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

3 Primary-side and Secondary-side Resonant Configurations

In Figure 2-1, the resonant capacitor is on the primary side. From its gain curves, it can be observed when the switching frequency is below the resonant frequency, the converter voltage gain rises. This occurs because the energy that is stored in the magnetizing inductor is transferred to the secondary side in each half-switching cycle. The lower the switching frequency, the more energy is stored in the magnetizing inductor. Therefore the voltage gain keeps going up.

When the resonant capacitor is moved on the secondary side, the gain curves are changed. One example of using secondary-side resonant is shown in Figure 3-1.

Figure 3-1 LLC Converter Using Secondary-side Resonant

Putting the resonant capacitor on the secondary side, when the switching frequency is below the resonant frequency, the energy stored in the magnetizing inductance can no longer be transferred to the secondary side. Instead, the energy is fed back to the input source. Due to this behavior, the voltage gain of the secondary-side resonant becomes flat when the switching frequency is below the resonant frequency, as shown in Figure 3-2.

(a) Primary-side Resonant (b) Secondary-side Resonant

Figure 3-2 Output Voltage Comparison between Primary-side Resonant and Secondary-side Resonant Configurations with 550-kHz Resonant Frequency, 24-V Input Voltage, and 1:1 Transformer Turns Ratio

Based on these curves, we can see if the LLC converter is operating with a fixed switching frequency and that frequency is slightly below the resonant frequency, the voltage gain is fixed, regardless of the frequency or the load condition, which means with a fixed input voltage, we get a fixed output voltage. This property also helps to maintain constant output voltage with the tolerances on the resonant components, which is equivalent to the switching frequency variation.