AP3302

Document number:DS38882 Rev. 4 - 2

12 of 17

www.diodes.com

March 2017

© Diodes Incorporated

AP3302

DEM

3

Line Voltage

Detector

Auxiliary

4

OCP

BNO

21

1

1

:

HV

Gate

Rs

SENSE

Inside AP3302

SOVP

4.2V

SUVP

1V

Vs

Figure 5

Constant Over Current Protection

Cycle-by-cycle current limit is a popular method to achieve output over current protection. Actually, the turn-off delay of the MOSFET and the

higher switching frequency always result in the higher OCP current at high line voltage. To obtain a constant OCP current value with universal

input voltage, AP3302 adopts an effective line compensation circuitry. The function block is illustrated in figure 5. The current

which reflects

line voltage is scaled down and inversed to

within AP3302, this

flows through the inner compensation resistor

and an external

compensation resistor

, and then the final line compensation voltage is formed as

As above formula indicates, changing the compensation voltage at different line voltage is a good way to balance the OCP current. In a real

system, usually keep the

value fixed when the BNO voltage is set up, to change the line compensation voltage, a good solution is to change

. Whenever the

is changed, adjust the

at the same time to offer an enough RC time to filter the spike on SENSE pin.

Secondary OVP and UVP

The AP3302 provides output OVP and UVP protection function. The auxiliary winding voltage during secondary rectifier conducting period reflects

the output voltage. Refer to figure 5,a voltage divider network containing

and

is connected to the auxiliary winding and DEM Pin, the

continuously by 6 switching cycles, the SOVP or SUVP protection will be triggered, the AP3302 will shut down and the system will restart when the

VCC voltage falls below the UVLO voltage.

ignored.