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

4.3.2 Magnetic Inductance

The calculation of LPRI can be the same as calculating the inductance for an ordinary buck regulator, basing on the desired primary ripple current.

Typically, a ripple current of between 20% and 40% of the primary current is used. Equation 6 determines the primary current in a fly-buck converter and Equation 7 determines the required primary inductance.

Equation 6. IPRI=IOUT1+IOUT2×N2N1+IOUT3×N3N1
Equation 7. LPRI=VIN-VOUT1K×IPRI×fSW×VOUT1VIN

Where

K=ripple current factor=20% to 40%.