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

3.4 MPPT and Input Voltage Ripple

Single phase power applications have significant power ripple. The frequency of this ripple is twice the grid frequency

Equation 12 shows the average power transferred to grid.

Equation 12. P A V G =   V R M S × I R M S × cos ⁡ ( φ )

Equation 13 calculates the voltage in the grid following the sine law.

Equation 13. V t = 2 × V R M S × sin ⁡ ( ω t )

When power factor equals 1, the current in the grid is following the sine wave too, see Equation 14.

Equation 14. I t = 2 × I R M S × sin ⁡ ( ω t )

Equation 15 is the equation for instantaneous power for a single-phase system.

Equation 15. P t = 2 × V R M S ×   I R M S × ⁡   s i n 2 ω t =   V R M S × I RMS × ( 1 +  cos ⁡ ( 2 × ω t ) )

Equation 15 reveals that the instantaneous power consists of a constant part, which represents the average power, and an alternating part possessing twice the grid frequency, characterized as power ripple. This equation also shows that the instantaneous power exhibits variations ranging from zero up to double the average power.

In traditional two-stage inverter approaches, such as LLC plus totem pole configurations, the power ripple is typically managed by the DC-Link capacitors located within the high-voltage DC-AC stage. These DC-Link capacitors can effectively handle voltage ripples of 20% or higher. By contrast, the low-voltage to high-voltage LLC stage is generally designed with a focus on maximum average power capability.

In the single-stage approach, there is no DC-Link capacitor to handle the power ripple, so the converter must be designed to transfer both average power and power ripple to the AC side.

The input of the inverter comes from a PV panel that can be considered as a current source. The power output of this source depends on several factors, including irradiation received by the panel, panel voltage, panel temperature, and so forth.

This indicates that the input capacitors serve as intermediate energy storage elements within each grid cycle. As a result, the input voltage of the converter experiences some ripple, which is directly reflected onto the operating voltage of the PV panel.

Figure 3-18 illustrates the P-V curve of a typical solar panel, demonstrating the relationship between the output of the panel and various parameters. As conditions affecting the PV panel change throughout the day, the power output also varies continuously.

The input power to the inverter remains relatively constant around the Maximum Power Point (MPP), while the output power exhibits a ripple due to these changing conditions. This causes the input voltage of the converter to experience some ripple, which is directly reflected onto the operating voltage of the PV panel. The I-V curve, shown alongside the P-V curve, shows the relationship between the output current of the panel and the output voltage. The intermediate energy storage elements within each grid cycle, provided by the input capacitors, serve to mitigate this ripple effect on the operation of the converter.

TIDA-010954 Input Voltage Ripple on P-V Curve Figure 3-18 Input Voltage Ripple on P-V Curve

As Figure 3-18 shows, any change of the panel voltage from VMPP lowers the power delivered from the panel and lowers the overall output power. Fortunately, across the MPP voltage the variation of the output power is relatively low. To understand the required input capacitance, define the maximum deviation across the power point. This parameter needs to be properly selected, because a deviation that is too low can cause excessive size and cost for capacitors A deviation that is too high can cause the PV panel to operate at an unfavorable point. Typically, the designer selects 99% of the MPPT efficiency as a starting point.

The minimum required capacitance can be calculated with Equation 16.

Equation 16. C ≥ P I N 2 × π × V I N × f A C × V R I P P L E

where

  • PIN is the PV panel maximum power
  • VIN is the PV panel voltage at MPP
  • fAC is the line frequency
  • VRIPPLE is the desired voltage ripple

The conclusion:

  • The converter needs to be designed to handle instantaneous power two times higher than average
  • The energy storage for this ripple is input capacitors
  • The value of the input capacitor needs to be selected to handle maximum voltage ripple required by MPPT