Zenith D60WLCD Series, Service Manual, Page: 31 / 88

CIRCUIT DESCRIPTIONS
D60WLCD - 923-03486
3-7
094A - SERVICING
CIRCUIT DESCRIPTIONS
POWER BLOCK
This circuit uses the FLY BACK CONVERTER for insulation.
It is composed of HYBRID IC STR F6668B (which includes
a POWER MOSFET) for switching power and STR G6153 as
the stand-by circuit. It is operated by an ERROR AMP
(IC802, SE105) which transfers a signal from the
transformer secondary back to the primary.
BLOCK DIAGRAM (STR F6668B)
# Symbol Name Function
1 O.C.P
/F.B
Over-current
/Feed-Back
Terminal Input signal of detecting over-
current & to input signal for
controlling constant voltage.
2 S Source
Terminal MOSFET source
3 D Drain
Terminal MOSFET drain
4 Vin Voltage Input
Terminal Input control circuit power
5 GND Ground
Terminal Ground
Internal Protect circuit against over-voltage
Internal Protect circuit against overheat
STR F6668B Pin Descriptions
Name
O.V.P.
T.S.D.
Function
FUNCTION & DESCRIPTION
VIN TERMINAL (PIN 4), START CIRCUIT
The Start circuit controls the operation start and stop
of the control IC. In power start, C825 is charged
through start resistor, R826 and R827. When the Vin
terminal voltage reaches 16V (typ), the control IC starts
operating. If this voltage decreases and reaches lower
than 10V (typ), the control IC stops and returns initial
states by the operation of UVLO (operation stop
circuit).
Once started, the control circuit receives rectified power
from the transformer output (Vovp = 22.5 typ).
Because the startup voltage begins to decay before
the transformer can reach its set value, operation stop
voltage is set at 11V (Max). This allows the control IC
to keep operating until the transformer voltage can
maintain it.
OCP / FB TERMINAL (PIN 1)
The Oscillator and Constant voltage control circuit uses
the charge and discharge of internal condensor C1 and
selects the PRC (Pulse Ratio Control) which fixes the
off time of the MOSFET and varies the on time.
The Over Current Protection (OCP) circuit detects the
peak value of MOSFET drain current on every pulse.
This is done by connecting R821 with the MOSFET
source terminal (Pin 2) and GND terminal (Pin 5) and
by inputting regulated voltage to the OCP/FB terminal.
The threshold voltage (Vth, 1) of the OCP terminal is
about 0.73V compared with GND. External components
R822 and R821 are a filter circuit to protect against a
surge current generated in MOSFET
on .
In light load, if the output voltage decreases, the
transformer voltage in the primary also decreases
proportionately. When the Vin terminal voltage in the
primary drops lower than the operation stop voltage,
the OCP circuit stops operating.
In this situation, the circuit current decreases so that
the Vin terminal voltage increases in proportion to
the charge current by start resistors R826 and R827
and intermittent operation occurs.
LATCH CIRCUIT
The latch circuit is to prevent power circuit operation
by keeping the output of the oscillator low in two
circuits (OVP & TSD) which protect against Over-voltage
and overheating.
This circuit starts to function when OVP and TSD
operate for more than 8 seconds. In this case, delay
time is set by a timer circuit in the control IC.
When the Latch circuit operates, the constant voltage
power (Reg) circuit in the control IC is also operating
and the circuit current is high. The Vin terminal voltage
decreases rapidly. If the Vin terminal voltage becomes
lower than the operation stop voltage (10V typ), circuit
current drops below 400
µ
A and the Vin terminal voltage
begins to increase. If the Vin terminal voltage reaches
operation start voltage (16V typ), circuit current begins
to increase and the Vin terminal voltage begins to
decrease. In other words, the Vin terminal voltage rises
and falls between 10V (typ) and 16V (typ). Doing this,
the latch circuit protects the Vin terminal voltage from
increasing abnormally.
Releasing the latch circuit is done by dropping the
Vin terminal voltage below 6.5V. The latch circuit is