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

2.2.1.12 Supercapacitor Charger Design

The shunt voltage reference (U6) sets the final charge voltage to 7.8 V. The NPN transistor (Q5) and 4.99-Ω resistor (R42) limit the charge current to approximately 120 mA (0.6 V / 4.99 Ω). The NMOS FET (Q4) operates in saturation region to maintain the required charge voltage drop. U6 pulls down the gate of Q4 as soon as Vbackup reaches 7.8 V. This way, Vbackup stays constant at the nominal 7.8 V. There are two charging scenarios:

  1. Charging time as first power supply turn-on: here Vbackup is zero. The ΔV that must be covered is from zero to 7.8 V; therefore, the charging time is ΔT = C × ΔV / I = 1.25 F × 7.8 V / 120 mA = 81.25 seconds (1 minute and 21 seconds).
  2. Charging time after energy storage release: here Vbackup is the UVLO of the boost converter (4.3 V). The ΔV that must be covered is from 4.3 V to 7.8 V; the charging time is
    ΔT = C × ΔV / I = 1.25 F × (7.8 V – 4.3 V) / 120 mA = 36.46 seconds.

In summary, the worst-case charging time is 1 minute and 21 seconds, while recharging between energy storage releases is 36.46 seconds.