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

3.3 Diode Selection

The rectifier diode on the secondary side must have a reverse voltage rating greater than the maximum input voltage plus the output voltage. The reverse voltage of the secondary diode is calculated as follows:

Equation 11. V D ,   r e v e r s e = V I N , M A X + V O U T   +   V D = 32   V + 12   V   +   0 . 5   V   = 44 . 5   V  

Due to leakage inductance, there is a negative spike when the primary side switch is being turned off. An RC snubber needs to be added across the diode to help minimize this voltage spike. Even if a snubber is added, a margin of at least 30% must be added to the value calculated in Equation 8. For this application, a diode with a reverse voltage rating of 60 V is selected.

The average current of the secondary side diode is estimated using

Equation 9. I D , a v g = I L O A D = 1   A

The diode must be able to conduct the value calculated in Equation 9 with some margin. For the design, the selected diode is capable of conducting 10 A of average forward current.

In addition, an appropriate package must be selected as the power dissipation can be estimated as:

Equation 10. P d i s s , D = I L O A D × V f

Where Vf is the forward voltage drop of the chosen diode.