AP3776 datasheet(8/17 Pages) DIODES

8 / 17 page

AP3776

Document number: DS36707 Rev. 4 - 2

8 of 17

www.diodes.com

February 2015

AP3776

A Product Line of

Diodes Incorporated

Function Description

Operation Description

Start-up Circuit

OUT

VCC

R13

R15

VDD

I S

VBULK

I SOURCE

OUT

VCC

R13

R15

I S

VBULK

OUT

(a)

(b)

Figure 1. The AP3776 Start-up Circuit

Figure1 (a) shows the Startup Phase

•

Before VCC reaches VTH(ST), VDD is zero and EM to IS pin is open.

•

C3 is charged by ISOURCE.

•

The resistance of R13 and R15 should be big enough to reduce the power dissipation.

Figure1 (b) shows the Normal Operation Phase

•

When VCC reaches VTH(ST), VDD is high and EM to IS pin is short.

•

The voltage of EM pin is lower than 1V so that the four diodes in chip are open.

•

C3 is supplied by Aux. winding of transformer.

Operation Mode

The typical application circuit of AP3776 is a conventional Flyback converter with a 3-winding transformer---primary winding (NP), secondary

winding (NS) and auxiliary winding (NAUX), as shown in typical application of AP3776 with AP4341. The auxiliary winding is used for providing

VCC supply voltage for IC and sensing the output voltage feedback signal to FB pin.

Figure 2 shows the typical waveforms which demonstrate the basic operating principle of AP3776 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

•

VS---The stable voltage at secondary winding when rectification diode is in conducting status, which equals the sum of output voltage

VOUT and the forward voltage drop of diode

•

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 voltage VCC 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”

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