SBAA541 December   2022 AMC1202 , AMC1302 , AMC1306M05 , AMC22C11 , AMC22C12 , AMC23C10 , AMC23C11 , AMC23C12 , AMC23C14 , AMC23C15 , AMC3302 , AMC3306M05

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
    1. 1.1 DC Charging Station for Electric Vehicles
    2. 1.2 Current-Sensing Technology Selection and Equivalent Model
      1. 1.2.1 Sensing of the Current With Shunt-Based Solution
      2. 1.2.2 Equivalent Model of the Sensing Technology
  4. 2Current Sensing in AC/DC Converters
    1. 2.1 Basic Hardware and Control Description of AC/DC
      1. 2.1.1 AC Current Control Loops
      2. 2.1.2 DC Voltage Control Loop
    2. 2.2 Point A and B – AC/DC AC Phase-Current Sensing
      1. 2.2.1 Impact of Bandwidth
        1. 2.2.1.1 Steady State Analysis: Fundamental and Zero Crossing Currents
        2. 2.2.1.2 Transient Analysis: Step Power and Voltage Sag Response
      2. 2.2.2 Impact of Latency
        1. 2.2.2.1 Fault Analysis: Grid Short-Circuit
      3. 2.2.3 Impact of Gain Error
        1. 2.2.3.1 Power Disturbance in AC/DC Caused by Gain Error
        2. 2.2.3.2 AC/DC Response to Power Disturbance Caused by Gain Error
      4. 2.2.4 Impact of Offset
    3. 2.3 Point C and D – AC/DC DC Link Current Sensing
      1. 2.3.1 Impact of Bandwidth on Feedforward Performance
      2. 2.3.2 Impact of Latency on Power Switch Protection
      3. 2.3.3 Impact of Gain Error on Power Measurement
        1. 2.3.3.1 Transient Analysis: Feedforward in Point D
      4. 2.3.4 Impact of Offset
    4. 2.4 Summary of Positives and Negatives at Point A, B, C1/2 and D1/2 and Product Suggestions
  5. 3Current Sensing in DC/DC Converters
    1. 3.1 Basic Operation Principle of Isolated DC/DC Converter With Phase-Shift Control
    2. 3.2 Point E, F - DC/DC Current Sensing
      1. 3.2.1 Impact of Bandwidth
      2. 3.2.2 Impact of Gain Error
      3. 3.2.3 Impact of Offset Error
    3. 3.3 Point G - DC/DC Tank Current Sensing
    4. 3.4 Summary of Sensing Points E, F, and G and Product Suggestions
  6. 4Conclusion
  7. 5References

2.2.4 Impact of Offset

Figure 1-2 shows the equivalent model of a real current sensor with offset. In this study, the current-sensor offset is modeled as a fixed-value normalized to the full scale of the measurement, see Equation 8.

Equation 8. IO=IMAX δO

where

  • I0 is the absolute offset value presented by the sensor
  • IMAX is the maximum of the measurement scale
  • ẟ0 is the per-unit value of the offset error introduced in the measurement

The goal of the current control loops of the AC/DC stage is to keep the currents detected by the MCU under control without determining the real currents in the system. If the measurement does not match the actual current due to an offset error, the current causes an undesired power disturbance in the system as is expressed with Equation 9.

Equation 9. ΔPO=V[IO1sin(ωt)+IO2sin(ωt-23π)+IO3sin(ωt+23π)]

where

  • ΔPO is the power disturbance caused by the offset errors as function of time
  • IO1, IO2, and IO3 are offset errors of each current sensor
  • V is the phase to neutral RMS voltage
  • ɷ is the electrical pulsation derived from the grid frequency

The power disturbance is not a function of the power conversion between the AC and DC stage, as opposed to the gain error case; therefore, the issue is always present for any operating condition. This reflects by always having voltage ripple in the DC link. The offset introduces a power disturbance in the system with a frequency equivalent to the line frequency of the grid. As mentioned in the gain error chapter, the DC bus voltage loop is not able to fully reject the power ripple coming out from the sensing point. For this reason control loop versus current-sensing performance must be simulated. Simulations were run for the following use-case and assumptions:

  • DC bus voltage working at the minimum rated voltage to maximize the ripple voltage (650 V)
  • Maximum power exchange between AC and DC side. This has no effect on the result. The results are the same for the no-load condition.
  • Offset error defined with respect to the full measurement scale per unit. When using a shunt-based design with ±50-mV isolated device, the maximum scale is ±32 A.
  • Offset for the three-phases applied to reach the worst-case scenario as follows: IO1 = –IO2 = –IO3
  • Current control-loop bandwidth kept constant in all the simulations (3 kHz)
  • AC filter designed with the aim to keep the grid THD at the nominal power at 3% when using ideal sensing
  • Power line frequency is 50 Hz

Figure 2-16 shows the simulation results of a AC/DC converter working with different current sensing and with different offset errors.

Figure 2-16 DC-Link Voltage Ripple Over Time With DC Link Bandwidth and Offset Error as Parameters

Observe the presence of a 50-Hz ripple voltage on the DC-link, caused by the power ripple injected by the current sensing stage with an offset. Additionally, the mean value of the voltage in all the cases is still the same when steady state is reached thanks to the integrative part of the PI controller.

The important correlation between the DC link voltage ripple and voltage control bandwidth is evident. If the bandwidth of the voltage control loop is high enough, the control loop tries to eliminate the ripple voltage by quickly controlling the current loops at the expense of the THD toward the grid. In fact, with a bandwidth of the control equal to 400 Hz, an offset of 1.4% offset error, leads to an increment of 10% of the THD (from 3% to 3.3%). Conversely, when the bandwidth of the voltage loop is not high, fluctuation in the DC link is very high because voltage loop is not trying to reject this variation, but this time not injecting any more harmonics in the grid. However, remember that having a voltage ripple in the DC-link can lead to power ripple on the battery which cannot be tolerated. Furthermore, if the voltage bandwidth is significantly reduced, the performance of the step load response becomes quite poor.

In conclusion, a current sensor located in the switching node with an offset error of 1.4% can lead to an increment of the grid current THD of more than 10%.