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UCC28782: How to adjust the negative exciting current

user4243385
Prodigy 60 points
Part Number: UCC28782

The negative exciting current is larger than expected.
Reducing the RDM resistance had no effect. Is there any possible cause?
The FETs used are as follows, but what is the appropriate negative exciting current for SJ_MOS?

QL: IPA65R225C7
QH: IPD70R360P7

7230.wavefome.pdf

  • 0
    TI__Expert 7080 points

    Hello,

    Achieving ZVS criteria : 1/2*Lm *Inegative^2 =1/2*Csw*Vsw^2.

    where:

    Lm : Transformer inductance,

    Inegative : what above mentioned "negative exciting current"

    Csw: switch node capacitance, The main part of Csw is low and high side FET output capacitor Coss.

    Vsw: switch node voltage.

    Once transformer design and input&output voltage are fixed , the negative current is only depended by high and low side FET Coss . large Coss , more negative current needed to achieving ZVS. Vice versa.

    And the negative current also variation at different input voltage , low line input , less negative current . high line input large negative current.

    Thanks.