A soft-start circuit is an electrical control function that gradually increases current or voltage when a power system is switched on, rather than applying full power instantly. Its purpose is to limit inrush current and reduce electrical and mechanical stress on components.
In EV charging equipment and power supplies, soft-start is commonly used to protect rectifiers, capacitors, transformers, contactors, and upstream protection devices during startup.
Why Soft-Start Circuits Matter in EV Charging Infrastructure
Charging systems contain power electronics and large capacitors that can draw very high inrush currents at energization.
– Prevents nuisance tripping of breakers and fuses during charger startup
– Reduces stress on contactors, relays, and rectifier components
– Improves reliability and extends component lifetime
– Reduces voltage dips that can affect other loads on the same site
– Supports smoother commissioning and power-up sequences for multi-charger sites
For sites with many chargers or DC power cabinets, soft-start helps avoid synchronized inrush events.
How a Soft-Start Circuit Works
Soft-start can be implemented in different ways depending on the power stage.
– Pre-charge resistors: charge DC link capacitors through a resistor before bypassing it with a relay/contactor
– NTC thermistors: initially limit inrush current, then warm up and reduce resistance
– Controlled switching: ramp gate drive or PWM duty cycle in power converters
– Sequential startup: power modules start one by one to avoid all loads energizing at once
– Inrush limiting devices: dedicated inrush limiters or electronic current limiters
In many DC chargers, a pre-charge circuit is used to bring the DC bus up safely before enabling the main converter stage.
Where Soft-Start Is Used in Charging Systems
– DC charger power modules (AC/DC rectifier and DC link)
– Auxiliary power supplies and control power circuits
– AC chargers with large filters or power factor correction stages
– Site switchgear sequences where multiple chargers are energized after outages
– Systems with contactless payment terminals and communication equipment that must boot reliably
Key Benefits of Soft-Start
– Reduced inrush current and less stress on electrical components
– Lower risk of upstream protection trips during energization
– Improved power quality at startup (less voltage sag)
– Better reliability, especially in multi-charger installations
– More predictable restart behavior after power interruptions
Limitations to Consider
– Added components and complexity (resistors, relays, control logic)
– Soft-start components can fail (e.g., pre-charge relay contacts) and cause startup faults
– Poorly tuned soft-start can increase startup time or cause repeated retries
– Does not replace correct breaker sizing and short-circuit protection
– Inrush issues can still occur if many chargers start simultaneously without sequencing
Related Glossary Terms
Inrush current
Pre-charge circuit
Power modules
Power factor correction (PFC)
Short-circuit protection
Breaker selectivity
Power quality
Startup sequencing
Overcurrent protection device (OCPD)
DC fast charging