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

8.1 EMI Performance With Standalone Converter

To compare the performance of the three different topologies (LLC, flyback, and push-pull), three boards are tested as stand alone converters, instead of using them to bias the gate drivers. The LLC converter is based on the UCC25800-Q1 with 24-V input and 24-V output, 6-W maximum power. The flyback converter uses the LM5180 EVM, also setup with 24-V input and 24-V output, 200mA maximum load. The push-pull converter is based on SN6505B, with 5-V input, with 22-V output voltage. All three converters are connected with a 150-Ω resistor load. The setup follows the CISPR 25 Class 5 (Conducted Emission) automotive EMI test standard.

Figure 8-1 through Figure 8-3 demonstrate the measured EMI performance from three different converters. The EMI filters are removed to show the noise generated by each converter itself. Due to the different bypass capacitor values, the differential-mode noise is not quite the same as the converter generated, while the common-mode and high-frequency noise should not be affected and are properly represented. From the noise measurement, it can be observed the LLC converter provides a much lower EMI noise level, especially at high-frequency range (around 20-dB reduction). As described earlier, the soft switching and lower parasitic capacitance help to achieve lower EMI noise.

Figure 8-1 EMI Noise Measured From The LLC Converter
Figure 8-2 EMI Noise Measured From The flyback Converter
Figure 8-3 EMI Noise Measured From The push-pull Converter