Inside AC EV Charger Safety: How Over-Temperature Protection Logic Prevents Thermal Runaway

This technical breakdown explains the Over-Temperature Protection (OTP) logic in AC EV chargers, a critical safety feature compliant with GB/T 18487 and IEC 61851 standards.

The system utilizes a high-frequency 2ms sampling cycle to monitor two key components with differentiated thresholds:

Control Board:​

Triggers OTP if the temperature exceeds 85°C for 8 seconds. It resets once the temperature drops below 70°C.

Charging Gun:​

Triggers OTP if the temperature exceeds 90°C for 3 seconds. It also resets at 70°C.

The design incorporates a 15°C hysteresis  (85°C down to 70°C) to prevent relay chattering caused by temperature fluctuations. Upon detecting an over-temperature condition in either zone, the system immediately opens the main relay to cut off power and stops the CP PWM signal  to instruct the vehicle to halt charging. The charger returns to standby mode only when both components have cooled below the reset threshold.

This logic aids engineers in component selection and helps maintenance teams quickly diagnose whether faults stem from the control board or the charging gun.