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Why is magnetising current seen in SMPS simulation too low?

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T

treez

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Hello,
The attached LTspice simulation (of a two transistor forward SMPS) shows that the magnetising current (shown by putting expression “I(L1)+0.189*I(L2)” into the waveform window) only rises to 188mA peak.

This is lower than calculated.
Why is this?

The primary voltage bus is 390V
Lp = 7mH
t(ON) = 4us

Therefore, the magnetising current peak, as seen in the primary, should be given by (V.dt)/Lp = 223mA.
Why do we only see 188mA peak magnetising current in the simulation.?
 

Attachments

  • Two Transistor forward SMPS.txt
    4.9 KB · Views: 41

Look at the current delta. It starts from a negative value.
 

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