Hitachi 46W500 Training page 14

DP-3X POWER SUPPLY SHUT DOWN EXPLANATION
VOLTAGE LOSS or SHORT DETECTION
(See Figure 4)
One circuit used is the Voltage Loss Detection cir-
cuit. This is a very simple circuit that detects a loss
of a particular power supply and supplies a Pull-
Down path for the base of a PNP transistor.
This circuit consist of a diode connected by its
cathode to a positive B+ power supply. Under nor-
mal conditions, the diode is reversed biases, which
keeps the base of Q1 pulled up, forcing it OFF.
However, if there is a short or excessive load on the
B+ line that's being monitored, the diode in effect
will have a LOW on its cathode, turning it ON. This
will allow a current path for the base bias of Q1,
which will turn it ON and generates a Shutdown
Signal.
B+ GENERATION FOR THE MAIN POWER SUPPLY DRIVER IC I901:
Vcc for the Driver IC is first generated by the AC input. This voltage is called Start Up Voltage. I901 normal pin 4 voltage is
23.3V. However, it will begin operation at 12V on pin (4) of I901. When the set is turned on by S901, AC is applied. AC is
routed through the main fuse F901 (a 10 Amp fuse), then through the Line filter L901 to prevent any internal high frequency
radiation for radiating back into the AC power line. After passing L901 filter it passes another filter L903. The HOT leg of
AC is routed through the relay S901 and AC arrives at the main full wave bridge rectifier D901 where it is converted to Raw
150V DC voltage to be supplied to the power supply switching transformer T901 pins (1 and 2).
However, one leg of the AC is routed to R906 and R907 (both a 3.9K ohm resistor), filtered by C911, clamped by a 36V
Zener D911 and made available to pin (4) of I901 as start up voltage. When this voltage reaches 13Vdc, the internal Regula-
tor of I901 is turned On and it begins to operate.
When the power supply begins to operate by turning on and off the internal Switch MOS FET, the Raw 150V DC routed
through T901, in on pin 3 (Drain) and out on pin 2 which is the Source. The Source of the internal Switch MOS FET is routed
out of pin (2) through four low ohm resistors (R908, R909, R910 and R911) to hot ground. When the internal Switch MOS
FET turns on, it causes the transformer to saturate building up the magnet field. When the internal Switch MOS FET turns off,
the magnet field collapses and the EMF is coupled over to the secondary windings, as well as the drive windings. The drive
windings at pin (8) produce a run voltage pulse which is rectified by D907, filtered by C911 then routed clamped by D911
and now becomes run voltage (23.3V) for I901 pin 4.
HOT GROUND SIDE SHUT DOWN SENSING CIRCUITS. (Specific to I901).
In addition, there are 4 Hot Ground side Shutdown inputs that are specifically detected by the main power driver IC I901.
These sensors circuits protect I901 from excessive current, temperature or over voltage.
LATCHED SHUT DOWN MONITORS: (AC must be removed to recover).
(OVP) Pin 4 is monitored for Over Voltage Protection at pin 4 of I901.
1.
2. (TSD) I901 itself is monitored for Excessive Heat. This block is labeled TSD. (Thermal Sensing Device).
3. (Trigger) Over Load Protection monitors the difference between the Hot Ground and Floating Ground.
RECOVERING SHUT DOWN INPUT: (Driver IC will recover on it's own when trouble is removed.)
(OCP) Pin 1 monitors the low ohm resistors, R908, R909, R910 and R911. If these resistors have an excessive current
4.
condition caused by monitoring the current through the internal Switch MOS FET, the voltage will rise and pin 1 has an
internal Over Voltage detection op-amp. If this voltage rises enough to trigger this op-amp, the IC will stop producing a
drive signal.
Voltage
Loss
Detector
Shut-Down Signal
Figure 4
Any Positive
B+ Supply
B+
Q1
(Continued on page 7)
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