About SMPS transformer properties ?

Discussion in 'General Electronics Chat' started by LETITROLL, Mar 5, 2014.


    Thread Starter Member

    Oct 9, 2013
    Hi everyone here .

    Just like usual am trying to get some informations on how SMPS works even the simplest explanations would be really appreciated .

    Now i'm really susprised by the performance of the the transformer used in SMPSUs , specially the size (like on mobile phone chargers), and even with that it can deliver a very important output current .

    Why they are so small in size despite the high current they deliver compared to a linear power supply transformer ?

    Can a normal 50/60 HZ frequency be applied to them like a linear one and still act as a linear ?
  2. crutschow


    Mar 14, 2008
    The size of a transformer is determined by two main factors:
    The amount of load current it carries.
    The inductance required to limit the no-load magnetizing current for the applied primary voltage and frequency to a reasonably low value. (Edit: The limit is that the magnetizing current must be keep below the saturation point of the core magnetic material.)

    Since the inductance required for a given magnetizing current is inversely proportional to frequency (inductive reactance is proportional to frequency) then the higher the frequency the lower the inductance required and the smaller the magnetic core can be. Since switching power supplies operate at tens of kilohertz there is a huge difference factor between that and the 50/60Hz main's frequency. Thus switching transformers can be much smaller than main's transformers for a given voltage and current.

    So obviously you can't apply mains voltage to a switching transformer as they will blow in short order due to very high magnetizing currents.
    Last edited: Mar 6, 2014

    Thread Starter Member

    Oct 9, 2013
    Good explanation , really straight forward .
    I see why they are smaller now .
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
    and the amount of iron in the core is determined by the frequency. 60 hz transformers are lighter than 50 hz. and 400 hz ( used in aircraft) transformers and motors are smaller and lighter than 60 Hz.
    smps transformers are smaller than linear supploy transformers for the same reason, higher frequency ferite cores instead of heavy iron laminated cores.

    Thread Starter Member

    Oct 9, 2013
    So when saturation occurs , the primary and secondary windings becomes shorted and then the transformer fails ? is that what you mean ?
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
    When the core is saturated, the inductors/transformers inductance drops.
    The core is fully magnetized and cannot be magnetized any further.

    Current will mainly be determined by the wire resistance. That's why it increases rapidly , if the source it permits.

    Less to no energy will be transferred from primary to secondary.

    In other words, it fails.