AN3095 datasheet(12/55 Pages) STMICROELECTRONICS

12 / 55 page

DC-DC converter

AN3095

12/55

Doc ID 16555 Rev 3

Due to symmetry during the two halves of the switching period, current expressions and

current paths may be derived with similar considerations for the second half of the switching

period.

If d >1 the current in the leakage inductor may reach zero and there is a boundary between

CCM and DCM.

In DCM there are also three modes of operation, as shown in Figure 10.

●

Mode 1, interval t0-t1:

At t=t0 inductor current is zero. After t=t0 devices M1and M4 are turned on under zero

current and inductor current rises according to the following equation:

Equation 5

Figure 10.

Modulation and transformer current in DCM

●

Mode 2, interval t1-t2:

At t1 M6 turns off and M5 turns on with zero current. Inductor current expression is given by:

Equation 6

and reaches zero at t=t3.

●

Mode 3, t2-t3

Equation 7

The boundary between DCM and CCM depends on the phase-shift angle, input voltage,

output voltage and transformer turns ratio and is given by:

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(

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AM05604v1

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(

Llk

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