TIDUF64 December   2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 DC-DC Boost Converter
      2. 2.2.2 Bidirectional DC-DC Converter
      3. 2.2.3 DC-AC Converter
    3. 2.3 Highlighted Products
      1. 2.3.1  TMDSCNCD280039C - TMS320F280039C Evaluation Module C2000™ MCU controlCARD™
      2. 2.3.2  LMG3522R030 650-V 30-mΩ GaN FET With Integrated Driver, Protection and Temperature Reporting
      3. 2.3.3  TMCS1123 - Precision Hall-Effect Current Sensor
      4. 2.3.4  AMC1302 - Precision, ±50-mV Input, Reinforced Isolated Amplifier
      5. 2.3.5  ISO7741 Robust EMC, Quad-channel, 3 Forward, 1 Reverse, Reinforced Digital Isolator
      6. 2.3.6  ISO7762 Robust EMC, Six-Channel, 4 Forward, 2 Reverse, Reinforced Digital Isolator
      7. 2.3.7  UCC14131-Q1 Automotive, 1.5-W, 12-V to 15-V VIN, 12-V to 15-V VOUT, High-Density > 5-kVRMS Isolated DC/DC Module
      8. 2.3.8  ISOW1044 Low-Emissions, 5-kVRMS Isolated CAN FD Transceiver With Integrated DC/DC Power
      9. 2.3.9  ISOW1412 Low-Emissions, 500kbps, Reinforced Isolated RS-485, RS-422 Transceiver With Integrated Power
      10. 2.3.10 OPA4388 Quad, 10-MHz, CMOS, Zero-Drift, Zero-Crossover, True RRIO Precision Operational Amplifier
      11. 2.3.11 OPA2388 Dual, 10-MHz, CMOS, Zero-Drift, Zero-Crossover, True RRIO Precision Operational Amplifier
      12. 2.3.12 INA181 26-V Bidirectional 350-kHz Current-Sense Amplifier
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Setup
      1. 3.2.1 DC-DC Boost Stage
      2. 3.2.2 Bidirectional DC-DC Stage
    3. 3.3 Test Results
      1. 3.3.1 DC-DC Boost Converter
      2. 3.3.2 Bidirectional DC-DC Converter
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Tools and Software
    3. 4.3 Documentation Support
    4. 4.4 Support Resources
    5. 4.5 Trademarks
  11. 5About the Author

2.2.3 DC-AC Converter

Figure 2-4 shows a block diagram for the DC-AC stage. The inverter stage is responsible for converting DC power to AC power. The topology is constituted by an H-Bridge with each group of diagonal switches operating at high frequency during one half-wave of output voltage. Additional switches placed in parallel to the grid allows an additional voltage-level across the output filter making this power conversion system a three-level topology. This enables constant common-mode voltage leading to negligible leakage current since the PV input stage is decoupled from the AC grid in the freewheeling phase.

GUID-20231206-SS0I-RQX5-JPB8-CH76L4JF0RTP-low.svg Figure 2-4 DC-AC Converter Block Diagram

This topology is an excellent choice for such transformerless string inverter applications where there is no isolation available between the AC grid and the PV panels. The common-mode currents are a well-known challenge in PV applications due to PV surfaces exposed over grounded roof or other surfaces in the proximity. The large surface areas can lead to high values of stray capacitance between the PV panel and ground, which can go as high as 200 nF / kWp in damp environments or on rainy days. This parasitic capacitance can cause high common-mode current flowing into the system when common-mode voltage of the converters is not well mitigated and can lead to EMI and issues such as grid current distortion.

This converter is operated at 100-kHz switching frequency for sinusoidal grid current control, allowing the EMI filter design to be compact. With the 230-V grid, an output power of 3.6 kW can be achieved with an output current of 15.6 ARMS. The EMI filter is composed of a boost inductor split between both rails, two common-mode chokes, Cx capacitors, and Cy capacitors. The EMI filter has been designed to attenuate both the differential-mode and common-mode noise injected into the grid. Additionally, electrolytic capacitors are present at the DC link to compensate for the power ripple present in such single-phase applications. For this application, the boost inductor chosen is Bourns 145453 (D6743) which is around 96 μH in value with a DC resistance of 30 mΩ. For the electrolytic capacitors, a combination of four of ALH82D161DD600 in parallel is considered, where each one is 160-μF rated leading to a total capacitance of 640 μF.