AP3987CT

Document number: DS41644 Rev. 2 - 2

7 of 15

www.diodes.com

June 2019

© Diodes Incorporated

AP3987CT

Operation Description (continued)

Figure 2. The Operation Waveform of Flyback PSR System

3.

Constant Voltage Operation

As to Constant Voltage (CV) operation mode, the AP3987CT 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 during tONS is:





D

OUT

S

AUX

AUX

V

V

N

N

V







(1)

Where VD is the conduction voltage drop of MOSFET in APR3415CT.

0V

tONS

tSAMPLE

Figure 3. Auxiliary Voltage Waveform

The voltage detection point is at the tSAMPLE of tONS. The voltage detection point is changed with the different primary peak current. The CV loop

control function of AP3987CT then generates a tOFF to regulate the output voltage.

4.

Constant Current Operation

The AP3987CT can work in Constant Current (CC) mode. Figure 2 shows the secondary current waveforms.

In CC operation mode, the CC control loop of AP3987CT will keep a fixed proportion between tONS and tOFFS. The fixed proportion is

4

4



OFFS

ONS

t

t

(2)

t

ONP

t

ONS

IPK

IPKS

VO

IP

IS

t

OFFS

t

OFF

VSEC

VA

VAUX

t

SW