FAN100_09 Datasheet, PDF(10/16 Page) Fairchild Semiconductor

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FAN100_09 Datasheet, PDF (10/16 Pages) Fairchild Semiconductor – Primary-Side-Control PWM Controller

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Functional Description

0 shows the basic circuit diagram of a primary-side

regulated flyback converter and its typical waveforms

are shown in 0. Generally, discontinuous conduction

mode (DCM) operation is preferred for primary-side

regulation since it allows better output regulation. The

operation principles of DCM flyback converter are as

follows:

During the MOSFET on time (tON), input voltage (VDL) is

applied across the primary side inductor (Lm). Then,

MOSFET current (Ids) increases linearly from zero to the

peak value (Ipk). During this time, the energy is drawn

from the input and stored in the inductor.

When the MOSFET is turned off, the energy stored in

the inductor forces the rectifier diode (D) to be turned

on. While the diode is conducting, the output voltage

(Vo), together with diode forward-voltage drop (VF), is

applied across the secondary-side inductor (LmÃNs2/

Np2) and the diode current (ID) decreases linearly from

the peak value (IpkÃ Np/Ns) to zero. At the end of

inductor current discharge time (tDIS), all the energy

stored in the inductor has been delivered to the output.

When the diode current reaches zero, the transformer

auxiliary winding voltage (Vw) begins to oscillate by the

resonance between the primary-side inductor (Lm) and

the effective capacitor loaded across the MOSFET.

During the inductor current discharge time, the sum of

output voltage and diode forward-voltage drop is

reflected to the auxiliary winding side as (Vo+VF)Ã Na/Ns.

Since the diode forward-voltage drop decreases as

current decreases, the auxiliary winding voltage reflects

the output voltage best at the end of diode conduction

time where the diode current diminishes to zero. Thus,

by sampling the winding voltage at the end of the diode

conduction time, the output voltage information can be

obtained. The internal error amplifier for output voltage

regulation (EA_V) compares the sampled voltage with

internal precise reference to generate error voltage

(VCOMV), which determines the duty cycle of the

MOSFET in CV mode.

Meanwhile, the output current can be estimated using

the peak drain current and inductor current discharge

time since output current is the same as average of the

diode current in steady state.

The output current estimator detects the peak value of

the drain current with a peak detection circuit and

calculates the output current using the inductor

discharge time (tDIS) and switching period (ts). This

output information is compared with the internal precise

reference to generate error voltage (VCOMI), which

determines the duty cycle of the MOSFET in CC mode.

With

Fairchildâs

innovative

technique,

TRUECURRENTâ¢, constant current (CC) output can

be precisely controlled.

Of the two error voltages, VCOMV and VCOMI, the smaller

determines the duty cycle. During constant voltage

regulation mode, VCOMV determines the duty cycle while

VCOMI is saturated to HIGH. During constant current

regulation mode, VCOMI determines the duty cycle while

VCOMV is saturated to HIGH.

Figure 23. Simplified PSR Flyback Converter Circuit

I pk

ID.avg = Io

Figure 24. Key Waveforms of DCM Flyback

Converter

FAN100 Rev. 1.0.2

www.fairchildsemi.com

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