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

2.2.1.11 Supercapacitor Selection

The supercapacitors supply the 12-V and the 5-V rails (TIDA-010939) in case of unexpected AC input shut down to turn off the main relay and unlock the plug. Assume 1 s duration time as the initial specification.

  • 12-V rail: Peak current 1.8 A for 200 ms (unlock plug and relay off)
  • 12-V rail: Average current 1.8 A × 0.2 s + 0.1 A × 0.8 s = 0.44 A
  • 5-V rail: Average current 0.275 A for 1 s

The total peak power required from supercap, PPK_SC:

Equation 48. PPK_SC = (V12Vp × IPK_1 + V5V × IPK_2 / ηBUCK) / ηBOOST
Equation 49. PPK_SC = (12 V × 1.8 A + 5 V × 0.275 A / 0.9) / 0.85 = 27.2 W

Total 27.2 W peak for 200 ms or a peak current of approximately 3.5 A (that is, 27.2 W / 7.8 V). Total average power required from supercapacitor, PAVE_SC:

Equation 50. PAVE_SC = (V12Vp × IAVE_1 + V5V × IAVE_2 / ηBUCK) / ηBOOST
Equation 51. PAVE_SC = (12 V × 0.44 A + 5 V × 0.275 A / 0.9) / 0.85 = 8 W (that is, 8 J in 1 s)

Consider that the supercapacitor is charged up to 7.8 V and then discharged down to 4.3 V (that is, UVLO of the TPS55330 boost converter):

Equation 52. CMIN_SERIES = 2 × (E) / ((V2)2 – (V1)2) = 2 × (8 J) / ((7.8 V)2 – (4.3 V)2) = 0.3778 F
Equation 53. CMIN = 2 × CMIN_SERIES = 0.76 F (for 1 s)

where

  • CMIN_SERIES is the minimum equivalent series capacitor
  • CMIN is the minimum individual capacitance

Now, for 3 s CMIN = 3 × 0.76 = 2.28 F is needed.

For this design, two 2.5-F in series that support up to 47.5 W and 4 A peak are selected.

The TL7705 voltage supervisor monitors for charge completion at an slightly lower voltage of 7.49 V. The supercapacitor energy available from 7.49 V to 4.3 V, ESC_7p5:

Equation 54. ESC_7p5 = 0.5 × C × (V12 – V22) = 0.5 × 1.25 F × (7.492 – 4.32) = 23.5 J

The energy available after accounting for boost efficiency, ESC_7p5_BOOST:

Equation 55. ESC_7p5_BOOST = ESC_7p5 × ηBOOST = 23.5 J × 0.85 = 20 J

The power available during 3 s, PSC_7p5:

Equation 56. PSC_7p5 = ESC_7p5_BOOST / time = 20 J / 3 s = 6.65 W

The supercapacitor energy available from 7.8 V to 4.3 V, ESC_7p8:

Equation 57. ESC_7p8 = 0.5 × C × (V12 – V22) = 0.5 × 1.25 F × (7.82 – 4.32) = 26.5 J

The energy available after accounting for boost efficiency, ESC_7p8_BOOST:

Equation 58. ESC_7p8_BOOST = ESC_7p8 × ηBOOST = 26.5 J × 0.85 = 22.5 J

The power available during 3 s, PSC_7p8:

Equation 59. PSC_7p8 = ESC_7p8_BOOST / time = 22.5 J / 3 s = 7.5 W