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Design on SMPS Transformer (Topology and Material Selection)
Typical locations where cores are used
Core Material
• Purpose of the core is:– To reduce the magnetizing current. (For topologies
such as Forward, Bridge etc we need the magnetizing current to be as small as possible. For fly-back topology, though the magnetizing current is used to transfer energy, the size of the transformer will be very large to get the required inductance if a core is not used.)
– To improve the linkage of the flux within windings if the windings are separated spatially.
– to contain the magnetic flux within a given volume– In magnetic amplifier applications a saturable core is
used as a switch.
Core Material
• Different types of material used for cores– Iron– Silicon Steel– Nickel-Iron– Iron-Cobalt– Ferrite– Molybdenum– Met-glas
Core Material
• Salient characteristics of a core material are– Permeability– Saturation flux density– Coercive force– Remnant flux– losses due to
• Hysteresis • Eddy Current
B-H loop of a magnetic material
B-H Minor loops
Core Material
• The power loss is a function of frequency and the ac flux swing and is given by the equation
• P = K1 * (frequency)K2 * (Flux Density)K3
• Typical power loss for different material are given below:
Material Type Loss (mw/cm^3)
Phillips, Ferrite 3C90, = 2300
110
Phillips, Ferrite 3C81
= 2700
220
Phillips, Ferrite 3F3
= 2000
120
Phillips, Ferrite 3F4
= 900
10 @ 200 G,500 KHz
180 @ 1000G,100KHz
1600 @1000G,500KHz
Frequency = 100 KHz ; B = 1000 Gauss.
Material Type Loss (mw/cm^3)
EPCOS, Ferrite N27, = 2000 231.5
EPCOS, Ferrite N67, = 2100 68.57
EPCOS, Ferrite N87, = 2200 50.7
Met-glas, 2714 AS 105
Met-glas , Powerlite, C core 840
Met-glas , Micolite = 245 560
Met-glas , Micolite = 270 630
Frequency = 100 KHz ; B = 1000 Gauss.
Material Type Loss (mw/cm^3)
Moly Powder Permalloy = 12 870
Moly Powder Permalloy = 60 360
Moly Powder Permalloy = 200 1440
Magnetics , Material K = 1500 110
Magnetics , Material R = 2300 70
Magnetics , Material P = 2500 125
Magnetics , Material F = 3000 225
Magnetics , Material H , = 15000
2200
Frequency = 100 KHz ; B = 1000 Gauss.
Material Type Loss (mw/cm^3)
Magnetics Tape wound Supermalloy
870
Magnetics Tape wound
Square-Permalloy
360
Magnetics , Material P = 2500 125
Magnetics , Material F = 3000 225
Magnetics , Material H , = 15000
2200
Frequency = 100 KHz ; B = 1000 Gauss.
Performance factor