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

2.2.2.1 Fault Analysis: Grid Short-Circuit

To evaluate the maximum latency required by the AC/DC to shut down safely, system simulations were performed by applying the following conditions:

  • DC bus voltage working at the maximum rated voltage (800 V)
  • Converter operating at the nominal current (16 ARMS)
  • Short circuit injected when the maximum current of a phase is drained
  • No linear inductance of boost inductors with flux versus current profile of soft-magnetics materials; the inductance versus current is optimized for an 11-kW AC/DC and the inductance decreases down to 30% of the nominal value when saturation is achieved
  • The overcurrent threshold of the current sensing in point B is set up at 30 A (93.7% of measurement range)
  • Based on available data sheets of power components used in 11-kW applications, a maximum-allowed current of 60 A was selected

When a short-circuit is happening in the grid the converter is still switching, thus leading to uncontrolled currents. Since the fault is happening suddenly, there is not enough time for the MCU to update and correct the duty cycles. PWM updates typically happen at a fixed frequency (70 kHz or every 14.2μs in this example). By following single and double update refresh techniques, the minimum reaction time of the MCU can be 1/fs or 1/2fs. Within this time, the current in the inductor can exceed the short-circuit current rating of the power switch.

Figure 2-11 depicts the voltage and currents of the AC/DC converter. Figure 2-11 shows that in the time frame between 0ms and 19ms, the converter is operating at the nominal condition with a grid voltage equal to 400 VRMS and a current transferred from the DC to the AC. At 19ms, a short-circuit event is simulated by dropping the phase voltage to 10% of the nominal value. Simultaneously to the grid fault, the currents in the switching node start to increase due to the voltage difference between the grid and the applied one from the switching stage, as shown in Figure 2-12.

Figure 2-11 Grid Voltages and Currents of AC/DC Converter: Short-Circuit Response of the AC/DC Converter
Figure 2-12 Zoomed-in Portion at t = 19.5ms (Span 120μs): Short-Circuit Response of the AC/DC Converter
Figure 2-13 Zoomed-in Portion of PWM at t = 19.5ms (Span 120μs): PWM Turn-off Behavior

At the beginning, the current start-to-rise linearly is because the core is not saturated and is following a fixed di/dt since the inductance is nearly constant:

Equation 4. didt=VDC(1.5L(i))

where

  • L is the AC/DC boost current in function of the current
  • VDC is the DC bus voltage at the moment of the fault

When the saturation current of the core is reached, the inductance value drops significantly, leading to a sudden increase of the current. When the real current in phase L3 reaches 30 A (overcurrent threshold), the MCU must be able to detect the overcurrent as soon as possible, since the MCU cannot detect higher currents, and shuts down before the current reaches a level above 60 A. Based on the simulation results, the current takes 4μs to reach the critical value. After this timing is reached, turn off the PWM signals as shown in Figure 2-13.

In conclusion, the system must turn off within 4µs to avoid damage to power switches. Consider the latency of the current sensing together with those of the MCU and driver stage shut down. Based on typical values of latency time of the MCU and driver stage, a maximum latency of 3.5µs must be provided by the current sensor.