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The transformer for a flyback converter is used as the converters inductor as well as an isolation transformer.
Contents
Variables and acronyms[edit  edit source]
 Universal constants
 Permittivity of free space (Wb A^{−1} m^{−1})
 (Wb A^{−1} m^{−1})
 Permittivity of free space (Wb A^{−1} m^{−1})
 Wire variables:
 , Wire resistivity (Ωcm)
 , Total RMS winding currents (A)
 , Peak magnetizing current (A)
 , Max RMS current, worst case (A)
 , Allowed copper loss (W)
 , Cross sectional area of wire (cm^{2})
 Xformer/inductor design parameters
 , turns (turns)
 , Magnetizing inductance (for an xformer) (H)
 , Inductance (H)
 , Winding fill factor (unitless)
 , Core maximum flux density (T)
 Core parameters
 EC35, PQ 20/16, 704, etc, Core type (mm)
 , Geometrical constant (cm^{5})
 , Geometrical constant (cm^{x})
 , Crosssectional area (cm^{2})
 , Window area (cm^{2})
 , Mean length per turn (cm)
 , Magnetic path length (cm)
 , or , Air gap length (cm)
 , Permittivity (Wb A^{−1} m^{−1})
 , Relative Permittivity (unitless)
 Acronyms
 RMS: rootmeansquared  (where denotes the arithmetic mean)
 MLT: mean length turn
 AWG: American wire gauge
Initial calculations[edit  edit source]
 Variables
  output voltage [V]
  input voltage [V]
  diode voltage drop [V]
  transistor on voltage [V]
  turns ratio [unitless]
  duty cycle [unitless]
 Calculate turns ratio
 Diode
 Rectifier:
 Schottky diode:
Inductance calculations[edit  edit source]
The inductance of the transformer, , controls the current ripple.
Say you want a current ripple 50% of average current.
 Solve for
let
The permittivity of freespace is so much larger than the permittivity the transformer material, that the magnetic path length, , can be estimated to be the air gap length, . so and
 Solve for
Minimize total power loss:
Core loss:
The and are in the core material's datasheets
Core calculations[edit  edit source]
Core selection[edit  edit source]
 Variables
  power loss in the core []
  saturation flux density []
  max flux density []
  change in flux density [], aka
  winding area []
  effective crosssetional area of the core []
  Area Product []
  window utilization factor, or fill factor [unitless]
  number of turns on the primary [unitless]
  number of turns on the secondary [unitless]
  number of turns on the bias [unitless]
  permittivity of free space (air) [H/m]
 Material specifications
Grade  [T]  Specific Power Losses @100 °C [W/cm3]  Manufacturer 

B2  0.36  THOMSON  
3C85  0.33  PHILIPS  
N67  0.38  EPCOS (ex S+M)  
PC30  0.39  TDK  
F44  0.4  MMG 
 Calculate minimal AP needed
[]
 should be less than , to avoid core saturation. for example , then for a conservative calculation use

 Generally and
 Using for offline power supplies is a good estimate
 Calculate minimum number of primary and secondary turns
 Calculate actual number of turn on the primary and secondary to be used.
 : Round up to the nearest integer
 Calculate air gap
Current calculations[edit  edit source]
 Variables
  Ripple current max peak
  Ripple current min peak
  pkpk ripple current
 Peak current
 DC current
 RMS current
 AC current
Power Loss[edit  edit source]
References[edit  edit source]
 U of Colorado  Flyback transformer design
 TI  "Magnetics Design 4  Power Transformer Design"  very good, long, description of transformers and design
 TDK ferrite materials
 IRF  Flyback Transformer Design  nice description of howto wind the transformer
 TI  Magnetics Design 5  Inductor and Flyback Transformer Design  describes various converters DCM and CCM
 OFFLINE FLYBACK CONVERTERS DESIGN METHODOLOGY WITH THE L6590 FAMILY  very good, full description of designing an offline flyback converter
 Isolated 50 Watt Flyback Converter Using the UCC3809
 TOPSwitch Flyback Transformer Construction Guide
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