transformer size and frequency

Discussion in 'General Electronics Chat' started by logicman112, May 17, 2010.

  1. logicman112

    Thread Starter Active Member

    Dec 27, 2008
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    I have heard that to have lighter and smaller step-down transformers in a computer switching power supply , the current of 60 HZ converts to much higher.

    How the size of a transformer depends on frequency?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    It can be proved mathematically but think of it logically. You have N primary turns and a core with cross sectional area A with an inductance L. If the transformer is operating at 60 Hz then a flux Ψ will flow through the core. If the frequency increases the current through the primary reduces and so does the flux level. To increase the new flux level to the flux level at 60 Hz, the core is made smaller (smaller A) and thus the inductance reduces. This increases the current and thus the flux level.
     
  3. logicman112

    Thread Starter Active Member

    Dec 27, 2008
    69
    2
    Current reduces because leakage flux increases , right?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    The current is the same. The difference is the physical size of the transformer.

    The small toroid is switched at a very high frequency compared to line voltage. That is why the relatively small toroidal transformer can achieve such high efficiency.
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Flux is proportional to current. Imagine a big transformer with a high inductance and small one with low inductance.

    To achieve the same current through them, the small transformer has to be operated with a higher frequency than the big one because of its smaller inductance (XL=ω*L).
     
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