IEEE Power Electronics Magazine - December 2020 - 37

Energy conversion in the flyback converter, is accomplished with the transformer magnetizing inductance, L m ,
by the action of the switching power transistor alternately
on and off. That energy is stored in the magnetizing inductance when the MOSFET turns on and then gets transferred
to the load when the switching power transistor turns off.
We will analyze the flyback converter in the discontinuous
conduction mode (DCM) operation by looking at both on
and off switching intervals of the switching power transistor, as shown in Figure 2.
When in the turn-on interval (Ton), the switching power
transistor is conducting and the input dc source current,
i pri and magnetizing inductance current, i m will linearly

f- lyback controller enables the output voltage to be above or
below the input voltage. Flyback architectures are especially nice when isolation is needed for either safety or
immunity to noise reasons [1]. The addition of a transformer
provides galvanic isolation. When using transformers, the
primary and secondary side leakage currents may cause
voltage spikes that may exceed the absolute maximum voltage rating of semiconductors and also may erroneously trip
the power switch control circuitry if the ringing continues
for long. The flyback design is also composed of a switching
power transistor, such as a MOSFET or a wide bandgap
device. There is also a power diode, and a pulse width modulator (PWM) controller, as shown in Figure 1.
Lk

Do

ipri

Rs

Cs

+

-
Vsn
+

Lm

ic

+

+

Vs

Vp

Cout

-

-

Ds

Vbulk

isec

n:1

im

+
Vout

Rload

-

-
PWM
Controller

+
+

SWpri

Vg

Vds
-

-
FIG 1 The basic flyback converter circuit. (Source: Silanna Semiconductor; used with permission.)

im
Vbulk/Lm

-n .Vout /Lm

Vbulk + 2n .Vout
Vds

Vbulk + n .Vout

Vbulk - n .Vout
t
Ton

Toff
Tsw

FIG 2 Switching intervals of flyback converter operating in DCM mode. (Source: Silanna Semiconductor; used with permission.)

December 2020

z IEEE POWER ELECTRONICS MAGAZINE

37

IEEE Power Electronics Magazine - December 2020

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