TIDUF04A December   2022  – December 2025

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1.     8
    2. 1.1 EV Charging Station Challenges
      1. 1.1.1 Efficient Relay and Contactor Drive
      2. 1.1.2 Contact Weld Detection
    3. 1.2 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
      1. 2.2.1 Isolated AC/DC Power Supply Design
        1. 2.2.1.1  Input Bulk Capacitance and Minimum Bulk Voltage
        2. 2.2.1.2  Transformer Turns-Ratio, Primary Inductance, and Primary Peak Current
        3. 2.2.1.3  Transformer Parameter Calculations: Primary and Secondary RMS Currents
        4. 2.2.1.4  Main Switching Power MOSFET Selection
        5. 2.2.1.5  Rectifying Diode Selection
        6. 2.2.1.6  Output Capacitor Selection
        7. 2.2.1.7  Capacitance on VDD Pin
        8. 2.2.1.8  Open-loop Voltage Regulation Versus Pin Resistor Divider, Line Compensation Resistor
        9. 2.2.1.9  Feedback Elements
        10. 2.2.1.10 Backup Power Supply
        11. 2.2.1.11 Supercapacitor Selection
        12. 2.2.1.12 Supercapacitor Charger Design
      2. 2.2.2 Relay Drive and Weld Detect
    3. 2.3 Highlighted Products
      1. 2.3.1 UCC28742
      2. 2.3.2 DRV8220
      3. 2.3.3 ATL431
      4. 2.3.4 TL431
      5. 2.3.5 TPS55330
      6. 2.3.6 TPS259470
      7. 2.3.7 TL7705A
  9. 3Hardware, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
    2. 3.2 Test Requirements
      1. 3.2.1 Power Supply Test Setup
      2. 3.2.2 Weld Detect Test Setup
    3. 3.3 Test Results
      1. 3.3.1 Isolated AC/DC Power Supply Based on UCC28742
        1. 3.3.1.1 Efficiency and Output Voltage Cross Regulation
        2. 3.3.1.2 Output Voltage Ripple Waveforms
        3. 3.3.1.3 Start, Shutdown, Backup Power, and Transient Response Waveforms
        4. 3.3.1.4 Thermal Performance
      2. 3.3.2 DRV8220-Based Relay Drive
      3. 3.3.3 Isolated Line Voltage Sensing
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 Bill of Materials
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  11. 5About the Author
  12. 6Revision History

2.2.1.7 Capacitance on VDD Pin

The capacitance on VDD needs to supply the device operating current until the output of the converter reaches the target minimum operating voltage in CC regulation. The capacitance on VDD must supply the primary-side operating current used during start-up and between low-frequency switching pulses. The largest result of two independent calculations denoted in Equation 36 determines the value of CVDD.

At start-up, when VVDD(on) is reached, CVDD alone supplies the device operating current and MOSFET gate current until the output of the converter reaches the target minimum operating voltage in CC regulation, VOCC. Now the auxiliary winding sustains VDD for the UCC28742 device above UVLO. The total output current available to the load and to charge the output capacitors is the CC-regulation target, IOCC. Equation 36 assumes that all of the output current of the converter is available to charge the output capacitance until VOCC is achieved. For typical applications, Equation 37 includes an estimated qG × fSW(max) of average gate drive current and a 1-V margin added to VVDD.

Equation 36. C V D D ≥   I R U N + q G f S W ( m a x ) × C O U T × V O C C I O C C V D D o n - V D D o f f + 1 V
Equation 37. C V D D ≥   2 m A + 10 n C × 37.7 k H z × 1360 µ F × 12 V 2.2 V 21 V - 8.5 V + 1 V ≥ 0.128 µ F

The current design uses 10-µF and 0.1-µF capacitors.