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

9 Common-Mode Transient Immunity (CMTI)

With the development of wide-bandgap power-semiconductor technology, inverters start to use SiC or GaN MOSFETs as the switching devices. These wide bandgap devices have faster switching speed and create higher dv/dt at the switch node, sometimes well above 50V/ns. This high dv/dt can couple through the transformer parasitic capacitance, as shown in Table 5-1. Common-mode transient immunity (CMTI) test becomes a standard requirement for this type of application. A UCC25800-Q1 based open-loop LLC converter uses a transformer with low parasitic capacitance. This makes the solution demonstrate excellent CMTI performance. The CMTI test setup is shown in Figure 9-1.

Figure 9-1 CMTI Test Setup

A CMTI generator was used to create CMTI strikes. It operates with 5-kHz switching frequency and 50% duty cycle. Its rise and fall rate are 165V/ns and 155V/ns respectively. With such a high voltage slew rate, the UCC25800-Q1 still operates normally. Its SW pin voltage remains a fixed switching frequency square wave.

(a) CMTI strikes don't interfere with converter switching (CH1: CMTI strike; CH2: UCC25800-Q1 SW-pin voltage; CH3: Not used)
(b) CMTI rise rate (CH1: CMTI strike; CH2: UCC25800-Q1 SW-pin voltage; CH3: Not used) (c) CMTI fall rate (CH1: CMTI strike; CH2: UCC25800-Q1 SW-pin voltage; CH3: Not used)
(d) CMTI postie strike (e) CMTI negative strike

Figure 9-2 CMTI Performance of UCC25800-Q1 Based Converter

The open-loop LLC converter uses a transformer with such a low parasitic capacitance. The current injection from the high dv/dt switch node becomes greatly reduced when compared to legacy solutions. Therefore, this solution can easily pass the CMTI test.