WS2268 datasheet(6/8 Pages) WINSEMI

6 / 8 page

6/8

WS2268

Steady, keep for your advance

Operration Description

1. Startup Mode

At the phase of power-on, the rectified high voltage DC

charges the capacitor of VP through the start-up resistor. When

VP rises to 20V, the IC enters normal operation, then switching

begins and the output voltage begins to rise.

The VP bypass

capacitor must supply the IC until the output voltage builds up

enough to provide power from the auxiliary winding to sustain

the VP. The under-voltage threshold of VP is 9V, and therefore,

the voltage on the VP capacitor must not drop more than 11V

while the output is charging up.

2. Constant Primary Peak Current

The primary current ip(t) is sensed by a current sense

resistor Rcs, the current rises up linearly at a rate of:

dip

)

(

)

(

（

1）

Primary current waveform

As illustrated in top figure，when the current ip(t) rises up to

Ipk, the switch turns off. The constant peak current is given by:

Rcs

Vcs

Ipk

（

2）

.The energy stored in the magnetizing inductance Lm each

cycle is :

Ipk

（

3）

So the power transferring from the input to the output is given

by:

Ipk

（

4）

Where

is the switching frequency. When the peak

current Ipk is constant, the output power depends on the

switching frequency

f .

3. Constant Voltage Operation

The WS2268 captures the auxiliary winding feedback voltage at

U pin and operates in constant-voltage (CV) mode to regulate

The output voltage. Assuming the secondary winding is

master, the auxiliary winding is slave during the D1 on-time

The auxiliary voltage is given by:

Naux

Vaux =

So,

)

(

Naux

Vaux

⋅

)

(

Vaux

Where Vd is the diode forward drop voltage.

Auxiliary voltage waveform

The output voltage is different from the secondary

voltage in a diode forward drop voltage that depends on

the current. If the secondary voltage is always detected at

a fixed secondary current, the difference between the

output voltage and the secondary voltage will be a fixed

Vd. The voltage detection point is at two-thirds of the D1

on-time. The CV loop control function of WS2268 then

generates a D1 off-time to regulate the output voltage.

4、Constant Current Operation

Secondary current waveform

In CC operation, the CC loop control function of

WS2268 will keep a fixed proportion between D1 on-time

Tons and D1 off-time Toffs by discharging or charging the

capacitance. The fixed proportion is:

WS2268 Datasheet(PDF) 6 Page - Shenzhen Winsemi Microelectronics Co., Ltd

WS2268 Datasheet(HTML) 6 Page - Shenzhen Winsemi Microelectronics Co., Ltd

Html Pages

1 2 3 4 5 6 7 8

Datasheet Download

Link URL