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