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

4.4.1 Primary Output Capacitor

As in the regular buck converter, the Fly-Buck primary output capacitor COUT1 must satisfyEquation 11 . In Fly-Buck™, The reflected secondary winding current adds to the primary winding current, leading to the condition that COUT1 must satisfy Equation 12 .

Equation 11. COUT1>∆im8×fSW×∆VOUT1
Equation 12. COUT1>IOUT2×N2N1+IOUT3×N3N1×TONmax∆VOUT1

More output capacitance can be used to improve load transient response. The optimization of output capacitor over the whole line and load ranges can be easily done experimentally.