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

3.2.2 Primary Winding Inductance Selection

Three main parameters are considered when selecting the inductance value of primary winding: primary winding current ripple ratio (ILRR), falling slope of the transformer current and the right half plane zero frequency (ωZ_RHP) of the control loop. Finding a balance between these three parameters helps to simplify the rest of the design process.

  • The primary winding ripple current ripple ratio is selected to balance the copper loss and core loss of the transformer. As the relative ripple current increases; the core losses increase and the copper losses decrease.
  • The falling slope of the transformer current should be small enough to prevent sub-harmonic oscillation in applications with a duty cycle greater than 50%. A relatively larger inductance value of the primary winding results in a smaller falling slope. The LM5155 provides fixed internal slope compensation as well as programmable slope compensation for these applications.
  • The right half plane zero should be placed at high frequency, allowing for a higher crossover frequency of the control loop. As the relative inductance value of the primary winding decreases the right half plane zero frequency increases.

A maximum ripple ratio between 30% and 70% results in a good balance of the total power loss of the transformer, matching the down slope of the transformer current to the internal slope compensation and the increasing the right half plane zero frequency. The maximum ripple ratio of the inductor current is set to 60%. In CCM operation, the maximum primary winding ripple current occurs when the supply voltage is at the maximum value. The primary winding inductance value for CCM operation is calculated using Equation 6.

Equation 6. GUID-1E73E310-5D7E-431F-BAD1-4EF713F32435-low.gifGUID-C39CFC92-74C4-4446-983A-38F0CE4FDB14-low.gif

where

  • ILRR is the ripple ratio
  • VSUPPLY_max is the maximum supply current
  • POUT_total is the maximum power delivered by the flyback regulator

The primary winding inductance is selected to be 21µH. The primary winding ripple current and primary winding peak current are calculated using Equation 7 and Equation 8, respectively. The peak primary winding current occurs at the minimum supply voltage.

Equation 7. GUID-29DFBBEA-681F-42D3-9EEE-6C408A07AA9D-low.gif
Equation 8. GUID-00B7E9BE-7F85-46C3-8325-8B1519BB4C7B-low.gif

ILPEAK is used to properly size the current sense resistor. Table 3-1 summarizes the key parameters of selected transformer.

Table 3-1 Selected Transformer Parameters
Parameter Value
Turns Ratio (NP:NS:NAUX) 1:0.5:1 (2:1:2)
Primary winding inductance (LM) 21 µH
Primary winding saturation current (ISAT) 6 A