AP3783 datasheet(9/17 Pages) DIODES

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AP3783

Document number: DS37429 Rev. 6 - 2

9 of 17

www.diodes.com

December 2015

AP3783

Operation Principle Description (Cont.)

Figure 2. The Operation Waveform of Flyback PSR System

For primary-side regulation, the primary current ip(t) is sensed by a current sense resistor RCS (as shown in Figure 1).The current rises up

linearly at a rate of:

)

(

)

(

dip



(1)

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 the input to the output is given by:





(4)

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

Constant Voltage Operation

As to constant-voltage (CV) operation mode, the AP3783 detects the auxiliary winding voltage at FB pin to regulate the output voltage. The

auxiliary winding voltage is coupled with secondary side winding voltage, so the auxiliary winding voltage at D1 conduction time is:





AUX







(5)

Where the VD is the diode forward voltage drop.

ONP

ONS

IPK

IPKS

OFFS

OFF

VSEC

VAUX

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