AN901 Datasheet, PDF (19/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
4.2. R12 Sense Resistor Selection
R12 is chosen to provide a cycle by cycle current limit. Equation 10 gives the average magnetizing current at
specified load.
The peak current in CCM is:
Im_AVE = _n----^I_--^L-₁--^O--–--^A---_D^D-----_ = ₃-----_---¹--₀---_.--₆-- = 556mA
Equation 51.
I_{m PK CCM} = I_{m AVE} + ^V-----^I-^N-₂---^D-_L---_m-^T----^s--^w-- = 0.556 + ²----⁴----₂-^----_⁰----^.2-⁴---₅--^-_----1²---0-^--_–-1-₆--0---^–---⁶- = 0.94A
Equation 52.
Allowing for some variation in performance from design calculations, 1 A current limit is chosen. Applying
Equation 13 and calculating for R12:
R12 = _I-¹-_m---⁰-_--⁰-_L---_I^m-_M----^V_I--_T- = ⁰---₁-^.--¹- = 100m
Equation 53.
Figure 9 shows the expected magnetizing current waveform at specified load.
Figure 9. Magnetizing Current
Rev. 0.1
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