SNVA866A February   2019  – January 2023 LM5155 , LM5155-Q1 , LM51551 , LM51551-Q1

 

  1.   How to design an Isolated Flyback using LM5155
  2.   Trademarks
  3. 1Introduction
  4. 2Example Application
  5. 3Calculations and Component Selection
    1. 3.1 Switching Frequency
    2. 3.2 Transformer Selection
      1. 3.2.1 Maximum Duty Cycle and Turns Ratio Selection
      2. 3.2.2 Primary Winding Inductance Selection
    3. 3.3 Current Sense Resistor Calculations
      1. 3.3.1 Current Sense Resistor and Slope Compensation Resistor Selection
      2. 3.3.2 Current Sense Resistor Filter Selection
    4. 3.4 MOSFET Selection
    5. 3.5 Diode Selection
    6. 3.6 Output Capacitor Selection
    7. 3.7 Input Capacitor Selection
    8. 3.8 UVLO Resistor Selection
    9. 3.9 Control Loop Compensation
      1. 3.9.1 Feedback Resistor Selection
      2. 3.9.2 RPULLUP Selection
      3. 3.9.3 Optocoupler Selection
      4. 3.9.4 RLED Selection
      5. 3.9.5 Crossover Frequency Selection
      6. 3.9.6 Determine Required RCOMP
      7. 3.9.7 Determine Required CCOMP
  6. 4Component Selection Summary
  7. 5Small Signal Frequency Analysis
    1. 5.1 Flyback Regulator Modulator Modeling
    2. 5.2 Compensation Modeling
  8. 6Revision History

3.6 Output Capacitor Selection

The output capacitor is required to smooth the load voltage ripple, provides an energy source during load transients and provides energy to the load during the on-time of the MOSFET. A practical way to size the output capacitor is based on the required load transient specification. The load transient specification is related to the control loop crossover frequency. For this estimate it is expected that the control loop cross over frequency is set to 1/5th the right half plane zero frequency. This right half plane zero frequency is calculated using Equation 21.

Equation 21. GUID-EEE13275-27F2-4094-A6EC-0B4E7BA3B1E8-low.gif

For this design example, the load transient specification indicates that the load voltage should not overshoot or undershoot more than 100mV during a load transient from 50% load current (2 A) to 100% load current (4 A) occurs. Equation 22 is used to calculate the estimated load capacitance to achieve the specified load transient load voltage ripple requirements.

Equation 22. GUID-033EA979-7063-44CC-984D-91C3A9EE0EDF-low.gif

where

  • ΔILOAD is the difference in the load current conditions (4 A - 2 A)
  • ΔVLOAD is the specified overshoot voltage specification and undershoot voltage specification

In this design CLOAD is selected to be 540µF.