SLUAAN0 December   2022 TPS62933F

 

  1.   Designing a Simple and Low-Cost Flybuck Solution With the TPS62933F
  2.   Trademarks
  3. 1Introduction
  4. 2Flybuck Converter Device Overview
    1. 2.1 Operation Description
    2. 2.2 Equations for Maximum Output Current
  5. 3Design Flybuck With TPS62933F
    1. 3.1 Primary Voltage and Turns Ratio
    2. 3.2 Feedback Resistor
    3. 3.3 Rectifier Diode
    4. 3.4 Primary Inductance
    5. 3.5 Primary Turns
    6. 3.6 Input and Output Capacitor
    7. 3.7 Pre-Load
    8. 3.8 Factors Affecting Voltage Regulation
    9. 3.9 Avoiding Low-Side Sink Current Limit
  6. 4Experimental Results
  7. 5Conclusion
  8. 6References

3.1 Primary Voltage and Turns Ratio

The primary-side, nonisolated output is set to 5 V for several considerations. First, the setting is below the minimum input voltage, 10 V, and the theoretical duty cycle varies from 20 to 50 percent at the full VIN range, which is a balanced duty cycle during normal operation. Because the isolated outputs only have the off-time window to transfer energy, for a duty cycle that is too high, the secondary winding current will have a huge spike, which leads to poor regulation. Next, the turns ratio of the transformer is not too high to handle for the 5 V voltage level steps up to 12 V. Last, 5 V is one of the most common voltage levels in many applications. In this design, a transformer turns ratio (N1:N2:N3) of 1:2.5:2.5 is selected (see Equation 10 and Equation 11).

Equation 10. V O U T 2 = V O U T 1 × N 2 N 1 - V F = 12 V
Equation 11. V O U T 3 = V O U T 1 × N 3 N 1 - V F = 12 V

In this design, the VF is targeted at 0.5 V.