AN892 Datasheet, PDF (12/16 Pages) Silicon Laboratories – DESIGN GUIDE FOR ISOLATED DC/DC USING THE

AN892
N  ^V-----^O----^U-_V--^T---_I-_N-+---_D--^V----^f--^D----¹-  ₅--⁵---_--+---₀--⁰-_.--_2.--⁵-₅--  4.4
Equation 38.
A 1:4 turns ratio was chosen.
The next parameter to choose is the primary inductance. Equation 16 gives the average magnetizing current.
Im_AVE = NI_LOAD = 40.4 = 1.6A
Equation 39.
Equation 15 shows that magnetizing current ripple is inversely proportional to primary inductance. There are
considerations for choosing the magnitude of the magnetizing current ripple. Choosing a very small primary
inductance leads to a large current ripple. Care must be taken not to approach the cycle-by-cycle current limit of
approximately 3A. For this design, peak current was chosen not to exceed 2.5A at specified maximum I_LOAD. A
ripple current of 1.8A was targeted. Rearranging Equation 15,
L_m + L_lkg = ^V-----^I-^N----_ID--_m---^_--¹-_R----_I^–-_P----^D_P---_L-^--_E-^T----^s---^w--- = ⁵-----^-----⁰---^.--²---⁵----^---⁰¹---^..--⁷⁸---⁵----^--⁴----^x---¹---⁰----^–---⁶- = 2.08H
Equation 40.
The result of Equation 40 suggests the combination of magnetizing current and leakage inductance should be
2.08 H. Leakage inductance is unavoidable in transformer design and it should be minimized for the best energy
transfer in an asymmetric half bridge flyback converter. A transformer was designed with primary inductance of
2 H and leakage inductance 100 nH. Figure 8 shows the expected magnetizing current during the portion of the
cycle that S1 is closed and S2 is open.
Figure 8. Magnetizing Current
12
Rev. 0.1