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FSEZ1317A

Primary-Side-Regulation PWM with POWER MOSFET Integrated

Fairchild Semiconductor 

Functional Description

Figure 24 shows the basic circuit diagram of primary-

side regulated flyback converter, with typical waveforms

shown in Figure 25. Generally, discontinuous

conduction mode (DCM) operation is preferred for

primary-side regulation because 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 (LmNs2/

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

the peak value (Ipk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) 

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 because output current is same as the average of

the diode current in steady state.

The output current estimator picks up 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 internal precise

reference to generate error voltage (VCOMI), which

determines the duty cycle of the MOSFET in CC mode.

With Fairchild’s innovative technique TRUECURRENT®,

constant current (CC) output can be precisely

controlled.

Among the two error voltages, VCOMV and VCOMI, the

smaller one determines the duty cycle. Therefore, 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.

VAC

Np:Ns

ID

IO

D

+

V DL

-

+

Lm

+ VF -

L

VO

O

A

D

-

EA_I

EstimIOator

VCOMI Ref

PWM

Control

DetteDcIStor

V COMV

EA_V

EstVimOator

Ref

Primary-Side Regulation

Controller

CS

RCS

VS

VDD

RS1

RS2

Ids

NA

+

Vw

-

Figure 24. Simplified PSR Flyback Converter Circuit

I pk

I

pk



NP

NS

VF



NA

NS

VO



NA

NS

I D.avg  Io

Figure 25. Key Waveforms of DCM Flyback

Converter

© 2010 Fairchild Semiconductor Corporation

FSEZ1317A • Rev. 1.0.1

10

www.fairchildsemi.com

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