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

1.7 How to Operate the UCC25640x Controller in the Open Loop?

Running UCC25640x open loop can require some modifications to the VCR circuitry as well as the FB pin to sink a constant current out of the FB pin (Figure 1-16). Since the FB pin is loosely regulated to approximately 5.6 V, connecting a resistor (RFB) from FB to ground sinks a fixed current (somewhere between 0 uA and 82 uA for 402/404 devices. 164 uA for 403 devices). The amount of current can determine the switching thresholds ((VTH-VTL) =Vvcr_pk_ pk) for VCR. And then depopulate the top VCR capacitance so that charge control is completely disabled. Now the switching frequency (fsw) is only dependent on the internal 2 mA ramp current and the lower VCR capacitance (C VCR_lower).

GUID-20230522-SS0I-K46D-Z7VP-ZC6DSW4J9Q7H-low.svg Figure 1-16 Operating the UCC25640x in the Open Loop by Connecting a Resistor at the FB Pin
Equation 31. The amount of current sunk out of the FB pin:    I FB_pin = V FB R FB = 5.6 R FB  
Equation 32. VCR peak to peak voltage:  V vcr_pk_pk =(82μA− I FB_pin )⋅100kΩ
Equation 33. Switching frequency:  f sw = 2mA 2⋅ V vcr_pk_pk ⋅ C VCR_lower

Design Example

Equation 34. Required Switching Frequency :   f s w = 100 k H z
Equation 35. Value of Capacitor connected at the VCR pin to ground :   C V C R _ l o w e r = 10 n F
Equation 36. VCR pin peak to peak voltage would be:  V v c r _ p k _ p k = 2 m A   2 × f s w × C V C R _ l o w e r = 2 m A   2 × 100 k H z × 10 n F = 1 V
Equation 37. Current that needs be pulled out from the FB pin:  I F B _ p i n = 82 μ A - V v c r _ p k _ p k 100 k Ω = 82 μ A - 1 V 100 k Ω = 72 μ A
Equation 38. Resistance that needs to be connected at the FB pin:  R F B = 5.6 V 72 μ A = 78 k Ω