PWM (Pulse Width Modulation) is a method of controlling electrical power by rapidly switching a signal on and off, varying the duty cycle (the percentage of time the signal is “on”). The higher the duty cycle, the more effectively the power is delivered. In EV charging, PWM is best known for the Control Pilot (CP) signal defined in IEC 61851, where PWM communicates the maximum available charging current from the EVSE to the vehicle.
Why PWM Control Matters in EV Charging
PWM enables safe, standardized control without complex data communication.
– Allows the EVSE to advertise the maximum allowable current to the EV
– Supports load management and load balancing by dynamically reducing the available current
– Helps prevent circuit overload by matching charging current to site capacity
– Enables interoperability between different EVs and chargers using IEC 61851 signaling
– Provides a predictable foundation for smart charging even without advanced protocols
How PWM Control Is Used on the Control Pilot
In IEC 61851 AC charging, the EVSE outputs a PWM signal on the CP line to indicate current availability.
– The EVSE generates a PWM waveform with a defined frequency and duty cycle
– The EV reads the duty cycle and calculates the maximum charging current it is allowed to draw
– The EV then sets its onboard charger to stay within this limit
– If site load changes, the EVSE can adjust the duty cycle to reduce or increase the limit in real time
PWM Control vs Power Electronics PWM
PWM appears in two different contexts in charging systems.
– Control PWM (IEC 61851 CP): a low-power signaling method that sets charging current limits
– Power conversion PWM: high-frequency switching inside inverters/rectifiers and DC power stages to control voltage/current delivery
Both use duty cycle modulation, but only the Control Pilot PWM is directly visible in EVSE-to-vehicle signaling for AC charging.
Practical Examples in Real Installations
– A site uses dynamic load management: the EVSE reduces PWM duty cycle when building load rises, lowering EV charging current
– A shared feeder serves multiple chargers: PWM limits each charger so the total stays below the main fuse rating
– A charger supports 32 A but detects a lower cable rating via Proximity Pilot (PP) and caps the PWM-advertised current accordingly
Key Benefits of PWM Control
– Simple, robust, and standardized current control for AC charging
– Fast response to site load changes without needing cloud connectivity
– Improves safety by enforcing current limits at the vehicle level
– Enables scalable multi-charger installations with predictable electrical behavior
Limitations and Considerations
– PWM on CP mainly controls current limits; it does not handle billing, authentication, or tariffs
– Charging speed may still be limited by the vehicle’s onboard charger, cable rating, or temperature conditions
– Advanced features like Plug & Charge rely on higher-layer standards such as ISO 15118 in addition to IEC 61851 signaling
Related Glossary Terms
– Control Pilot (CP)
– IEC 61851
– Proximity Pilot (PP)
– AC charging
– Load management
– Load balancing
– Type 2 Connector
– ISO 15118