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SEPIC with coupled inductors vs uncoupled..?

Discussion in 'General Electronics Discussion' started by eem2am, Feb 24, 2011.

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  1. eem2am


    Aug 3, 2009

    I am confused about the comparative performance of SEPICs with coupled inductors, compared with SEPICs with uncoupled inductors....

    ..can any reader shed any light here......

    if a SEPIC converter uses coupled 1:1 inductors, then it behaves the same as does a SEPIC with uncoupled inductors of twice the value.

    For example, consider a SEPIC with 1:1 coupled inductors (each winding measuring 250uH with the other one open)............

    ....then this gives the same inductor ripple current as does a SEPIC with two uncoupled inductors of value 500uH.

    If you wish , you can confirm this with this LTSPice simulation:-

    (-just change the inductor values and the coupling from 1 to 0, eg "K L1 L2 0" or "K L1 L2 1")

    My question is......why is the factor of *two* involved here?

    ...i would have thought that the coupled inductors would appear to have *four* times the inductance of the case of uncoupled inductors.

    ....after all, the coupled 1:1 inductor effectively has twice the number of windings , and so that means *four* times the inductance....

    So why is it that 1:1 coupled inductor SEPICs don't behave like uncoupled SEPICs with *four* times the winding inductance?
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