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

2.2.1.4 Main Switching Power MOSFET Selection

The drain-to-source RMS current, IDS_RMS, through switching FET is calculated using:

Equation 27. I D S _ R M S =   I P R M S = 0.641 A

Select a MOSFET with five times the IDS_RMS calculated. The maximum voltage across the FET can be estimated using:

Equation 28. V D S P K =   V I N ( m a x ) × 2 + V O C V + V F × N P S + V L K = 460 V × 2 + 12 V + 0.8 V × 7 + 63 V = 803.1 V

Considering a de-rating of 15% and leakage spike of around 150 V, the voltage rating of the MOSFET must be around 925-V DC. A 950-V MOSFET is selected.