Zenith D60WLCD Series Service Manual page 32

restarted after cutting AC input.
OVERHEAT PROTECTION
This circuit activates the latch circuit when the frame
temperature of the Hybrid IC rises above 140°C.
The actual temperature is detected by a control circuit
element. This is to protect the MOSFET from overheating.
OVER-VOLTAGE PROTECTION
This circuit activates the latch circuit when the Vin
terminal voltage exceeds 22.5V (typ). This is used to
protect the Vin terminal in the control circuit.
The Vin terminal is supplied by a secondary of the
transformer so it is proportional to the output voltage
of the transformer. Thus it is able to detect and protect
the circuit against an over-voltage condition.
OPERATION STOP CIRCUIT (UVLO)
This begins operating if voltage is supplied to the low
subsidiary current terminal (Vcc), Pin 5. If the supplied
voltage in Vcc reaches 17.5V typ., current is supplied
to circuits within the IC and then the operation stop
circuit (UVLO) begins operation control and high
impedence is sustained until the supplied voltage
reaches the same voltage. At this time the supplied
current is circuit current (Icc), in not operating, and
is 70µa (Max).
If the operation stop circuit begins operation control,
circuit current (Icc) is in operation and about 15mA
flows in the same terminal. In controlling operation, if
the supplied voltage in this terminal goes below
operation stop current voltage (Vcc), 11.5V (typ), the
IC stops functioning. Like this, this terminal functions
to detect supplied voltage and to control operation
STR G6153T Pin Descriptions
# Symbol Name
1 D Drain
terminal
2 OCP Over-current
terminal
3 GND Ground
terminal
4 FB Feed-Back
terminal
5 Vcc Power
terminal
Symbol Function
OVP Protect circuit against over-voltage
OLP Protect circuit against overload
TSD Protect circuit against overheat
SS Soft start function
D60WLCD - 923-03486
CIRCUIT DESCRIPTIONS
Function
MOSFET drain
Input signal of detecting
over-current in primary
part
Ground
Input signal of controlling
constant voltage
Input control circuit power
start in low voltage input.
INTERNAL OSCILLATOR (OSC)
The OSC uses a capacitor (Ct) to determine oscillation
frequency. When the IC begins operating, Ct is charged
internally by constant current I1. This causes both SW1
and SW2 to become open. When the electric potential
of the charge reaches about 4V, the comparator
reverses, and SW1 and SW2 are on (closed)
simultaneously.
If I1 is lower than I2, SW1 becomes on, Ct discharges
and Vt decreases. Maximum Duty cycle of the oscillator
is set to 80% (typ) and the frequency is set to 100KHz
(typ). The globular shaped wave output is used as a
timing clock in the internal circuit and the triangle
shaped wave output is used as a standard signal for
transforming pulse width.
CONTROL MODE (PWM LATCH / FEED BACK)
The control mode of the IC uses MIX Mode which is
switched to the voltage control mode in light load
and and current control mode in heavy load, linearly,
according to load. The MIX control mode operates PWM
control, comparing a standard triangle shaped wave
with the current waveform imposed on DC FB voltage.
Input voltage difference of PWM Comparator in
waveform cross becomes larger. As a result, a stable
PWM signal is gained. In MIX control mode, enough
noise margin can be obtained with changing the
voltage PWM of standard triangle shaped wave.
Also,even in continuation inductor mode in which On
Duty is over 50%, Sub-harmonic phenomenon is
suppressed and every essential part of internal oscillator
becomes optimized.
CIRCUIT PROTECTION
1) PROTECT CIRCUIT AGAINST OVERLOAD (OLP)
This IC has a protection circuit against over-current
which checks over-current on every pulse. In over-
current, the FB system cuts the Photo Coupler for FB
off because the output voltage doesn't reach the set
voltage. If an Over-current signal and an FB Current
Mirror signal are detected, an internal timer starts
655ms later.
When the Count Out signal is output, the main switch
is set to off and the latch circuit is activated.
When the latch circuit is activated, it needs 400µA
(Max) for maintenance. This current is influenced by
the electric potential of the Vcc terminal.
If the supplied voltage in the Vcc terminal goes below
power voltage for releasing latch circuit, 8.7V (typ),
latch operation can be released. When the latch
operation is released, this IC begins start operation
from Soft Start.
2) PROTECT CIRCUIT AGAINST OVER-VOLTAGE (OVP)
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094A - SERVICING