AN892 Datasheet, PDF (5/16 Pages) Silicon Laboratories – DESIGN GUIDE FOR ISOLATED DC/DC USING THE

AN892
2.4. Magnetizing Current
If during the t_S1 portion of the cycle (S1 closed, S2 open), V_IN charges C1 in a linear fashion. Rearranging Equation
1 and substituting Equation 7 and Equation 14, ripple magnetizing current is:
I_{m RIPPLE} = -^--^V----^I-_L-^N---_m--–---+-^V----_L^C---_l-¹-_k--^_g---^t-^S----¹- = -^--^V----^I--^N-_L---_m-–----^V-+---^I-_L-^N---_l-^D_k---_g--^--^t--^S---¹- = -^V----^I-^N----^D-_L----_m^--¹---+--–---_L--^D-_l--_k-^-_g--^T----^S---^W---
Equation 15.
The average magnetizing current is related to the output current as
The peak magnetizing current is given by:
l_{m AVE} = I_LOADN
Equation 16.
l_{m PK} = l_{m AVE} + ^V-----^I-^N-₂---^D_---_L--^-_m-¹-----+–-----^DL---_l-^-_k--^T_g---_-^S---^W---
Equation 17.
Si882xx/Si883xx controller limited the peak magnetizing current to approximately 3A. If more current than 3A is
sensed during S1 on and S2 off mode, the controller immediately switches to the S1 off and S2 on mode. The
controller maintains the same switching period, but reduces the duty cycle D to limit peak current.
2.5. Input Capacitor
The purpose of C2 input capacitor is to provide current during switching cycles. During S1 closed, S2 open, C2
provides current to C1 in series with L_m + L_lkg.
l_IN = I_{m AVE}D = I_LOADDN
Equation 18.
During the t_S1 portion of the cycle (S1 closed, S2 open), V_IN recharges C2. The voltage ripple on C2 can be written
as:
Substituting Equation 18 into 19:
V_{IN RIPPLE} = ^I--^C----²---^--¹-----_C^–-----_2D-----^--^T----^S---^W---
Equation 19.
V_{IN RIPPLE} = -^I-^L---^O----^A----^D----^D----^---¹-_C---–-₂----^D-----^--^T----^S---^W-----^N---
Equation 20.
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