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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
- Wire variables:
- , Wire resistivity (Ω-cm)
- RMS winding currents (A) , Total
- , Peak magnetizing current (A)
- , Max RMS current, worst case (A)
- , Allowed copper loss (W)
- , Cross sectional area of wire (cm2)
- 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 (cm5)
- , Geometrical constant (cmx)
- , Cross-sectional area (cm2)
- , Window area (cm2)
- , Mean length per turn (cm)
- , Magnetic path length (cm)
- , or , Air gap length (cm)
- , Permittivity (Wb A−1 m−1)
- Acronyms
- RMS: root-mean-squared - arithmetic mean) (where denotes the
- 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 free-space is so much larger than the permittivity the transformer material, that the magnetic path length,
- 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 cross-setional 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 off-line 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
- - pk-pk 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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