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

7.3 Multiple Transformers

Multiple output voltages can also be accomplished by using one UCC25800-Q1 to drive multiple transformers. In this case, each transformer can be configured as secondary-side resonant and the tolerance on the resonant frequency can be mitigated. The secondary-side resonant capacitor can be chosen to achieve the same resonant frequency for each transformer. The example circuit configuration is shown in Figure 7-3.

Figure 7-3 UCC25800-Q1 Drives Multiple Transformers

This implementation method is verified by modifying PMP22835 6-W 24V input, 24V output reference design. Using one primary-side circuit to drive three sets of transformers and secondary-side circuits, as shown in Figure 7-4.

Figure 7-4 Hardware of One UCC25800-Q1 Driving Three Sets of Transformers

Three output voltages at different load conditions are summarized in Figure 7-5. It can be seen with different transformers, the output voltage remains the same when they are loaded with the same load current. It can be noticed that when the load is extremely light (less than a few mA), the output voltage rises significantly. This is due to the extra energy delivered to the secondary side through charging the secondary-side diode junction capacitors. The energy delivered is limited and it can be ignored when the output voltage is loaded. However, when the output load is very light, the delivered energy might be more than the load can consume. This causes the output voltage to rise. Since the extra energy is limited, normally the gate driver IC quiescent current is large enough to consume the energy. If the quiescent power is not sufficient to consume the energy, a Zener diode or dummy-load resistor can be used to reduce the output voltage rise. Only a few mA load can bring the output voltage to the regular levels.

Figure 7-5 Output Voltage Regulation For One UCC25800-Q1 Driving Three Sets Of Transformers