AP3785T

Document number: DS37552 Rev. 4 - 2

9 of 18

www.diodes.com

September 2015

© Diodes Incorporated

AP3785T

3.

Constant Voltage Operation

As to constant-voltage (CV) operation mode, the AP3785T 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 the conduction time of

secondary rectification diode D1 is:





Vd

V

N

N

V

OUT

S

AUX

AUX







(1)

Where Vd is the diode forward voltage drop.

0V

Figure 3. Auxiliary Voltage Waveform

CV loop control function of AP3785T then generates a D1 off-time to regulate the output voltage.

4.

Constant Current Operation

The AP3785T can work in constant-current (CC) mode. Figure 2 shows the secondary current waveforms.

proportion is

4

4

t

t

OFFS

ONS



(2)

OFFS

ONS

ONS

PKS

OUT

t

t

t

I

2

1

I









(3)

As to tight coupled primary and secondary winding, the secondary peak current is

PK

S

P

PKS

I

N

N

I





(4)

Thus the output constant-current is given by:

PK

S

P

OFFS

ONS

ONS

PK

S

P

OUT

I

N

N

8

2

t

t

t

I

N

N

2

1

I

















(5)

Therefore, AP3785T can realize CC mode operation by constant primary peak current and fixed diode conduction duty cycle.