AP3776B

Document number: DS36737 Rev. 6 - 2

10 of 17

www.diodes.com

December 2015

© Diodes Incorporated

AP3776B

0V

Tons

Tsample

Figure 3. Auxiliary Voltage Waveform

The voltage detection point is at 2/3 of the D1 on-time. The voltage detection point is changed with the different primary peak current. The CV

loop control function of AP3776B then generates a D1 off-time to regulate the output voltage.

Constant Current Operation

The AP3776B can work in constant-current (CC) mode. Figure 2 shows the secondary current waveforms.

In CC operation mode, the CC control loop of AP3776B will keep a fixed proportion between D1 on-time Tons and D1 off-time Toffs. The fixed

proportion is

4

4



Toffs

Tons

(6)

Toffs

Tons

Tons

I

I

PKS

OUT









2

1

(7)

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

PK

S

P

PKS

I

N

N

I





(8)

Thus the output constant-current is given by:

PK

S

P

PK

S

P

OUT

I

N

N

Toffs

Tons

Tons

I

N

N

I

















8

2

2

1

(9)

Therefore, AP3776B can realize CC mode operation by constant primary peak current and fixed diode conduction duty cycle.

Multiple Segment Constant Peak Current

As to the original PFM PSR system, the switching frequency decreases with output current decreasing, which will encounter audible noise issue

since switching frequency decrease to audio frequency range, about less than 20kHz.

In order to avoid audible noise issue, AP3776B uses 3-segment constant primary peak current control method. At constant voltage mode, the

It can be seen from the following Figure 4 that with multiple segment peak current control, AP3776B power system can keep switching

frequency above 24kHz from light load to heavy load and guarantee the audible noise free performance.