TIDUFD2 May   2025

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Terminology
    2. 1.2 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Input Capacitors Selection
      2. 2.2.2 DC Side
      3. 2.2.3 AC Side
    3. 2.3 Highlighted Products
      1. 2.3.1 TMDSCNCD28P55X - controlCARD Evaluation Module
        1. 2.3.1.1 Hardware Features
        2. 2.3.1.2 Software Features
      2. 2.3.2 LMG2100R026 - 100V, 53A GaN Half-Bridge Power Stage
      3. 2.3.3 LMG365xR035 - 650V 35mΩ GaN FET With Integrated Driver and Protection
      4. 2.3.4 TMCS1123 - Precision 250kHz Hall-Effect Current Sensor With Reinforced Isolation
      5. 2.3.5 TMCS1133 - Precision 1MHz Hall-Effect Current Sensor With Reinforced Isolation
      6. 2.3.6 INA185 - 26V, 350kHz, Bidirectional, High-Precision Current Sense Amplifier
      7. 2.3.7 LM5164 – 100V Input, 1A Synchronous Buck DC-DC Converter With Ultra-Low IQ
      8. 2.3.8 ISO6762 – General-Purpose Six-Channel Reinforced Digital Isolators With Robust EMC
  9. 3System Design Theory
    1. 3.1 Isolation for Solar Inverters
    2. 3.2 Topology Overview
    3. 3.3 Control Theory
      1. 3.3.1 Single and Extended Phase Shift Modulation Technique
      2. 3.3.2 Zero Voltage Switching and Circulating Current
      3. 3.3.3 Optimized Control Method
      4. 3.3.4 Dead-Time Compensation
      5. 3.3.5 Frequency Modulation
      6. 3.3.6 Controller Block Diagram
    4. 3.4 MPPT and Input Voltage Ripple
  10. 4Hardware, Testing Requirements, and Test Results
    1. 4.1 Hardware Requirements
    2. 4.2 Test Setup
      1. 4.2.1 Board Check
      2. 4.2.2 DC-DC Tests
      3. 4.2.3 DC-AC Tests
    3. 4.3 Test 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

3.3.5 Frequency Modulation

As the trajectories show, in Section 3.3.3, ZVS realization is different in different loads, voltages, and modes. At light load in mode III, the controller has wide room between primary and secondary ZVS requirements, this means that there is a lot of current circulating in the system. By changing the frequency, the distance between the primary and secondary ZVS requirements can be narrowed and reduce the circulating current.

In grid sine waves with high peaks, a low load causes a lack of primary side ZVS. The load coefficient M is defined as the ratio of IREF to IN. IN is dependent on LK and the switching frequency. The controller can increase switching frequency at light loads to increase relative load M by decreasing IN.

The best switching frequency is defined by the frequency that achieves the desired distance Δ between primary and secondary ZVS requirements. This Δ is selected by the system designer following a thorough characterization of the system.

Derive the equations for the best switching frequency from Equation 4 and Equation 6. Equation 10 and Equation 11 are the final equations.

Equation 10. FSW,III=N×VDC×m2-2*Δ2×1-m24×LK×IREF×m
Equation 11. FSW,II=N×VDC×(8×Δ2×m2+16×Δ2×m +16×Δ2-8×Δ×m -16×Δ- m2+4)16×LK×IREF×m2

An increase of FSW is because of an increase in the relative load M of the converter. The increased load in mode III of operation reduced the distance between the primary and secondary ZVS requirements but at the same time reduced the RMS current. In mode II the increase of FSW helps to have the minimum load required by the primary lagging arm to keep the current positive at the positive voltage pulse, thus achieving ZVS.

The trajectories clearly show that with heavy loads, the calculated switching frequency is low, limited by the FSW_MIN requirement. Conversely, when working under light loads, the frequency rises to maximum (FSW_MAX) saturation mark. With moderate loads, however, the frequency exhibits significant variability and achieves ZVS over a broad range while maintaining circulation current at a low level.