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AN1262
APPLICATION NOTE
May 2001
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 proc ess. Durin g 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 inductor s). A t the secondary side, the catch diode is reverse-
biased, thus the load is being supplied by the energy stored in the output bulk capacitor.
Figu re 1. Fly ba c k Top ol ogy and ass oc i ate d wave form s.
Q
CLOCK
Ip
Is
Vdrain
Vin
n
•
Vout
DCM oper ati on
Q
CLOCK
Ip
Is
Vdrain
∆
Ip
CCM operation
Q
CLOCK
Ip
Is
Vdrain
TRANSITION
Vin
ISOLATED
FEEDBACK
Lp
Vout
Is
L6590
L6590D
L6590A
Clock
+
-
DRAIN
GND
1
1/100
S
R
Q
Max. Dut
c
cle
OSCILLA TOR
Driver
Rsense
OCP
+
-
0.5 V
+
-
E/A
2. 5 V
VFB
COMP
FREQUENCY
COMPENSA TION
Vcc
LEB
Vac
Ls
n:1
Ip
PWM
Clock
by Claudio Adragna
OFFLINE FLYBACK CONVERTERS DESIGN
METHODOLOGY WITH THE L6590 FAM ILY
The design of flyback conver ters is quite a demandi ng task that requires SMPS engineers to cope with sev-
eral problem ar eas such as magnetics, control loop analys is, power devices, as well as re gulations 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.
