FSEZ1016AMY datasheet(10/16 Pages) FAIRCHILD

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FSEZ1016A • Rev. 1.0.2

Functional Description

Figure 24 shows the basic circuit diagram of a primary-

side regulated flyback converter, with typical waveforms

shown

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 turn on.

While the diode is conducting, the output voltage (VO),

together with diode forward-voltage drop (VF), are

applied across the secondary-side inductor (Lm×Ns

2) 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 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. 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 the average of

the diode current in steady state.

The output current estimator detects the peak value of

the drain current by a peak detection circuit and

calculates the output current by 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

Fairchild’s

innovative

technique

TRUECURRENT™, 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.

p:Ns

Primary-Side Regulation

Controller

VDD

F -

Estimator

DIS

Detector

PWM

Control

VAC

Ref

EA_V

EA_I

COMV

COMI

Figure 24. Simplified PSR Flyback Converter Circuit

ds (MOSFET Drain-to-Source Current)

DI S

(Diode Current)

(Auxiliary Winding Voltage)

•

Davg

•

Figure 25. Key Waveforms of DCM Flyback

Converter

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