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

2.2.1.8 Open-loop Voltage Regulation Versus Pin Resistor Divider, Line Compensation Resistor

The resistor divider at the VS pin determines the output voltage regulation point of the flyback converter. Also, the high-side divider resistor (RS1) determines the line voltage at which the controller enables continuous DRV operation. RS1 is initially determined based on transformer auxiliary-to-primary turns ratio and desired input voltage operating threshold.

Equation 38. R S 1 = V I N ( r u n ) × 2 N P A × I V S L ( m i n )

where

  • NPA is the transformer primary-to-auxiliary turns ratio
  • VIN(run) is the ACRMS voltage to enable turn-on of the controller (run); in case of DC input, leave out the √ 2 term in the equation
  • VSL(run) is the run threshold for the current pulled out of the VS pin during the switch on-time (see the Electrical Characteristics section of the UCC28742 data sheet)
Equation 39. R S 1 = 80 V × 2 4.81 × 210 µ A = 112 k Ω
Equation 40. R S 1 ( s e l e c t e d ) = 121 k Ω

The low-side VS pin resistor is selected based on the desired VOUT regulation voltage in open-loop conditions and sets the maximum allowable voltage during open-loop conditions.

Equation 41. R S 2 =   R S 1 × V O V P T H N A S × V O V + V F - V O V P T H  

where

  • VOV is the maximum allowable peak voltage at the converter output
  • VF is the output-rectifier forward drop at near-zero current
  • NAS is the transformer auxiliary-to-secondary turns ratio
  • VOVPTH is the overvoltage detection threshold at the VS input (see the Electrical Characteristics section of the UCC28742 data sheet)
Equation 42. R S 2 =   121 k Ω × 4.65 V 1.455 × 15 V + 0.8 V - 4.65 V = 30.7 k Ω
Equation 43. R S 2 ( s e l e c t e d ) =   33.2 k Ω

The UCC28742 device maintains tight CC regulation over varying input lines by using the line-compensation feature. The line-compensation resistor (RLC) value is determined by the current flowing in RS1 and the total internal gate drive and external MOSFET turn-off delay. Assuming an internal delay of 50 ns in the UCC28742 device:

Equation 44. R L C =   K L C × R S 1 × R C S × t D × N P A L P

where

  • tD is the current-sense delay including MOSFET turn-off delay
  • KLC is a current-scaling constant (see the Electrical Characteristics section of the UCC28742 data sheet)
Equation 45. R L C =   25 × 121 k Ω × 0.5 Ω × ( 46 n s + 50 n s ) × 4.81 700 µ H = 998 Ω
Equation 46. R L C ( s e l e c t e d ) =   1 k Ω