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

2.2.1.2 Transformer Turns-Ratio, Primary Inductance, and Primary Peak Current

The target maximum switching frequency at full load, the minimum input-capacitor bulk voltage, and the estimated DCM resonant time determine the maximum primary-to-secondary turns-ratio of the transformer. Initially, determine the maximum-available total duty-cycle of the on-time and secondary conduction time based on the target switching frequency (fMAX) and DCM resonant time (tR).

At the transition-mode operation limit of DCM, the interval required from the end of the secondary current conduction to the first valley of the VDS voltage is ½ of the DCM resonant period (tR), or 1 µs assuming 500-kHz DCM resonant frequency. The maximum allowable MOSFET on-time DMAX is determined using Equation 4:

Equation 4. D M A X = 1 - D M A G C C - t R 2 × f M A X = 1 - 0.475 - 38   k H z × 2   µ s 2 = 0.485

where

  • tR is the estimated period of the LC resonant frequency at the switch node
  • DMAGCC is defined as the duty cycle of the secondary-diode conduction during CC operation and is fixed internally by the UCC28742 device at 0.475

When DMAX is known, the maximum primary-to-secondary turns ratio is determined with Equation 5. The total voltage on the secondary winding must be determined, which is the sum of VOCV and VF.

Equation 5. N P S ( m a x ) = D M A X × V B U L K _ V A L L E Y D M A G C C × V O C V + V F

Assuming VF = 0.8 V:

Equation 6. N P S ( m a x ) =   0.485 × 90.7 V 0.475 × 12 V + 0.8 V = 7.24
Equation 7. N P S ( s e l e c t e d ) =   7

A higher turns-ratio generally improves efficiency, but can limit operation at a low input voltage. Transformer design iterations are generally necessary to evaluate system-level performance trade-offs.

The primary transformer inductance is calculated using the standard energy storage equation for flyback transformers. The primary current, maximum switching frequency, output voltage and current targets, and transformer power losses are included in Equation 8:

Equation 8. L P = 2 × V O C V + V F × I O C C ŋ X F M R × I 2 P P ( m a x ) × f M A X =   627.7   μ H
Equation 9. L P ( s e l e c t e d ) = 700   μ H

The UCC28742 CC regulation is achieved by maintaining DMAGCC at the maximum primary peak current setting. The product of DMAGCC and VCST(max) defines a CC-regulating voltage factor VCCR which is used with NPS to determine the current-sense resistor value necessary to achieve the regulated CC target, IOCC (see Equation 10).

Equation 10. R C S =   V C C R × N P S 2 × I O C C × ŋ X F M R
Equation 11. R C S =   0.363 V × 7 2 × 2.2 A × 0.9 = 0.547   Ω
Equation 12. R C S ( s e l e c t e d ) =   0.5   Ω

where

Equation 13. I P P ( m a x ) = V C S T ( m a x ) R C S = 0.83 V 0.5 Ω = 1.66   A
Equation 14. I P P ( n o m ) = V C S T ( n o m ) R C S = 0.77 V 0.5 Ω = 1.54 A

NAS is determined by the lowest target operating output voltage while in CC regulation and by the VDD UVLO turnoff threshold of the UCC28742 device. Additional energy is supplied to VDD from the transformer leakage-inductance which allows a lower turns ratio to be used in many designs.

Equation 15. N A S = V D D ( o f f ) + V F A V O C C + V F = 8.15 V + 0.8 V 5 V + 0.8 V = 1.54

where

  • VDD(off) is UCC28742 turnoff threshold (from the data sheet)
  • VOCC is the lowest output voltage target of the converter while in constant-current regulation
  • VFA is voltage drop across rectifier diode on auxiliary side of flyback stage
Equation 16. N A S ( s e l e c t e d ) =   1.455

This implies:

Equation 17. N P A ( s e l e c t e d ) = 4.8 1

Since the ±14-V rails are unregulated, the turn ratio determines their output voltage:

Equation 18. N P T = N P S ( V O V 14 + V F ) / ( V O C V + V F ) = 7 ( 14 V + 0.8 V ) / ( 12 V + 0.8 V )   = 6.05  
Equation 19. N P T ( s e l e c t e d ) = 5.92