WS2268
Operration Description
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:
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.
Vaux Vs
=
Naux Ns
Vs = Vo +Vd
Naux
So,
Vaux =
⋅(Vo +Vd)
Ns
R2
Vaux = Vu *(1+
)
R3
Where Vd is the diode forward drop voltage.
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(t) Vg(t)
（1）
=
dt
Lm
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.
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:
Vcs
（2）
Ipk =
Rcs
.The energy stored in the magnetizing inductance Lm each
cycle is :
1
Eg = × Lm× Ipk 2
（3）
2
So the power transferring from the input to the output is given
by:
4、Constant Current Operation
1
P = × Lm× Ipk 2 × fsw
（4）
2
Where
is the switching frequency. When the peak
fsw
current Ipk is constant, the output power depends on the
switching frequency
.
fsw
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:
3. Constant Voltage Operation
The WS2268 captures the auxiliary winding feedback voltage at
U pin and operates in constant-voltage (CV) mode to regulate
6/8
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WINSEMI [ SHENZHEN WINSEMI MICROELECTRONICS CO., LTD ]
