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

2.2.1.1 Input Bulk Capacitance and Minimum Bulk Voltage

The minimum voltage on the input bulk capacitance is needed to determine the maximum primary-to-secondary turns-ratio of the transformer. The input power of the converter based on target full-load efficiency, the minimum input RMS voltage, and the minimum AC input frequency determine the input capacitance requirement. Maximum input power is determined based on Equation 1:

Equation 1. P I N = V O C V × I O C C η = 12 V × 2.2 A + - 14 V × 0.1 A + + 14 V × 0.1 A 0.8 ≈ 36.5   W

where

  • VOCV is the regulated output voltage of the converter
  • IOCC is the converter total output CC target
  • η is the converter overall efficiency at full-power output

Equation 2 provides an accurate solution for the total input capacitance based on a target minimum bulk capacitor voltage. Alternatively, to target a given input capacitance value, iterate the minimum capacitor voltage to achieve the target capacitance value.

Equation 2. C B U L K = 2 P I N × 0.25 + 1 2 π × a r c s i n V B U L K ( d e s i r e d ) 2 × V I N ( m i n ) 2 V I N ( m i n ) 2 - V B U L K ( d e s i r e d ) 2 × f L I N E ( m i n ) ≈ 58.9   μ F
Equation 3. C B U L K ( s e l e c t e d ) = 68   μ F

Four 68-µF electrolytic capacitors were used at the input to create an equivalent 68-µF bulk capacitor to support a maximum input voltage of 460 VRMS. This selection changes the minimum VBULK voltage to 90.7 V (also called VBULK_VALLEY) per the UCC28742 design calculator.