Using TOP269EG off-line
switcher IC, (U1), in a flyback configuration can be designed a very
simple high efficiency notebok laptop power adapter.TOP269EG IC has an
integrated 725 V MOSFET and a multi-mode controller. It regulates the output by adjusting the MOSFET duty cycle, based on the current fed into its CONTROL
pin.This laptop power adapter circuit will provide a fixed 19 volts
output voltage at a maximum current of 3.5A. input voltage range is
between 90 to 265VAC.
Common-mode inductors L3 and L4 provide filtering on the AC input. X
class capacitor C1 provides differential filtering, and resistors R1
and R2 provide safety from shock if the AC is removed, by ensuring a
path for C1 to discharge. This is required by safety agencies when the
capacitor value exceeds 100 nF. Bridge rectifier D1 rectifies the AC
input, and bulk capacitor C2 filters the DC.
Y capacitor C11, connected between the primary and secondary side
provides common mode filtering.Capacitor C7 provides the auto-restart
timing for U1. At startup this capacitor is charged through the DRAIN
(D) pin. Once it is charged U1 begins to switch. Capacitor C7 stores
enough energy to ensure the power supply starts up. After start-up the
bias winding powers the controller via the CONTROL pin. Bypass capacitor C6 is placed as physically close as possible to U1.
U1 utilizes an output overvoltage shutdown function. An increase in
output voltage causes an increase in the bias winding on the primary
side, sensed by VR1. A sufficient rise in the bias voltage causes VR1 to
conduct and bias Q1 to inject current into the Voltage Monitor (V) pin
of U1. When the current exceeds 336 uA, U1 enters the overvoltage
shutdown mode and latches off.