AN1262
APPLICATION NOTE
OFFLINE FLYBACK CONVERTERS DESIGN
METHODOLOGY WITH THE L6590 FAMILY
by Claudio Adragna
The design of flyback converters is quite a demanding task that requires SMPS engineers to cope with sev-
eral problem areas such as magnetics, control loop analysis, power devices, as well as regulations concern-
ing safety, EMC and the emerging standby consumption requirements. Lots of variable are involved and
complex tradeoffs are necessary to meet the goal.
In this scenario, the high-voltage monolithic switchers of the L6590 family greatly simplify the task and, at
the same time, allow to build robust and cost-effective low-power systems.
In this application note, after a review of flyback topology, a step-by-step design procedure of an offline sin-
gle-output flyback converter will be outlined. As an example, the design of the test board will be carried out
in details.
1
FLYBACK BASICS
Flyback operation will be illustrated with reference to the basic circuit and the waveforms of fig. 1. It is a two-
step process. During the ON-time of the switch, energy is taken from the input and stored in the primary winding
of the flyback transformer (actually, two coupled inductors). At the secondary side, the catch diode is reverse-
biased, thus the load is being supplied by the energy stored in the output bulk capacitor.
Figure 1. Flyback Topology and associated waveforms.
Vin
Is
Lp
Vout
Vac
n:1
Vcc
DRAIN
Ls
L6590
L6590D
L6590A
OSCILLATOR
2.5 V
Clock
Ip
Max. Duty cycle
S
R
Driver
Q
1
1/100
ISOLATED
FEEDBACK
+
VFB
-
E/A
+
PWM
-
Clock
LEB
+
OCP
-
Rsense
0.5 V
COMP
GND
FREQUENCY
COMPENSATION
CLOCK
CLOCK
CLOCK
Q
Q
Q
Ip
Ip
Ip
∆Ip
Is
Is
Is
Vdrain
Vin
n•Vout
Vdrain
Vdrain
DCM operation
TRANSITION
CCM operation
May 2001
1/42
STMICROELECTRONICS [ STMICROELECTRONICS ]
