AN4070 datasheet(9/53 Pages) STMICROELECTRONICS

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AN4070

DC-DC converter design

Doc ID 022934 Rev 1

9/53

Figure 6.

MOSFET MOS1 and MOS2 currents

During the second switching interval MOS1 is still on while MOS2 is turned off. The energy,

previously stored in L2 is now transferred to the secondary while inductor L1 is charged by

the input voltage and its current continues to build up. The primary current is delivered to the

transformer secondary winding and output capacitor C2 is charged since diode D2 is in

conduction (Figure 7). During the third time interval both MOSFETs conduct again.

Therefore, energy is stored both in L1 and L2 while the load is supplied by the output

capacitors. During the fourth time interval MOS1 is off and MOS2 is kept on, which means

that the energy previously stored in L1 is discharged on C3 with the transformer secondary

current flowing through D1.

In reality, some additional operating intervals must be taken into account. This is mainly due

to the presence of system parasitic elements such as transformer leakage inductance and

semiconductor output capacitances.

Figure 7.

Rectifier diode currents

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