SLUAAB9A March   2021  – December 2021 UCC25800-Q1

 

  1.   Trademarks
  2. 1 Introduction
  3. 2 Open-Loop LLC Converter Operation Principle
  4. 3 Primary-side and Secondary-side Resonant Configurations
  5. 4 Rectification Methods
    1. 4.1 One Resonant Capacitor, Voltage Doubler
    2. 4.2 Two Resonant Capacitors, Voltage Doubler
    3. 4.3 One Resonant Capacitor, Full-wave Rectifier
  6. 5  LLC Transformer Design Steps
    1. 5.1 Transformer Turns Ratio Selection
    2. 5.2 Transformer Volt-second Rating Calculation
    3. 5.3 Transformer Construction
    4. 5.4 Transformer Winding Selection
    5. 5.5 Transformer Inductance
      1. 5.5.1 Leakage inductance
      2. 5.5.2 Magnetizing inductance
    6. 5.6 Transformer Selections
  7. 6 Negative Voltage Generation
    1. 6.1 Using a Zener Diode
    2. 6.2 Using a Shunt Regulator
    3. 6.3 Using a Shunt Regulator and Linear Regulator
  8. 7 Multiple-output Design
    1. 7.1 One UCC25800-Q1 Drives Each Output
    2. 7.2 Transformer With Multiple Secondary-side Windings
    3. 7.3 Multiple Transformers
  9. 8 EMI Performance
    1. 8.1 EMI Performance With Standalone Converter
    2. 8.2 EMI performance with an inverter power stage
  10. 9 Common-Mode Transient Immunity (CMTI)
  11. 10Summary
  12. 11Revision History

5.1 Transformer Turns Ratio Selection

When an LLC converter operates at its resonant frequency, the impedance of the resonant tank is equal to zero. The input voltage and output voltage are virtually connected together. Given this property, we can easily determine the transformer turns-ratio needed. Using the example shown in Figure 5-1, the primary side uses a half bridge. Therefore, the transformer primary side only sees half of the input voltage. The secondary side uses a voltage doubler (Here, the resonant capacitor is the sum of two capacitors ). Therefore, the transformer secondary side only sees half of the output voltage. In this case, considering the converter operates close to the resonant frequency, the voltage gain is equal to 1. We can get Equation 5.

Equation 5. V I N = n × V O U T
Figure 5-1 LLC Resonant Converter with Secondary-side Resonant and Voltage Doubler

Considering the voltage drop on the diodes, the transformer and the primary-side switches, we can set the transformer turns-ratio n using Equation 12. The extra 1 V can be adjusted as needed to account for the actual voltage drop or extra voltage needed for the post regulator.

Equation 6. n = V I N V O U T + 1 V