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

4.3.3 Check Ipk

Both the positive and negative peak value through the primary winding must not exceed the current limits of the LMR38020.

This peak-to-peak magnetizing current ripple is:

Equation 8. ∆im=VIN-VOUT1×DLPRI×fSW

The positive peak current of IPRI is given by Equation 9 :

Equation 9. IPRI_PK_POS=(IOUT1+IOUT2×N2N1+IOUT3×N3N1)+(∆im2)

The negative peak current of IPRI depends on different leakage inductance and parasitic parameters and finding an explicit expression is difficult. Commonly, an empirical and conservative linear approximation can be used, in which the secondary current is represented as a linear function and ramps up from zero at the start of Toff time[1].

Equation 10. IPRI_PK_NEG=IOUT1-(∆im2)-(IOUT2×N2N1+IOUT3×N3N1)×(1+D)(1-D)

This gives a simple estimation of the negative peak current in worst case. However, the conservative estimate may lead to an unnecessary over design of the transformer. In fact the secondary current is usually a convex curve, as shown in Figure 3-3, therefore the actual IPRI_PK_NEG is actually smaller than the calculated.

Equation 10 is more close to reality when the leakage inductance is very large, and when IOUT1=0, VIN=VINMIN.