TIDUF63 December   2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 PV or Battery Input With DC/DC Converter
    2. 1.2 Isolation and CLLLC Converter
    3. 1.3 DC/AC Converter
    4. 1.4 Key System Specifications
  8. 2System Design Theory
    1. 2.1 Boost Converter Design
    2. 2.2 MPPT Operation
    3. 2.3 CLLLC Converter Design
      1. 2.3.1 Achieving Zero Voltage Switching (ZVS)
      2. 2.3.2 Resonant Tank Design
    4. 2.4 DC/AC Converter Design
  9. 3System Overview
    1. 3.1 Block Diagram
    2. 3.2 Design Considerations
      1. 3.2.1 DC/DC Converter
        1. 3.2.1.1 Input Current and Voltage Senses and MPPT
        2. 3.2.1.2 Inrush Current Limit
      2. 3.2.2 CLLLC Converter
        1. 3.2.2.1 Low-Voltage Side
        2. 3.2.2.2 High-Voltage Side
      3. 3.2.3 DC/AC Converter
        1. 3.2.3.1 Active Components Selection
          1. 3.2.3.1.1 High-Frequency FETs: GaN FETs
          2. 3.2.3.1.2 Isolated Power Supply
          3. 3.2.3.1.3 Low-Frequency FETs
        2. 3.2.3.2 Passive Components Selection
          1. 3.2.3.2.1 Boost Inductor Selection
          2. 3.2.3.2.2 Cx Capacitance Selection
          3. 3.2.3.2.3 EMI Filter Design
          4. 3.2.3.2.4 DC-Link Output Capacitance
        3. 3.2.3.3 Voltage and Current Measurements
    3. 3.3 Highlighted Products
      1. 3.3.1  TMDSCNCD280039C - TMS320F280039C Evaluation Module C2000™ MCU controlCARD™
      2. 3.3.2  LMG3522R050 - 650-V 50-mΩ GaN FET With Integrated Driver
      3. 3.3.3  LMG2100R044 - 100-V, 35-A GaN Half-Bridge Power Stage
      4. 3.3.4  TMCS1123 - Precision Hall-Effect Current Sensor
      5. 3.3.5  AMC1302 - Precision, ±50-mV Input, Reinforced Isolated Amplifier
      6. 3.3.6  AMC3330 - Precision, ±1-V Input, Reinforced Isolated Amplifier With Integrated DC/DC Converter
      7. 3.3.7  AMC1311 - High-Impedance, 2-V Input, Reinforced Isolated Amplifier
      8. 3.3.8  ISO6741 - General-Purpose Reinforced Quad-Channel Digital Isolators with Robust EMC
      9. 3.3.9  UCC21540 - Reinforced Isolation Dual-Channel Gate Driver
      10. 3.3.10 LM5164 - 100-V Input, 1-A Synchronous Buck DC/DC Converter with Ultra-low IQ
  10. 4Hardware, Software, Testing Requirements, and Test Results
    1. 4.1 Hardware Requirements
    2. 4.2 Test Setup
      1. 4.2.1 DC/DC Board
      2. 4.2.2 DC/AC Board
    3. 4.3 Test Results
      1. 4.3.1 Input DC/DC Boost Results
      2. 4.3.2 CLLLC Results
      3. 4.3.3 DC/AC Results
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
    2. 5.2 Tools and Software
    3. 5.3 Documentation Support
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Author

1.2 Isolation and CLLLC Converter

Micro inverters require isolation between PV panels and the AC grid because of a variety of reasons such as the following:

  • Electrical safety
  • Mitigation of common-mode currents flowing between the panels and the grid
  • High input or output voltage ratio

From a safety point of view, PV panels can be touched by the end-user, thus isolation can mitigate the electrical shock hazard. The common-mode currents are a well-known challenge in PV applications due to PV surfaces exposed over grounded roofs or other surfaces in the proximity. This enormous quantity of surface leads to high parasitic capacitance between the panels and the ground (200 nF / kW). This parasitic capacitance can cause high common-mode current flowing into the system when common mode voltage of the converters is not mitigated enough. A common strategy to significantly reduce the parasitic currents flowing in the system is to add an isolation stage between the panels and the grid.

GUID-20231204-SS0I-H7GP-NPRB-4P8FMSBTX85F-low.svg Figure 1-2 PV Panel Parasitic Capacitance
GUID-20231204-SS0I-VJSS-GJNJ-G12S2SLT2T5W-low.png Figure 1-3 Blocking Common-Mode Noise

The third reason to use an isolated transformer is to efficiently convert power from 75 V to 400 V.

When converting from 75 V to 400 V, using non-isolated DC/DC, challenges like very short duty cycle and high losses in inductor and switches are found. To improve efficiency and thermal performance of the conversion stage, a transformer CLLLC was used.

The input and output voltage for the CLLLC converter is fixed and regulated by PV or battery inputs and DC/AC converter, respectively. This means that no voltage regulation is needed in this stage.

To address all these requirements, CLLLC topology with fixed-frequency was selected, thus leading to small magnetic size and high efficiency. This converter can be optimized to operate in the most favorable point and achieve Zero Voltage Switching (ZVS) in the entire load range.

To increase efficiency and provide bidirectional power flow, this design uses CLLLC topology with Synchronous Rectification (SR). When power flows from the LV to the HV, SR is implemented on the HV side. In reverse power flow, the excitation is on HV side and SR on the LV side.

GUID-20231206-SS0I-CXSL-VRPQ-DQVRSFVVQM8Q-low.svg Figure 1-4 CLLLC Block Diagram

Driving the transformer in a CLLLC converter can be achieved with two possible configurations: full-bridge and half-bridge configurations. The full-bridge requires twice the amount of switches with respect to the half-bridge configuration. Conversely, half-bridge has 2 times more current with the same power level.

On the LV side current is higher, thus making the full-bridge converter the best option. The HV side has much higher voltage and lower current levels, thus making a half-bridge converter the optimum design.