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

3.1 Basic Intervals of Steady State Operation

Figure 3-1 shows the typical steady state waveforms of a Fly-Buck™ converter in which Vpri is the primary voltage across the coupled inductor, im is the magnetizing current, im and isec are primary side current and secondary side current.

The operation of the Fly-Buck™ converter basically has two modes: TON and TOFF.

GUID-20230801-SS0I-XMSK-685T-K1S7F7SJG2Q4-low.svgFigure 3-1 Fly-Buck™ Steady State Operation Waveforms

TON Mode

This mode is the same as traditional synchronous buck converter when the main switch(HS) is ON. The voltage stress of the low-side (LS) switch is the input voltage(VIN). The magnetizing inductance, Lm is charged by the input voltage minus the primary output voltage as in the regular buck converter. The secondary winding current remains zero for the diode D2 is reverse biased according to the winding polarity configuration, and D2 sees voltage stress of (N2/N1)×(VIN-VOUT1)+VOUT2. The isolated output capacitor COUT2 is supplying the load current.

TOFF Mode

In this mode LS is ON and HS is OFF. Vpri becomes negative, forward biasing D2 to force a secondary current to flow to transfer part of the stored energy in the coupled inductor to the secondary output capacitor, COUT2 and the load, RLoad2.

Unlike the buck converter, ipri in Fly-buck decreases at a faster rate, owing to supplying current to both loads, IOUT1 and IOUT2.

The secondary current waveform is determined by the load, leakage inductance, and output capacitance. The current direction of ipri at the end of one switching cycle (positive or negative) depends on factors including the current ratio of IOUT2: IOUT1 and current ripple.

The primary output voltage is the same as a buck converter and is given by Equation 1.

Equation 1. VOUT1=TONTON+TFFVIN=D×VIN

The secondary output voltage is given by Equation 2.

Equation 2. VOUT2=VOUT1×N2N1-VF

where

  • N1 and N2 are the turns of the primary winding and secondary winding
  • VF is the forward voltage drop of the secondary rectifier diode