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

4.2 Switching Frequency Set

The switching frequency(fsw) of the LMR38020 is programmable with the resistor RT placed across the RT/SYNC pin and GND pin. Equation 3 determines the value of RT for a targeted switching frequency.

Equation 3. RTkΩ=30970×fSWkHz-1.027

Smaller sized transformer can be selected when fsw is higher. But when fsw is high, which means the Ts is small, if the D is large, there might not be enough energy transferring to isolated side during the small toff.

Considering D>0.5 when VIN=VINMIN=16 V in this application, 250 kHz is chosen. If Dmax is no larger than 0.5, higher fsw can be selected.

It is normally recommended DMAX<0.5 in Fly-Buck design. A greater duty can cause lower secondary Vout than the set point. The worst situation happens when VIN =VIN_MIN, IOUT1=0, IOUT2= IOUT1_MAX. Refer to this article Pick the Right Turns Ratio for a Fly-Buck™ Converter for detailed discussion on this topic.