SLUAAL2 june   2023 UCC256402 , UCC256403 , UCC256404

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1UCC25640x Frequently Asked Questions
    1. 1.1  For the Time Domain Simulation and Fundamental Harmonic Analysis of LLC Resonant Converters, What Model of the Transformer Should be Used?
      1. 1.1.1 LLC Design Using T Type Transformer Model
    2. 1.2  How to Connect External Gate Drivers to the UCC25640x for High Gate Driver Current Capability?
    3. 1.3  When Powering on the PFC-LLC AC-DC Converter, What Sequence is Recommended?
    4. 1.4  How to Eliminate the Nuisance ZCS Detection During the Light Load?
    5. 1.5  What is the Purpose of Maintaining the FB Pin Voltage of the UCC25640x Controllers at a Constant Level?
    6. 1.6  How to Improve the Slew Rate Detection at HS Pin of the UCC25640x Controller?
    7. 1.7  How to Operate the UCC25640x Controller in the Open Loop?
    8. 1.8  What Happens if the VCR Pin Peak to Peak Voltage of the Controller Exceeds 6 V?
    9. 1.9  What UCC25640x settings effect the startup duration of the LLC?
    10. 1.10 What is Causing the Current Imbalance in the LLC's Secondary Side Windings?
    11. 1.11 How to Design TL431 Compensator for LLC With UCC25640x Controller
      1. 1.11.1 LLC Plant Transfer Function Under HHC Control
      2. 1.11.2 Type 2 and Type 3 Compensator with TL431 [20]
        1. 1.11.2.1 Type 2 Compensator
        2. 1.11.2.2 Type 2 Compensator Without Fast Lane
        3. 1.11.2.3 Type 3 Compensator with Fast Lane
        4. 1.11.2.4 Type 3 Compensator Without Fast Lane
      3. 1.11.3 Type 3 Compensator Design Example
    12. 1.12 How to Design LLC for Battery Charging and LED Driver Applications?
      1. 1.12.1 LED Driver Design Example
      2. 1.12.2 Battery Charger Design Example
    13. 1.13 How to Implement CC-CV Feedback Control?
      1. 1.13.1 Voltage Feedback Loop (Type 2) Transfer Function
      2. 1.13.2 Current Feedback Loop (Type 2) Transfer Function
    14. 1.14 What is the Simplest Approach to Configure the Burst Mode Thresholds for UCC25640x Based on the Load Power?
    15. 1.15 How to Avoid the UCC25640x Controller to Enter into Burst Mode?
    16. 1.16 What are the Methods for Preventing VCC From Decreasing Below the VCC Restart Threshold During Burst Mode?
    17. 1.17 How Does BMTL Threshold Value Impacts the Output Voltage Ripple and the VCC Pin Voltage and Magnetizing Current?
    18. 1.18 How to Design Magnetics for LLC?
      1. 1.18.1 LLC Resonant Inductor Design
      2. 1.18.2 LLC Transformer Design
    19. 1.19 How is the Dead Time in UCC25640x Determined During ZCS Detection and in the Absence of Valid Slew Rate Detection?
  5. 2References

1.3 When Powering on the PFC-LLC AC-DC Converter, What Sequence is Recommended?

Normally, LLC can start switching before the PFC output voltage starts rising or vice versa depending on system requirement.

In general, PFC output voltage starts rising before the LLC converter starts switching. The output voltage rising is done to make sure the LLC is able to achieve regulation when starting into heavy load without risking going into the capacitive region. UCC25640x RVCC is designed for this purpose. In case of UCC256402/404, once the AC input is provided and VCC reaches 26-V level, RVCC voltage is generated to power up PFC controller for PFC to boost. Once PFC boost to a level above UCC25640x BLK setting, LLC DC-DC start to operate. For any reason, if the BLK voltage reaches below BLKstop, switching stops.

In the case of TVs, during light load, LLC converter need to startup before PFC output voltage rises. In this type of scenario, UCC256404 is recommended, since the BLK turn on threshold of this controller is only 1 V. Since LLC transformer was designed for input voltage>300 V and full load, there is not any issue of triggering OCP protection.