FSEZ13X7 データシートの表示（PDF） - Fairchild Semiconductor

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FSEZ13X7

Design Guideline for Primary Side Regulated (PSR) Flyback Converter Using FAN103 and FSEZ13X7

Fairchild Semiconductor 

AN-8033

2. Operation Principle of Primary-

Side Regulation

Figure 2 shows the simplified circuit diagram of a primary-

side regulated flyback converter and its typical waveforms

are shown in Figure 3. Generally, discontinuous

conduction mode (DCM) operation is preferred for

primary-side regulation since it allows better output

regulation. The key of primary-side regulation is how to

obtain output voltage and current information without

directly sensing them. Once these values are obtained, the

control can be accomplished by the conventional feedback

compensation method.

The operation principles of DCM flyback converter are as

follows:

ƒ 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.

ƒ When the MOSFET is turned off, the energy stored in

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

on. During the diode conduction time (tD), the output

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

(VF), are 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 tD, all the energy stored in the inductor has

been delivered to the output.

ƒ 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 MOSFET output capacitor.

During the diode conduction 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. 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 an error voltage (VCOMV), which determines the

duty cycle of the MOSFET, as shown in Figure 2.

Meanwhile, the output current can be estimated through

calculation. Assuming that output current is same as the

average of the diode current in steady state, the output

current can be estimated as:

IO

= I PK

NP

NS

• tD

2tS

(1)

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 diode conduction time (tD) and

switching period (ts). These output information is

compared with internal precise reference to generate error

voltage (VCOMI), which determines the duty cycle of the

MOSFET, as shown in the block diagram of Figure 2.

Among the two error voltages, VCOMV and VCOMI, the

smaller one actually 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.

Figure 2. Primary-Side Regulated Flyback Converter

I pk

I

pk

⋅

Np

Ns

VF

⋅

Na

Ns

VO

⋅

Na

Ns

ID.avg = Io

Figure 3. Key Waveforms of Primary-Side Regulated

Flyback Converter

© 2009 Fairchild Semiconductor Corporation

Rev. 1.0.1 • 5/6/10

2

www.fairchildsemi.com

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