SNVAA43 March   2023 LM5157 , LM5157-Q1 , LM51571-Q1 , LM5158 , LM5158-Q1 , LM51581 , LM51581-Q1

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Application Parameters
  5. 3Calculations and Component Selection
    1. 3.1 Switching Frequency
    2. 3.2 Coupled Inductor Selection
      1. 3.2.1 Coupling Capacitor (AC Cap)
    3. 3.3 Diode Selection
    4. 3.4 MOSFET Selection
    5. 3.5 Output Capacitor Selection
    6. 3.6 Input Capacitor Selection
    7. 3.7 Output Voltage Setting
  6. 4Simplified SEPIC Schematic
  7. 5References

3.2.1 Coupling Capacitor (AC Cap)

The flying capacitor is chosen so that its ripple voltage is no more than 5% of the maximum input voltage:

Equation 5. CAC≥IOUT×DMAX0.05×VIN,MAX×fSW

For this design we will choose a value of 10uF. The AC capacitor is charged to the input voltage, therefore its voltage rating must take into account enough margin with respect to the maximum input voltage. In this example the voltage rating for the AC capacitor is chosen to be 50 V.

The RMS current flowing through this capacitor is also an important parameter to take into account as it might cause overheating and damange:

Equation 6. IRMS, CAC=IIN,DC×1-DMAXDMAX
Equation 7. IIN, DC=VOUT×IOUTη×VIN,min