© 2012 Fairchild Semiconductor Corporation

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FL103 • Rev. 1.0.1

10

Functional Description

Figure 20.

Basic Circuit of a PSR Flyback Converter for LED Illumination

Figure 20 shows the basic circuit diagram of a primary-

side regulated flyback converter with typical waveforms

shown

in

Figure

21.

Generally,

Discontinuous

Conduction Mode (DCM) operation is preferred for

primary-side regulation since it allows better output

regulation.

S

A

F

N

N

V

×

S

P

PK

N

N

I

×

S

A

O

N

N

V

×

Figure 21.

Waveforms of DCM Flyback Converter

The operation principles of DCM flyback converter are

as follows:

Stage I

from the input and stored in the inductor.

Stage II

When the MOSFET (Q1) is turned off, the energy stored

on. While the diode is conducting, the output voltage

applied across the secondary-side inductor and the

inductor has been delivered to the output.

Stage III

When the diode current reaches zero, the transformer

the effective capacitor loaded across MOSFET (Q1).

Constant Voltage Regulation