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

3.9.7 Determine Required CCOMP

The RCOMP resistor and CCOMP capacitor set a low frequency zero of the compensation network, providing a phase boost. Placement of this zero frequency largely impacts the transient response of the control loop. A good strategy to help ensure adequate phase margin is to place the zero at geometric mean of the crossover frequency (fCROSS) and the low frequency pole of the modulator. Equation 31 places the low frequency zero of error amplifier a the geometric mean of fCROSS and low frequency pole of the plant (ωP_LF).

Equation 31. GUID-BC46E323-D152-4372-ACE2-F38FD52ED060-low.gif

where

  • D is the duty cycle at the maximum supply voltage (0.217)

For this design CCOMP is selected to be 220 nF to help ensure extra phase margin.