Hitachi 46W500 Training page 21

DP-3X SW +115V POWER SUPPLY REGULATION EXPLANATION
Hi-Voltage Power Supply Circuit Diagram explanation:
(See Power Supply SW+115V Regulation Circuit Diagram for details)
THIS POWER SUPPLY RUNS ONLY WHEN THE SET IS TURNED ON:
TURNING ON THE SW +115V POWER SUPPLY:
When the Set is turned on, the Microprocessor I001 Outputs a Power On/Off 1 high command via pin 59. This
Power On command is routed through Q027 and Q028 to the PPS4 connector pin 7. This High will be passed to
the base of Q902 provided the Shut Down sensor Q904 isn't activated. When the base of Q902 goes high, it's
collector will go low. This will supply a ground path for the power on Relay S901 turning it on. When the relay is
energized, AC is supplied to the Bridge rectifier D901.
D901 develops raw 150V which is routed through F903 to Pins 1 and 2 of T901. This voltage is routed through
the primary coil inside T901 and out pins 5 and 6 to pin 3 of I901 which is the Drain of the internal Switch MOS
FET. The Ground return path for the primary voltage is out pin 2 of I901 which is the Source of the internal
Switch MOS FET and then through four 0.22 ohm resistors R908, R909, R910 and R911.
See SW+115V Regulation Circuit Diagram for details.
SW +115 REGULATION
SW +115V pulse is generated from pin 11 of T901. This pulse is rectified by D925, filtered by C943 and then
routed through the Excessive Current sensing circuit R944 and Q905.
The primary route for the SW +115V is through E907, L923 to pin 9 and 10 of PPD5 and output as SW +115V
to the Deflection circuit and High Voltage generation circuit.
However, the regulation route is through E906 to pin 1 of I904. Internally, the regulator transistor works as a
variable resistor whose resistance is dependant upon the SW +115V voltage fluctuations. The internal variable
resistor manipulates the current flow from pin 2 to pin 3 ground. This will cause the voltage at pin 2 of I903 to be
manipulated. Internally, the LED is illuminated by degrees dependant upon the SW +115V voltage fluctuations.
The internal receiver receives this light and acts as a variable resistor from pin 4 to pin 3 which is the regulation
control signal.
This action causes pin 1 of I901 to manipulate the internal oscillator within I901. This in turn causes the fre-
quency of the drive pulse delivered to the Gate of the internal SMOSFET (Switch Metal Oxide Semiconductor
Field Effect Transistor) to manipulate the frequency of the pulse generated on the primary of T901. The current
drain of the internal SMOSFET is monitored by four low ohm resistors mentioned above. If this current exceeds
a specific value, the voltage developed by these low ohm resistors is routed through R912 back into pin 1 which
is the Over Current Protection circuit as well as the Regulation Control pin. This pin will inhibit the drive signal
to the gate of the SMOSFET. As soon as the excessive current situation is eliminated, the IC will recover and
continue functioning.
B+ GENERATION FOR THE POWER SUPPLY DRIVER IC:
Vcc for the Driver IC is first generated by the AC input. This voltage is called Start Up Voltage. I901 requires
23.3V DC to operate normal. However, it will begin operation at 12V DC on pin 4 of I901.
When AC is applied to 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 pin 1 and 2.
However, one leg of the AC is routed to a half wave filter consisting of R906 and R907 (both a 3.9K ohm resis-
tor), 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 12Vdc, the internal Regulator of I901 is turned On and begins operation.
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 1 (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 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 clamped by D911 and now becomes run voltage (23.3V) for I901 pin 4.
The RED LED D912 can be used to determine if the B+ to pin 4 of I901 is present.
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