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3DQDVRQLF
3.8. Protection Circuitry
3.8.1. Over-voltage Protection (OVP)
Over-voltage Protection is used to protect IC801 if V
IN
pin 4 terminal rises to approximately 22V. Although it
basically functions as protection for pin 4 against
overvoltage, it is also used to protect against
overvoltage of the secondary output (in the event of
failure of the regulation, for example). This is because
pin 4 is supplied by winding B1-B2 of transformer
T802, this voltage being proportional to the output
voltage of the secondary side.
basically functions as protection for pin 4 against
overvoltage, it is also used to protect against
overvoltage of the secondary output (in the event of
failure of the regulation, for example). This is because
pin 4 is supplied by winding B1-B2 of transformer
T802, this voltage being proportional to the output
voltage of the secondary side.
3.8.2. Over-current Protection (OCP)
Overcurrent Protection is performed pulse-by-pulse
by directly detecting the drain current of the internal
power MOSFET. Since the detection voltage is
monitored by an internal comparator of IC801,
constant temperature stabilisation is also achieved.
by directly detecting the drain current of the internal
power MOSFET. Since the detection voltage is
monitored by an internal comparator of IC801,
constant temperature stabilisation is also achieved.
The
Drain-Source
current
through
the power
switching MOSFET is passed via the resistor R809,
which develops a voltage across it. The input voltage
to IC801 pin 1 (OCP/FB) is passed to an internal
comparator. When this input voltage exceeds a
pre-determined value, the drive output is pulled LOW,
resulting in the power MOSFET switching OFF.
which develops a voltage across it. The input voltage
to IC801 pin 1 (OCP/FB) is passed to an internal
comparator. When this input voltage exceeds a
pre-determined value, the drive output is pulled LOW,
resulting in the power MOSFET switching OFF.
3.8.3. Latch
The latch circuit is used to pull the output of the
oscillator LOW (switching MOSFET OFF) when the
over-voltage protection or thermal shutdown circuits
are activated.
oscillator LOW (switching MOSFET OFF) when the
over-voltage protection or thermal shutdown circuits
are activated.
16V
10V
t
In this condition the V
IN
terminal pin 4 decreases until
the shutdown voltage of 10V is reached. At this point
pin 4 begins to rise again but when it reaches the start
up level of 16V, the latch circuit continues to stop the
drive.
When the latch is on, V
pin 4 begins to rise again but when it reaches the start
up level of 16V, the latch circuit continues to stop the
drive.
When the latch is on, V
IN
voltage at pin 4 increases
and decreases within the range 10V to 16V, as shown
in the above diagram, and is prevented from rising
normally.
in the above diagram, and is prevented from rising
normally.
Cancellation of the latch circuit operation is achieved
by restarting the AC input to the circuit after switching
off the TV.
by restarting the AC input to the circuit after switching
off the TV.
3.8.4. Thermal Shutdown
This circuit triggers the latch when the body
temperature
temperature
of
the
IC
exceeds
140
E
C.
The
temperature is sensed by the control IC, but also
works against overheating of the MOSFET, as both
are mounted on the same lead frame.
works against overheating of the MOSFET, as both
are mounted on the same lead frame.
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