AN901 Datasheet, PDF (5/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
2.3. Voltage Transfer
Let duty cycle D be defined as the ratio of time S1 is closed over the complete switching period T_sw:
D = -_t-_S----₁--^t--^S_+---¹--_t--_S---₂-
Equation 3.
Now t_S1 and t_S2 can be expressed in terms of D and switching period as:
t_S1 = DT_sw
Equation 4.
t_S2 = 1 – DT_sw
Equation 5.
and assume diode D1 has no voltage drop when conducting the volt-second balance equation for L_m, which in
CCM operation can be written as:
Equation 6 simplifies to:
V_INDT_sw – nV_OUT1 – DT_sw = 0
Equation 6.
V_OUT  _n----_-^V--₁--^I--^N_–---^D--_D-----_
Equation 7.
For DCM, current does not flow out of the secondary over the entire (1-D) portion, which changes the voltage
transfer function shown in Equation 7. Unlike CCM, the voltage transfer characteristics in DCM are dependent on
factors such as R_LOAD and switching period. The governing equation is:
V_OUT  V_IND ^R-----^L---^O-₂---^A-_L--^D--_m--^T----^s---^w--
Equation 8.
Rev. 0.1
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