Sony TRINITRON BVM-14G1A Operation And Maintenance Manual page 94

3. HIGH-B Regulator
The HIGH-B power supply is composed of IC301, IC302,
PH301, PH801, Q305, T301, and T301 secondary rectifica-
tion circuits.
Like the LOW-B power supply, IC301 which is a PWM con-
trol IC is added with the control voltage by IC302 perform-
ing constant voltage control of the +120V line via the isolater
PH301. Q305 FET which is a converter out is switched by
the PWM controlled pulse by IC301 via the Q303 and Q304
buffers. As a result, +120V used for the voltage for the CRT
heater and voltage used by the deflection/high voltage out-
put blocks are generated.
4. Protection Circuits
4-1. PFC Over-voltage Protection (OVP) Circuit
This circuit is composed of IC101 (1/2), Q105, Q106, and
the Vpfc voltage detector (R53 to R57). When the Vpfc volt-
age rises abnormally due to malfunctions of the feedback
block for Vpfc voltage control, it stops PFC operations. IC101
(1/2) which is the detector observes the voltage obtained by
resistance-dividing the Vpfc voltage at a certain percentage.
When the Vpfc voltage rises in malfunctions, exceeds the
reference voltage set, it sets the output to HI. As a result, the
latch output composed of Q105 and Q106 is passed through
D102 to set the ENABLE terminal of IC31 to "Low" and
stops the PFC operations.
At the same time, D103 (red LED) is lit to inform of the
error.
4-2. LOW-B Power Supply Protection Circuit
The protection circuit of this power supply is composed of
the power supply circuit for IC201 composed of Q4, Q5, Q6,
IC1, PH901, Q802, Q817, etc. and the following detection
circuit.
The detection circuit detects the over-voltage of the standby
+5V and +15V lines. The OVP circuit of the standby +5V is
composed of the reference voltage IC802, detector IC801
(2/4) and switch Q801 while the +15V line OVP circuit is
composed of the detector IC804. The protection circuit op-
erates when the power supply voltage to IC201 PWM con-
trolled is stopped.
Specifically, when the +15V voltage rises due to an error of
the voltage feedback block, and the value determined by the
percentages of R825 and R826 is exceeded, the output of
IC804 becomes "Low", and Q816 is turned OFF via D810,
to turn OFF the transistor in PH803. As a result, Q4 and Q5
turn ON, Q6 goes OFF so that no power supply voltage is
supplied to IC201 and PWM operations stop.
When the OVP circuit of the standby +5V works, Q802 is
turned OFF and the same operations are performed. At the
same time, D4 (red LED) is lit to in form of the error.
6-8
4-3. HIGH-B Power Supply Protection Circuit
The protection circuit of this power supply is composed of
the PWM control pulse shut-off circuit composed of Q300,
Q301, PH802, Q814, Q815, etc. and the following detection
block.
The detection circuit is composed of the +120V OVP and
OCP (over-current). The OVP circuit is composed of the de-
tection block IC801 (3/4), and latch block Q806 and Q807,
while the OCP circuit is composed of the detector IC801 (4/
4), latch block Q809 and Q810. The reference voltage used
is shared with IC802.
Taking the OCP circuit as an example to describe the circuit
operations, when the +120V current increases due to load
problems, etc., the voltage of the R339 current-voltage con-
version resistor also drops significantly, and the voltage value
observed by IC801 (4/4) also drops.
When the voltage value drops below the value obtained by
resistance-dividing by R819 and R820, the detector IC801
(4/4) output becomes "High", turns ON the Q809 and Q810
latch via D810, and turns ON Q815 to turn OFF the transis-
tor in PH802. As a result, Q301 is turned ON by the current
from R290, Q300 turns ON, and the Vref voltage (5V) (volt-
age of pin !¢) is input to pin 4 of the PWM control IC301
via D300. pin 4 of IC301 performs duty limitation of the
output pulse. When voltages above 3V are supplied, the out-
put pulse is shut-off.
Therefore, the power supply is stopped by shutting-off the
PWM output pulse.
When the OVP circuit operates, the output of IC801 (3/4)
becomes "High" and the same operations are performed
again.