Dti Psu - Hitachi CL32W30TAN Wartungshandbuch

Winding 2/11 produces approximately 16V via D952, which is smoothed by C955 and then applied to Q921 through R974, which is
used as a current sense. The output from Q921 supplies 16V for the audio amplifier I401.
H.T. regulation is controlled by Q954 stage. The base of Q954 is set at a pre-determined level by the resistor network R950, R982
and R953. The emitter of Q954 is held at approximately 6V2 by Z950. Should the H.T. rise, the base voltage becomes more positive
than the emitter, and this difference is amplified by the transistor and applied to optocoupler I901. The output from I901 is then
applied to pin 1 of I900 which regulates the H.T. by altering the duty cycle of the waveform output from pin 6 of I900.
STANDBY SWITCHING
The low voltage supplies are switched off in standby, this is controlled by the micro processor which outputs a high in standby and a
low when the set is out of standby. This is then applied to resistors R923 and R940/R977. This 'high' is also applied to the base of
Q915, Q916, Q910 which in turn terminate operation of the +8, +5 DC-DC converters, and pull the gate of Q921 low so switching
the +16V audio supply to low state. In standby the +B voltage rises slightly to maintain bias winding voltage to pin 7 of I900. To
switch the set out of standby the on/off line is switched high by the micro, and Q916, Q915, Q910 are deactivated, causing the +8,
+5, +16V rails to return to normal operation.
POWER GOOD AND PROTECTION
I903 is made up of 4 comparators, the power good line uses comparator 2. Pin 5 is used as the reference which is held at 2.5V by
I905 supplied by pull up resistor R968. Pin 4 uses R961/R962 and R955/R959 as a potential divider which is connected between the
+10V and T900 winding through D959 which is in forward converter mode. In operation this means pin 4 is held below the reference
level of pin 5 until the mains supply is interrupted or the set is switched off at which time pin 4 rises above pin 5 and the output pin 2
is pulled low. This low is sensed by the micro. In normal operation pin 2 is held high by pull up resistor R990 from the 5V supply.
The protection line (pin 14) is held high under normal running conditions by R971 from the 5V supply, this high is applied to the
protection line to the micro. When the protection line is pulled low the set goes into standby mode, the set can be restarted by the
usual methods of bringing the set out of standby, but until the cause of the protection circuit operation is removed the set will return
to its standby state.
The over current for the 16V audio supply uses comparator 1, a reference voltage is set up on pin 6 by resistors R967, and R964.
The voltage being compared is fed to pin 7 using the potential divider R968 and R966 which is supplied from the output side of the
current sense resistor R974. Should the voltage on pin 7 fall below that of pin 6 pin 1 will be pulled low , thus pulling the protection
line low via D958 putting the set into standby.
Comparator 3 is used to protect against a layer short within the F.B.T. it will also act as protection for a short on the secondary
outputs of the F.B.T. I903 is supplied with approximately 16V via D960 to pin 3, from this supply a reference voltage is fed to pin 9
using potential divider R972/R969, fed via Z948 supply. Pin 8 is supplied by another potential divider this time made up of three
resistors R973/R970 and R760 samples the current flowing through Q752 should this increase the voltage drop across R760 will
increase and raise the voltage of pin 8 when it exceeds pin 9 and pin 14 will be pulled low putting the set into standby.
Comparator 4 is used for EHT/over voltage protection, as all the F.B.T. secondary voltages are proportional, the 200V supply to the
CRT base is used to generate the voltage to be used in comparison, this is done by using a potential divider made up of R718,
R749 and R719. Z708 monitors the voltage at the junction of R718 and R749, if this exceeds 36V the Zener diode conducts
applying a high on pin 10 of I903 this is compared with the reference voltage on pin 11 which also uses the 2.5V set up by I905.
When pin 10 is higher than 2.5V, pin 13 is pulled low in turn pulling the protection line low via D957 putting the set into standby.
The L.T. lines are given protection using diodes D931, D932 and D930 these are connected in reverse bias from the prot sense line
to the 8V and 5V. The +5V standby link is protected by D986 pulling the reference Z950 low, if a short is seen on the output of I952.
The +16V phono out has a diode back to pin 5 I903 in case of short.
If the +B or audio supply lines become short circuit to ground before the protection on the secondary of the power supply, the
primary over current protection (pin 3 I900) will operate turning off the drive output from pin 6 I900. A latch circuit
Q955/R998/RP01/C964, will operate to turn the power supply into standby ( under fault condition) if the software fauls to act on pin
14 I903 low. Delay before latch set by C964/R994. The +B is given protection from overvoltage, via Z907, Z907 goes short circuit if
the +B voltage rises above 180V DC.

DTI PSU

CIRCUIT DESCRIPTION
The mains is switched on via the { TV on }. At this point the A.C mains is rectified by D8, D10, 11, 12 which produces approximately
340v D.C across C58. This is then applied to the drain of Q4 via the primary winding of T1. Initially the voltage on pin 7 of IC10 will
rise to approximately 16v via R15, D24, and C54 this allows the internal circuitry of IC10 to generate a sawtooth waveform at pin 4
from which a square wave output is obtained at pin 6. This output is applied to the gate of Q4 turning the mosfet on and off,
generating the outputs in the secondary windings of T1.
After initial power up pin 7 of IC10 is supplied from the bias winding pin 4 of T1 via D31 for continued operation hence R15 unable to
provide running current demand for IC10 [20mA].
A current sense circuit consisting of R74, R75 & R79 feeds back a voltage to pin 3 of IC10, this voltage is compared with an internal
reference voltage, of approximately 0.8V should the applied exceed the reference, the pulse width is limited from output pin 6 [chip
in over current mode]. In this way Q4 is offered protection from changes in primary current. Secondary overvoltage protection is
offered also by R77, should the feedback be disabled then the secondary voltage will be limited to safe levels, the output from pin 6
will be limited by the internal error amplifier and internal reference.
R21 and C31 act as a soft start circuit, this limits the pulse width output from pin 6 during the initial start up period, allowing a
gradual rise to full power. ZD3 & 4 will go short if Q4 should go short, protecting IC10.
The secondary voltage induced in T1 winding 11/14 is rectified by D15 producing 38V which is smoothed by C40. This voltage is
then stepped down to provide 30V at 20mA via R96 and clamped by ZD21.
Winding 10/14 produces 5V at 1.5A when rectified by D17 and smoothed by C37 & 38. This voltage rail being the highest current rail
is used for regulation via IC8 providing feedback via IC4 to pin 1 of IC10. From the 5V rail a 3V3 rail is extracted at 2.7A. The current
output is controlled by a pwm circuit consisting of switching mosfet Q9, gate drive is provided Q10, and pin 1 of IC6, pin 2 of IC6 is
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