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 EMI Performance

Traditionally, the isolated bias supply is created through PWM converters like flyback or push-pull converters. These converters have a triangular or trapezoidal shape input current, which has more harmonic content and creates more EMI noise. Besides, both of these converters typically operate as hard switching and generate higher dv/dt on the switch node, which creates more common mode noise. Furthermore, due to the topology limitations, they need to work with the transformer with fairly low leakage inductance, which means the transformer primary-side to secondary-side parasitic capacitance will be large (10 ~ 30 pF range). The high dv/dt, together with the large parasitic capacitance make the flyback and push-pull perform poorly from an EMI standpoint.

Unlike the PWM converters, the LLC converter has a sinusoidal shape current. It is also capable of achieving ZVS with lower switch node dv/dt. Furthermore, the LLC converter is able to use a transformer with split chamber bobbin and significantly reduce the transformer primary-side to secondary-side parasitic capacitance (below 2 pF). The sinusoidal shape current, lower dv/dt, and the low parasitic capacitance make the LLC a much better solution from the EMI noise point of view.

Table 8-1 summarizes the parasitic capacitance comparison between different solutions.

Table 8-1 Transformer comparisons among different topologies
LLC flyback push-pull
Construction Split chamber bobbin One chamber bobbin Toroid
Primary-side to secondary-side parasitic capacitance < 2 pF ~ 20 pF ~ 10pF