AN901 Datasheet, PDF (6/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
2.4. Magnetizing Current
Substituting Equation 4 into Equation 1, the ripple magnetizing current is:
I_{m RIPPLE} = ^V-----^I-_L^N---_m-^t--^S---¹- = ^V-----^I-^N---_L-^D--_m---^T----^s--^w--
Equation 9.
The average magnetizing current is related to the output current as:
I_{m AVE} = _n----^I_--^L-₁--^O--–--^A---_D^D-----_
Equation 10.
When a flyback converter is operating in CCM, the peak magnetizing current is given by the average current plus
one half of the ripple current:
I_{m PK CCM} = I_{m AVE} + ^V-----^I-^N-₂---^D-_L---_m-^T----^s--^w--
Equation 11.
When a flyback converter is operating in DCM, the peak magnetizing current is equal to the ripple current:
I_{m PK DCM} = I_{m RIPPLE}
Equation 12.
Si884xx/Si886xx controller limits the peak magnetizing current by comparing the voltage across the current sense
resistor R12 to an internal reference voltage of approximately 100 mV. If more than 100 mV is developed across
R12 during S1 closed, the controller immediately switches S1 open. The controller maintains the same switching
period, but reduces the duty cycle D to limit peak current. The cycle by cycle current limit is given by:
I_{m LIMIT} = ¹----⁰-_R--⁰--₁--^m--₂---^V---
Equation 13.
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