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

3.1 Switching Frequency

Selecting the switching frequency is the first step in the design process. Higher switching frequencies yield a smaller total solution size. However, the small size comes at the cost of increased switching losses, decreasing the efficiency of the regulator. Higher efficiency is achieved by selecting a relatively lower switching frequency but requires physically larger components. Harmonics of the switching frequency should be considered in designs that have strict EMC requirements. Equation 1 is used to set the frequency of the internal oscillator of the LM5155. The example application is selected to have a switching frequency of 250kHz.

Equation 1. GUID-F671C808-241A-479C-9897-56EFFC7236E8-low.gif

A standard value of 86.6kΩ is chosen for RT.

Note that the internal oscillator of the LM5155 can be synchronized to an external clock as described in the data sheet. The LM5155 has a maximum duty cycle limit that is frequency dependent. See the LM5155 data sheet for details on the maximum duty cycle limit.