AN901 Datasheet, PDF (4/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
2.1. S1 Closed
V_IN is applied to the primary inductance L_m. As a result, current flows through inductance L_m and energy is stored
in the magnetic field of the transformer T1:
V_IN = L_m-^I-^m----^---^R-_t--_SI--^P-₁--^P----^L---^E-
Equation 1.
I_m,RIPPLE is the magnetizing current ramp during t_S1, and t_S1 is the time that S1 is closed. In Discontinuous
Conduction Mode (DCM), I_m,RIPPLE is equal to I_m,PK as primary and secondary currents returning to zero before
the next cycle. In Continuous Conduction Mode (CCM), the currents do not reach zero before the next switching
cycle.
2.2. S1 Open
The instant S1 opens, current can no longer flow through the primary and the magnetic field collapses, transferring
energy to the secondary, causing current to flow out of the dot of the ideal transformer. The energy stored in the
leakage inductance is not transferred and it must be dissipated in the primary through the snubber network. The
voltage at the secondary will be impressed on the primary. The governing current equation is:
I_{m RIPPLE} = – -ⁿ----^-----^V----^O--_L--^U--_m--^T----^-----^t--^S----²-
Equation 2.
where n and t_S2 are primary to secondary turns ratio and time that S1 is open, respectively.
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Rev. 0.1