WS3254 Datasheet, PDF (7/10 Pages) Shenzhen Winsemi Microelectronics Co., Ltd – High Precision Primary-Side Off-line PWM Power Switch

WS3254 Product Description
Function Description
WS3254 is a high-integrated offline PWM power switch,
optimized for high performance low power AC/DC charger
and adapter application. WS3254 operates in primary-side
sensing and regulation. Consequently, opto-coupler and
TL431 could be eliminated, thus reduce the cost. High
precision CC/CV control can meet most adapter and charger
application requirements.
Startup Current and Startup Control
Startup current of WS3254 is designed to be extremely low at
5uA, so that VCC could be charged up above UVLO
threshold level and device starts up quickly. A large value
startup resistor can therefore be used to minimize the power
loss, predigest the design of startup circuit and provides
reliable startup in application.
Operating Current
The operating current of WS3254 is low at 2mA. Excellent
efficiency is achieved with low operating current together and
multi-mode control circuit.
CC/CV Operation
WS3254 is designed to produce good CC/CV control
characteristic as shown in the fig.1. In charger applications, a
discharged battery charging starts in the CC portion of the
curve until it is nearly full charged and smoothly switches to
operate in CV portion of the curve. In an AC/DC adapter, the
normal operation occurs only on the CV portion of the curve.
The CC portion provides output current limiting. In CV
operation, the output voltage is regulated through the primary
side control. In CC operation mode, WS3254 will regulate the
output current constant regardless of the output voltage drop.
Principle of Operation
To achieve high precision CC/CV control, system needs to be
designed in DCM mode for flyback system. In DCM mode, the
output voltage can be sensed via the auxiliary winding. During
MOSFET turn-on time, the load current is supplied from the
output filter capacitor Co. The current in the primary winding
ramps up. When MOSFET turns off, the primary current
transfers to the secondary at the amplitude of
I_s
=
N_P
N_s
*I_p
The auxiliary voltage reflects the output voltage, given by
V _AUX
=
N _AUX
N_S
* (V_o
+ ∆V )
Where △V indicates the drop voltage of the output Diode.
Via a resistor divider connected between the auxiliary
winding and INV, the auxiliary voltage is sampled at the end
of the demagnetization and it is hold until the next sampling.
The sampled voltage is compared with Vref (2V) and the
error is amplified. The error amplifier output COMP reflects
the load condition and controls the PWM switching
frequency to regulate the output voltage, thus the constant
output current can be achieved.
When sampled voltage is below Vref and error amplifier
output COMP reaches its maximum, the switching frequency
is controlled by the sampled voltage thus the output voltage
to regulate the output current, thus the constant output
current can be achieved.
Adjustable CC point and Output Power
In WS3254, the CC point and maximum output power can be
externally adjusted by external current sense resistor Rs at
CS pin. The output power is adjusted through CC point
change. The larger Rs, the smaller CC point is, and the
smaller output power becomes, and vice versa.
Vo
Large Rs
Small R s
Io
Fig.2 Adjustable output power by changing Rs
WIN SEM I M ICROELECTRON ICS
WIN SEM I M ICROELECTRON ICS
WIN SEM I M ICROELECTRON ICS
www.winsemi.com Tel : [phone] Fax : [phone]
WIN SEM I M ICROELECTRON ICS
WIN SEM I M ICROELECTRON ICS
7/10