How does the communication charging station motherboard limit the charging current? The relationship between CP duty cycle and 7kW/11kW/22kW

Conclusion: AC charging stations do not directly control the current, but tell the vehicle "how much current can be used at most" through the PWM duty cycle of the CP signal, which is followed by the vehicle OBC itself. The essence of limiting current on the motherboard is to adjust this duty cycle.

1、 The correspondence between CP duty cycle and current

• The CP signal is a PWM square wave, and the duty cycle determines a larger available current.

• The corresponding relationship specified in the international standard IEC 61851-1 is: a duty cycle of 10% corresponds to 6A, and for every 1% increase in duty cycle thereafter, the current increases by 0.6A.

• For example, a duty cycle of 53.3% corresponds to approximately 32A, and a duty cycle of 26.7% corresponds to approximately 16A.

• A duty cycle below 10% indicates non rechargeable, while a duty cycle above 85% enters a special reserved area.

2、 Duty cycle corresponding to 7kW/11kW/22kW

• 7kW single-phase 220V, rated current of about 32A, CP duty cycle of about 53%.

• 11kW three-phase 380V, with a current of approximately 16A per phase and a CP duty cycle of approximately 27%.

• 22kW three-phase 380V, with a current of approximately 33A per phase and a CP duty cycle of approximately 55%.

• Note: The duty cycle of the three-phase pile corresponds to the current of each phase, not the total current.

• When designing the motherboard, it is necessary to clearly indicate the rated current to avoid misalignment between duty cycle and power calibration.

3、 How to limit the current on the motherboard

• There is a PWM generator inside the motherboard, which calculates the target current based on the rated power of the pile, grid capacity, temperature, scheduling instructions, etc., and then converts it into a duty cycle to output to the CP line.

• After receiving the duty cycle, the vehicle OBC interprets it as "greater current per phase" and controls the actual power supply not to exceed this value.

• The motherboard does not directly cut off the current, but narrows the license range.

4、 Scenarios with dynamic restrictions

• When the motherboard detects that the temperature is too high, the total power exceeds the limit, or receives a background derating command, it will lower the duty cycle.

• For example, if the current drops from 53% to 27%, the vehicle's OBC will then decrease the current from 32A to 16A.

• If the vehicle does not comply, the motherboard will trigger the protection disconnect relay if it detects that the actual power supply exceeds the limit through current sampling.

Summary:

The CP duty cycle is the "current permission" for the vehicle, and the motherboard limits the charging current by adjusting the duty cycle. 7kW corresponds to approximately 53%, 11kW corresponds to approximately 27%, and 22kW corresponds to approximately 55%, all of which are currents per phase. The motherboard does not directly control the current, but sets an upper limit, which is executed by the vehicle.

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