AP3984 datasheet(6/15 Pages) DIODES

6 / 15 page

AP3984

Document number: DS37655 Rev. 3 - 2

6 of 15

www.diodes.com

December 2016

AP3984

Operation Description

RFB1

RFB2

RCS

GND

IC1

AP3984

NAUX

IOUT

VAUX

vIN

CBC

RCBC

OUT

GND

VCC

IC2

AP4341

Figure 1. Simplified Flyback Converter Controlled by AP3984

Constant Primary Peak Current

The primary IP(t) current is sensed by a current sense resistor RCS as shown in Figure 1.

The current rises up linearly at a rate of:

BULK

)

(

)

(



……… (1)

IP(t)

See equation 2

IPK

Figure 2. Primary Current Waveform

As illustrated in Figure 2, when the current IP(t) rises up to IPK, the switch Q1 turns off. The constant peak current is given by:



……………… (2)

The energy stored in the magnetizing inductance LM each cycle is therefore:





………… (3)

So the power transferring from input to output is given by:





………(4)

Where fSW is the switching frequency. When the peak current IPK is constant, the output power depends on the switching frequency fSW.

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