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

5.4 Transformer Winding Selection

The transformer winding design starts with estimating the RMS currents of the primary-side and secondary-side windings . As discussed earlier, at the resonant frequency, the primary-side and secondary-side currents are both sinusoidal. There is a phase shift between these two currents, due to the magnetizing current. If the magnetizing current is ignored, the primary-side current and secondary-side current are scaled by the transformer turns-ratio. Since the output current is equal to the average value of the rectified transformer secondary-side current, the RMS current of the transformer windings can be easily estimated by Equation 8 and Equation 9. Considering the tolerances on the switching frequency and component values, an extra 20~30% design margin is recommended for these currents.

Secondary-side winding RMS current:

Equation 8. I r m s S = π 2 I O

Primary-side winding RMS current:

Equation 9. I r m s P = I r m s S / n

Even though the converter gain is equal to the transformer turns-ratio when the switching frequency is equal to the resonant frequency, the output voltage will be slightly lower than the theoretical value due to the loss elements in the converter, including the diode voltage drop (Vf), the primary-side switch on-state resistance (Rdson), the transformer AC resistance Rac, the resonant capacitor ESR (RESR), as well as the diode ESR (RDiode). The overall output voltage can be estimated based on Equation 10. Given the transformer AC resistance affects the voltage gain, to improve the load regulation, it is desired to minimize the transformer AC resistance.

Equation 10. V O U T ≅ V I N n - 2 V f - π 2 2 × R d s o n n 2 + R a c + R E S R + R D i o d e × I O

The transformer AC resistance should be measured from the secondary side, with primary side shorted, and at the resonant frequency, as show in Figure 5-3.

Figure 5-3 Transformer AC Resistance Measurement

Ideally, this impedance should be as low as possible. Based on the equation, we can estimate the voltage drop caused by the load current and estimate how much AC resistance can be allocated for the transformer. The Rdson is coming from UCC25800-Q1 and it can be estimated using 0.3 Ω. The ESR of the capacitor can be ignored if the NP0 capacitor is used. The X7R capacitor would have a larger ESR. The diode resistance can be estimated based on the diode forward voltage drop curves, normally it is about 0.3 Ω.