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

3.9.3 Optocoupler Selection

When selecting a optocoupler a few major parameters need to be considered: current transfer ratio (CTR), diode voltage drop on the secondary side, and the capacitance of the BJT on the primary side. In the following equations CTR is synonymous with kOPTO, the diode drop with VD and the BJT capacitance with COPTO in Figure 3-2.

  • The CTR varies drastically based on the selected component. There can be as large as 600% variance in the CTR value over operating conditions. Due to this large tolerance in CTR, the loop compensation component selection needs to account for the minimum CTR value and maximum CTR value. In this design the CTR of the selected optocoupler is 100% to 200%.
  • The diode voltage drop impacts the selection of the RLED value. The diode drop need to be small enough to allow the voltage reference to be supplied with adequate voltage to ensure operation. The diode drop of the selected optocoupler is 1.4V.
  • The pull-up resistor (RPULLUP) and the parasitic capacitance (COPTO) form a pole, limiting the maximum crossover frequency of the control loop. Once the pull-up resistor is selected the capacitance of the optocoupler is calculated from the optocoupler data sheet. The selected optocoupler the capacitance is calculated to be approximately 3.3nF. The cross over frequency should be less than the pole, which is roughly 9.66kHz for this design.

Table 3-2 summarizes the key parameters of the selected optocoupler.

Table 3-2 Selected Optocoupler parameters
Parameter Value
kOPTO_min 100%
kOPTO_max 200%
VD 1.4V
COPTO 3.3nF
VCE(sat) 200mV