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

5.2 Start Up

GUID-20230802-SS0I-HBP1-JG50-0PFMRJ6MRHX4-low.pngFigure 5-7 Start Up When VIN=16 V, Full Load
GUID-20230802-SS0I-2T1S-FLBS-QJ0L2M1CGTTS-low.pngFigure 5-8 Start Up When VIN=60 V, Full Load

Figure 5-7 and Figure 5-8 show typical start-up behavior. The secondary output voltage tracks the primary output voltage during the soft start sequence.

The secondary voltages' drop during startup for about 18ms can be explained by the IC OCP blaning time (please refer to the data sheet). The heavier secondary load lighter primary load brings more secondary voltage drops during this time. Adding a small amount of preload on the primary side can help reduce these secondary voltage drop during startup..