Forward converter design - current through switching MOSFET

Discussion in 'General Electronics Chat' started by rochak.chadha, Mar 17, 2012.

  1. rochak.chadha

    Thread Starter New Member

    Mar 17, 2012
    7
    0
    Hi everyone,

    I am a newbie here so apologizes if I miss something. I am designing a forward converter and I am simulating the circuit in PowerSim. What I am surprised to see is that the current through the MOSFET switch is about 3 times the output current of the converter. Although, the converter is providing the required output current, but it seems weird if so much current is just being 'burnt' via MOSFETs resistance.
    I am using an application note by On Semi for the design, here is the link to the pdf http://www.onsemi.cn/pub_link/Collateral/NCP1216AFORWGEVB_MANUAL.PDF

    Will be grateful for your help folks,

    Thanks !

    Rochak
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    In a forward conv, current flows in the primary and secondary of the transformer at the same time. You should be able to calculate the currents based on the output side current and the transformer turns ratio. Make sure your simulation has the correct turns ratio. This design has a max duty cycle of only 50%, so the peak currents on the primary will be high since the transistor only works half the time.
     
    Last edited: Mar 17, 2012
  3. rochak.chadha

    Thread Starter New Member

    Mar 17, 2012
    7
    0
    Thanks BountyHunter, I checked the turns ratio and it is fine. The primary: tertiary winding's ratio is 1:0.8 and while primary to secondary ratio is 1:1.78. This does mean that the current in the tertiary winding will be high and so the current in the MOSFET.
    Is this a correct conclusion ?
    The transformer is a commercial coilcraft product, designed for 30W applications
    Here is the link to the product page http://www.coilcraft.com/fct1xxd3sl.cfm
    The part number is FCT1-240D3SL.

    Thanks for your help !

    Rochak
     
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