voltage utilization rate below 50%, requiring more series-connected switches and increasing overall cost.

Technology

This active snubber technology for solid-state circuit breakers (SSCBs) facilitates ultrafast interruption of direct current (DC) faults through specialized circuit topologies. The solution integrates a metal-oxide varistor (MOV) with networks of resistors, capacitors, diodes, and either full-controlled (MOSFET) or half-controlled (thyristor) switches, adaptable for unidirectional or bidirectional applications. Operating across synchronized detection, reaction, and preparation intervals, the system coordinates the main and snubber switches to manage fault currents. When a fault occurs, the main switch opens while the snubber switch closes, allowing the capacitor to control the voltage rise rate before the MOV absorbs the surge energy. Once the fault is cleared, the snubber switch opens to completely isolate the MOV from the power line during the breaker’s OFF-state.

This technology is differentiated by its ability to simultaneously eliminate MOV degradation and maximize the main switch voltage utilization rate. Unlike conventional snubbers that leave the MOV exposed to continuous voltage stress—leading to leakage currents and thermal runaway—this active approach disconnects the MOV during the OFF-state to ensure long-term reliability. Furthermore, it boosts the switch voltage utilization rate from 37.5% to 60%, significantly reducing the number of costly series switches required for high-voltage applications. Tested prototypes demonstrated increased power density (from 50 kW/L to 72 kW/L) and improved efficiency (99.92%). Additionally, the snubber switches operate under zero-voltage turn-on and zero-current turn-off conditions, eliminating the need for extensive cooling and enabling fault isolation even in zero-inductance DC grids.

Benefits

• Enhances metal-oxide varistor (MOV) reliability and lifespan by isolating it from the power line during the off-state to prevent degradation and thermal runaway
• Increases the main switch voltage utilization rate from 37.5% to 60%, enabling operation at higher DC bus voltages
• Reduces overall system cost by decreasing the number of series and parallel switches required in the main branch
• Improves system power density (up to 72 kW/L) and overall operational efficiency (up to 99.92%)
• Minimizes switching stress and eliminates the need for extensive cooling by operating snubber switches under zero-voltage turn-on and zero-current turn-off conditions
• Provides ultrafast DC fault interruption capable of isolating faults even in zero-inductance or low-inductance DC grids
• Supports flexible implementation with both unidirectional and bidirectional configurations using either full-controlled or half-controlled switches

Applications

• DC microgrid fault protection
• Electric vehicle fast charging infrastructure
• Data center DC power distribution
• Renewable energy grid integration
• Electric aircraft and shipboard power

Additional info

Fei Lu is an Associate Professor in the Department of Electrical and Computer Engineering at Lehigh University. His areas of research include: Electrical Engineering, Power Electronics, High Voltage DC Power Systems, Semiconductor Device Packaging, Renewable Energy

Assignee: Lehigh University; the patent record also identifies Drexel University as an assignee