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

3.3.2 Zero Voltage Switching and Circulating Current

Zero Voltage Switching (ZVS) can significantly reduce switching losses and increase efficiency. To realize ZVS, the current flows from drain-to-source and discharge COSS capacitance prior to turn on. In the DAB converter, the realization of ZVS plays a critical role because the switching frequency is high.

Consider a switching event for the Q1, Q2 leg of the DC side. Figure 3-6 shows that in the prior switching event, Q2 turned on and the current in the inductor flows. In the figure, parasitic COSS and body diode is shown for Q1.

TIDA-010954 Prior Switching Figure 3-6 Prior Switching

At the switching event, the controller turns off Q2 first and the dead time starts. The current flowing from transformer is almost constant because the current is driven by Lk on the secondary side. When Q2 is turned off, the current must go through Q1. This current starts to flow into the parasitic COSS capacitance of Q1 and is discharged.

TIDA-010954 COSS Discharge Figure 3-7 COSS Discharge

When the COSS is fully discharged, the body diode becomes negative biased and the diode starts to conduct. For GaN devices, this mode is commonly called third quadrant conduction. This mode can have significant losses due to high source-to-drain voltage in the third quadrant. TI recommends keeping the dead time reasonable so the losses remain low.

TIDA-010954 Third Quadrant Conduction Figure 3-8 Third Quadrant Conduction

Finally, when the dead time is finished the controller turns Q1 on. The current starts to flow through the switch. This turn on is called a soft switch because there is almost no voltage loss, no current overlapping loss, and no COSS losses.

TIDA-010954 Soft Switching Figure 3-9 Soft Switching

There are three switching conditions:

  • Full-ZVS
  • Partial-ZVS
  • Hard switch

If the current flowing from source-to-drain is able to fully discharge parasitic COSS capacitance within the dead time, the ZVS condition is satisfied and this switching is called full soft switch.

In case the current at the switching event is not enough to discharge COSS within dead time - the switching is partial ZVS. The parasitic capacitor still has stored energy that is dissipated in the switch at the turn-on event. This decreases efficiency and increases switch temperature. The increase of the dead time and turn-off current helps to make the switching softer.

In case the current at the switching event flows in the opposite direction, the switching is fully hard. Full energy stored in the COSS is dissipated in a switch. The increase of DT has no effect on switching behavior in this case. The controller needs to optimize the modulation parameters to achieve the correct direction of the current at the switching event. In many cases, phase shift angles or frequency can be optimized to increase range when ZVS is feasible. Nevertheless, in some conditions (for example, very light load) it is not possible to achieve ZVS for all legs.

Circulating current is very important for DAB. The circulating current is analogous with reactive current in grid systems. Like reactive current, the circulating current causes additional conduction losses. But unlike reactive current in a grid, the circulating current in DAB can be useful in some cases.

To illustrate the role of circulating current consider the SPS mode of DAB. The current transferred to the secondary side increases with the increase of the fundamental phase shift. Together with the active current increase, the circulating current is increased. When the primary- and secondary-side voltages have different polarities, the converter changes the direction of the current in the inductor (circulating time). During circulating time, DAB discharges the inductor current to zero and then recharges the inductor back with opposite polarity. This discharge and recharge does not contribute to the power transfer and has two negative effects. The first effect is basically conduction losses during the discharge and recharge process. The longer this process takes, the bigger the circulating losses are in the system. Second, the circulating time reduces active time in the power transfer cycle. This means that with shorter active time, the converter needs higher current amplitude to deliver the same average current to the output.

TIDA-010954 Output Current vs Circulating CurrentFigure 3-10 Output Current vs Circulating Current

The output current is increasing if the fundamental phase shift increases from 0 to 0.25 of the switching period. Nevertheless, the RMS current increases drastically if PS is higher than 0.125 while the active current increase is not so significant. So using fundamental phase shift higher than ±0.125 is not recommended in SPS mode.

Conversely, this circulating current provides the turn-off current for ZVS realization in DAB. Typically, the converter is designed to keep circulating current as small as needed to achieve ZVS. The internal phase shift in EPS method and frequency variation can be leveraged to control the amount of circulating current in the system.