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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 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.

Q

CLOCK

Ip

Is

Vdrain

Vin

n

•Vout

DCM operation

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. Duty cycle

OSCILLATOR

Driver

Rsense

OCP

+

-

0.5 V

+

-

E/A

2.5 V

VFB

COMP

FREQUENCY

COMPENSATION

Vcc

LEB

Vac

Ls

n:1

Ip

PWM

Clock

by Claudio Adragna

OFFLINE FLYBACK CONVERTERS DESIGN

METHODOLOGY WITH THE L6590 FAMILY

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.