TIDUF03 December   2022

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 System Design Theory
      1. 2.2.1 Detection Principals
      2. 2.2.2 Saturation
      3. 2.2.3 General Mode of Operation
    3. 2.3 Highlighted Products
      1. 2.3.1 DRV8220
      2. 2.3.2 OPAx202
      3. 2.3.3 TLVx172
      4. 2.3.4 TLV7011
      5. 2.3.5 INA293
      6. 2.3.6 SN74LVC1G74
      7. 2.3.7 TLV767
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware
      1. 3.1.1  Board Overview
      2. 3.1.2  Filter Stage
      3. 3.1.3  Differential to Single-Ended Converter
      4. 3.1.4  Low-Pass Filter
      5. 3.1.5  Full-Wave Rectifier
      6. 3.1.6  DC Offset Circuit
      7. 3.1.7  Auto-Oscillation Circuit
        1.       31
      8. 3.1.8  DRV8220 H-Bridge
      9. 3.1.9  Saturation Detection Circuit
      10. 3.1.10 H-Bridge Controlled by DFF
      11. 3.1.11 MCU Selection
      12. 3.1.12 Move Away From Timer Capture
      13. 3.1.13 Differentiating DC and AC From the Same Signal
      14. 3.1.14 Fluxgate Sensor
    2. 3.2 Software Requirements
      1. 3.2.1 Software Description for Fault Detection
    3. 3.3 Test Setup
      1. 3.3.1 Ground-Fault Simulation
    4. 3.4 Test Results
      1. 3.4.1 Linearity Over Temperature
    5. 3.5 Fault Response Results
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5About the Author

3.1.11 MCU Selection

For this design, the MSP430F5132 MCU was originally chosen for the high clock speed required for timer capture DC fault detection. Use a lower-cost MCU instead, since a high clock speed was a requirement for duty cycle detection using the timer capture method. The timer capture method directly measured duty cycle changes by triggering a timer on each duty cycle edge, so a high clock speed was required. This timer capture method is used in many existing RCD modules. With testing, it was determined that a lower BoM cost is achieved by reading DC fault with an ADC instead of using the timer capture method.

The most important specification of MCU choice is the integrated ADC.

The ADC must have an effective resolution small enough to consistently differentiate between a fault. This design sees a 200-mV filter output during a 6-mA DC fault, and 600-mV maximum filter output during an 30-mARMS AC fault(1). This design uses a 10-bit ADC with a full scale range of 1.5 V integrated in the MSP430F5132 MCU.

The ADC needs a sample speed faster than 2000 samples per second. The software stores the low value read by the ADC and uses the low value to determine if an AC or DC fault occurred. The ADC must sample quickly enough to consistently detect a low value during an AC waveform to differentiate AC versus DC fault.

The largest source of noise is the ADC reference voltage error. This design has a total reference voltage error of 1.5%. This is the most significant source of error. The fault detection signal needs to be gained enough to make this error insignificant.

1. These fault threshold values can be increased by increasing the gain of the filter stage.