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

3.6 Input and Output Capacitor

The TPS62933F device requires an input decoupling capacitor and, depending on the application, a bulk input capacitor. The typical recommended value for the decoupling capacitor is 10 μF, and an additional 0.1-µF capacitor from the VIN pin to ground is recommended to provide high frequency filtering.

The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the capacitor. X5R and X7R ceramic dielectrics are recommended because they have a high capacitance-to-volume ratio and are fairly stable over temperature. The capacitor must also be selected with the DC bias taken into account. The effective capacitance value decreases as the DC bias increases. The capacitor voltage rating needs to be greater than the maximum input voltage. The input capacitor must be large enough to limit the input voltage ripple, see Equation 24.

Equation 24. C I N ≥ I O U T 1 + N 2 N 1 I O U T 2 + N 3 N 1 I O U T 3 4 × f S W × ∆ V I N

Choosing ΔVIN= 0.2 V gives a minimum of CIN= 4.9 μF. Considering derating, two standard value 10-μF, 50-V capacitors are selected for better input ripple performance. Another 0.1μF capacitor has been added as a bypass capacitor to clear high-frequency noise.

Energy is transferred from primary to secondary when the synchronous switch of the buck converter is on. The primary output capacitance is given by Equation 25.

Equation 25. C O U T 1 ≥ N 2 N 1 I O U T 2 + N 3 N 1 I O U T 3 × D m a x f S W × ∆ V O U T 1

Choosing ΔVOUT1= 0.05 V gives a minimum of COUT1= 19.2 μF. Considering derating, two standard value 22-μF, 35-V capacitors are selected.

The secondary output current is sourced by COUT2 during TON. Ignoring the current transitions time in the secondary winding, the value of the secondary output capacitor can be calculated using Equation 26.

Equation 26. C O U T 2 ≥ I O U T 2 D m a x f S W × ∆ V O U T 2

Choosing ΔVOUT2= 0.05 V gives a minimum of COUT2= 4 μF. Considering derating, two standard value 10-μF, 50-V capacitors are selected, the same as the COUT3.