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

3.4 MOSFET Selection

The MOSFET switch (SW) on the primary side must have a drain-source voltage rating greater than the maximum input voltage plus the output voltage. The drain-source voltage is calculated as follows:

Equation 11. V D S = V I N , M A X + V O U T

A 30% margin must be added to the previous value.

The current through the MOSFET is calculated as follows:

Equation 12. I p e a k = I L O A D + I I N , D C +   ∆ I L

For the power dissipation the RMS current is needed:

Equation 13. I p e a k , R M S = I I N D M A X

In this case, a MOSFET is chosen that is rated for 80 V, 10 A.