AN1262 APPLICATION NOTE
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 stored energy is delivered to the
output capacitor and the load. The output voltage V
out
is reflected back to the primary through the turns ratio n
(V
R
, reflected voltage) and adds up to the input voltage V
in
, giving origin to a much higher voltage on the drain
of the MOSFET.
Flyback is operated in DCM (Discontinuous Conduction Mode) when the input -or primary - current starts from
zero at the beginning of each switching cycle. This happens because the secondary of the transformer has dis-
charged all the energy stored in the previous period. If this energy transfer is not complete, the primary current
will start from a value greater than zero at the beginning of each cycle. Then flyback is said to be operated in
CCM (Continuous Conduction Mode). DCM is characterized by currents shaped in a triangular fashion, whereas
CCM features trapezoidal currents.
The boundary between these two types of operation depends on several parameters. For a given converter,
that is, as the switching frequency, inductance of the primary winding, transformer turns ratio and regulated out-
put voltage are defined, it depends on the input voltage and the output load.
At design time, whether the converter will be operated in CCM or in DCM and where the boundary will be located is
up to the designer. Usually CCM is selected with the objective of maximizing converter's power capability or minimiz-
ing primary RMS current. However, in CCM operation the system's dynamic behavior is considerably worse.
Usually, the converters based on the L6590 family devices are able to deliver the desired output power even
with DCM operation, thus CCM will not be considered.
Table 1. Converter specification data and pre-design choices
Converter Electrical Specification
V
ACmin
V
ACmax
f
L
N
H
V
out
∆V
out%
V
r%
P
outmax
η
T
amb
Minimum mains voltage
Maximum mains voltage
Mains frequency (@ min. mains)
Number of holdup cycles
Regulated output voltage
Percent output voltage tolerance
(±)
Percent output voltage ripple
Maximum output power
Expected converter efficiency
Maximum ambient temperature
Pre-design Choices
V
R
η
T
V
spike
V
cc
V
F
V
BF
2/42
Reflected voltage
Transformer efficiency
Leakage inductance overvoltage
IC supply voltage
Secondary diode forward drop
Bridge Rectifier + EMI filter voltage drop
STMICROELECTRONICS [ STMICROELECTRONICS ]
