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

5.5.2 Magnetizing inductance

Traditionally in LLC converters, the magnetizing inductance serves two purposes: to help achieve ZVS (Zero Voltage Switching) and to help boost the voltage gain up when the switching frequency is below the resonant frequency, to compensate the input voltage variation. In the open-loop LLC bias supply designs, there is no need to boost the voltage gain up. The only purpose of the magnetizing inductance is to help achieve ZVS. Based on the ZVS criteria, the design target of the magnetizing inductance can be calculated based on Equation 12. In this equation, Lm is the magnetizing inductor value, td is the dead-time, fSW is the switching frequency, and CSW is the SW-pin parasitic capacitance (it has a typical value of 170 pF).

Equation 12. L m = t d 8 C S W f S W

Furthermore, because it is a low voltage input, even without achieving the ZVS, the switching loss is not significant. We could live with the magnetizing inductance without using the air-gap.

If the magnetizing inductance is much larger than the leakage inductance (> 20 times), the transformer can use the core without an air-gap.

If the leakage inductance is a much larger percentage of the magnetizing inductance, to minimize the error caused by the magnetizing inductance tolerance, it is desired to insert an air-gap in the transformer core. In this case, the air-gap should be minimal without causing additional manufacturing cost.