L4990 - L4990A
Figure 29. Half duty cycle option.
tc + td
2 ·tc + td
To evaluate the device performance, a demon-
stration board has been realized. Despite its sim-
plicity, it exploits most of the features of the
The board embodies an application based on the
following specification of universal mains AC-DC
Input voltage range: 85-270 Vac (50/60 Hz)
Output voltage: 15 V
Output current: 0.5 to 2A
Output voltage ripple : 300 mV (max.)
Load regulation: ± 5% (0.5 to 2 A load change)
Target efficiency @ Iout = 2 A: ; 80% over the
input voltage full range
Some preliminary decisions, concerning topology,
operating mode, switching frequency, maximum
duty cycle allowed and control technique, have
As for topology, at this power level and output
voltage, flyback is the most advantageous one,
mainly because of its simplicity, which means low
parts count, low cost and inherent high efficiency.
A peculiar design choice aiming at optimizing the
overall system concerns the operating mode: the
converter will work in continuous current mode at
low input voltages, when input current is greater,
and in discontinuous mode at higher input volt-
ages. Numerous benefits originate from that.
Compared to discontinuous current mode, con-
tinuous operation involves lower peak currents
(typ. -40%) at the same throughput power. This
implies less stress for all power components.
The transformer inductance is higher and, there-
fore, a smaller air gap is required for a given core:
this increases primary-to-secondary coupling and,
as a consequence, reduces leakage inductance
and improves energy transfer. Both efficiency and
load regulation will take advantage of that.
Another point in favor is a reduction of the mini-
mum output power that the system is able to de-
liver keeping the output well regulated.
Few components are required in addition for
Actually, continuous mode flyback suffers also
from a poor dynamic behavior during load tran-
sients because of the narrow bandwidth of the
control loop due to stability problems. However,
great dynamic performance is not required to AC-
DC adapters, so this problem is of no concern.
The boundary between the two operating modes
has been set at about 150 Vac (@ Iout=2A).
The selection of the switching frequency is a mat-
ter of trade-off between achieving a small trans-
former size and high efficiency. 200 kHz seems to
be a good compromise.
In this application, the wide input voltage range
requires a large duty cycle sweep. The higher is
the maximum duty cycle, the larger is the operat-