AP3781

Document number: DS42518 Rev. 2 - 2

8 of 15

www.diodes.com

May 2020

© Diodes Incorporated

AP3781

Operation Principle Description

1.

The Conventional PSR Operating Waveforms

Figure 1. The Operation Waveform of Flyback PSR System

Figure 1 shows the typical waveforms which demonstrate the basic operating principle of AP3781 application. And the parameters are defined as

following.

IP---The primary side current

IS ---The secondary side current

IPK---Peak value of primary side current

IPKS---Peak value of secondary side current

VSEC---The transient voltage at secondary winding

VO---The output voltage

VAUX---The transient voltage at auxiliary winding

VA--- The stable voltage at auxiliary winding when rectification diode is in conducting status, which equals the sum of VCC voltage and the forward

voltage drop of auxiliary diode

tSW ---The period of switching frequency

tONP ---The conduction time when primary side switch is “ON”

tONS ---The conduction time when secondary side diode is “ON”

tOFF ---The dead time when neither primary side switch nor secondary side diode is “ON”

tOFFS --- The time when secondary side diode is “OFF”

For primary-side regulation, the primary current ip(t) is sensed by a current sense resistor RCS connected to CS pin. The current rises up linearly at

a rate of:

M

L

t

dt

t

dip

)

(

V

)

(

IN



(1)

As illustrated in Figure 1, when the current ip(t) rises up to IPK, the primary MOSFET turns off. The constant peak current is given by:

CS

CS

PK

R

V

I



(2)

t

ONP

t

ONS

IPK

IPKS

VO

IP

IS

t

OFFS

t

OFF

VSEC

VA

VAUX

t

SW