DAEWOO ELECTRONICS DTE-21U6TH, Service Manual

Page: 41 / 63

Service manual CP-785AF
- 40 -
When the power switch is pushed on, V
IN
increases slowly. During this time, C806 is charged through
R802.
As soon as V
IN
reaches 16V, the STR-F6654 control circuit starts operating. Then, V
IN
is obtained by
smoothing the winding voltage which appears between pin6 and pin7 of the SMPS transformer.
As this winding voltage does not increase to the set voltage immediately after the control circuit starts
operating, V
IN
starts dropping. However, as this winding voltage reaches the set value before V
IN
voltage drops to the shutdown voltage (at 11V), the control circuit continues operating (see below V
IN
voltage at start-up). R805 resistor prevents that V
IN
pin voltage varies according to the secondary side
output current.
V
IN
must be set higher than the shutdown voltage (V
IN
(off) = 11V
max
) and lower than the O.V.P.
(overvoltage protection) operating voltage
(V
OVP
= 20.5V
min
)
Shutdown voltage 11V
16V (TYP.)
O.V.P.voltage 20.5V
Vin
t
Waveform of Vin pin voltage at start-up