When the switch turns off, the primary circuit is open and the energy stored in the primary is transferred

to the secondary by magnetic coupling. The catch diode is forward-biased, and the energy is delivered

to the output capacitor and to the load (recirculation). The output voltage is reflected back to the primary

through the turns ratio and adds up to the input voltage (typically, the filtered rectified mains), giving ori-

gin to a much higher voltage on the drain of the MOSFET.

Flyback topology is operating in DCM (Discontinuous Conduction Mode) when the input -or primary -

current starts from zero at the beginning of a given switching cycle. This happens because the secon-

dary of the transformer has discharged all the energy stored in the previous period. If this energy trans-

fer is not complete, then the primary current will start from a value greater than zero at the beginning of

each cycle. Then the flyback is said to be operating in CCM (Continuous Conduction Mode). DCM is

characterized by currents shaped in a triangular fashion, whereas CCM features trapezoidal currents

(see fig. A1).

The boundary between these two types of operation depends on several parameters. Some of them are

structural, that is parameters that identify the flyback converter: inductance of the primary of the trans-

former, transformer turns ratio and regulated output voltage. Others are related to the external world and

are subject to changes: input voltage and output load. The switching frequency is usually a structural pa-

rameter, unless it is synchronized to an external signal.

√

2

(A1).

The point is: in a given flyback, when operating in DCM, the peak input current depends solely on the

power drawn from the input.

which the MOSFET is OFF and the catch diode is conducting) are respectively:

=

n

(A2)

The quantity n

(A3).

transition between DCM and CCM implies:

(A4)

and, by combining equations (A1), (A2), (A3) in (A4), it is possible to determine the "Transition Power"

input power at which it works in CCM) for a given input voltage (and a given switching frequency, if this

can vary):

=

1

2

⋅ 









1

+











2

(A5);

AN1049 APPLICATION NOTE

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