SNVAA82 august   2023 LMR38020

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Fly-Buck Converter
  6. 3Fly-Buck Basic Operation
    1. 3.1 Basic Intervals of Steady State Operation
    2. 3.2 Impact Of Leakage Inductor On Fly-Buck Operation
  7. 4Design A Fly-Buck Converter with LMR38020
    1. 4.1 IC Select
    2. 4.2 Switching Frequency Set
    3. 4.3 Transformer Design
      1. 4.3.1 Turns Ratio
      2. 4.3.2 Magnetic Inductance
      3. 4.3.3 Check Ipk
    4. 4.4 Output Capacitor Selection
      1. 4.4.1 Primary Output Capacitor
      2. 4.4.2 Secondary Output Capacitor
    5. 4.5 Secondary Output Diode
    6. 4.6 Preload Resistor
  8. 5Bench Test Results
    1. 5.1 Typical Switching Waveforms Under Steady State
    2. 5.2 Start Up
    3. 5.3 Efficiency
    4. 5.4 Load Regulation
    5. 5.5 Short Circuit
    6. 5.6 Thermal Performance
  9. 6Design Considerations
  10. 7Summary
  11. 8References

5.1 Typical Switching Waveforms Under Steady State

GUID-20230813-SS0I-SFCG-XZ89-X9XCPFSXGBVD-low.pngFigure 5-3 Steady State When VIN=16 V, IOUT1=0.4 A, IOUT2=IOUT3=0.1 A
GUID-20230813-SS0I-Z5NK-KGG8-XFQR3G6LCCGD-low.svgFigure 5-5 Steady State When VIN=16 V, IOUT1=0.4 A, IOUT2=IOUT3=0 A
GUID-20230813-SS0I-ZBFK-D6CC-KJVD7G04KRNB-low.pngFigure 5-4 Steady State When VIN=60 V, IOUT1=0.4 A, IOUT2=IOUT3=0.1 A
GUID-20230813-SS0I-X6LG-DDJ9-49GXKQV9Q6ZQ-low.pngFigure 5-6 Steady State When VIN=60 V, IOUT1=0.4 A, IOUT2=IOUT3=0 A

In Figure 5-5, switching frequency fsw becomes higher. This normally happens when VIN=VIN_MIN and D>0.5, and toff becomes smaller, hence the primary current hits the negative peak current limit. Therefore, theLMR38020 turns off the LS and start a new cycle at earlier time, causing the switching frequency to increase.