(L-4) Low Voltage Chopper Regulator Circuit - NEC MultiSync 4FGe Service Manual

1-4. Low voltage chopper regulator circuit
The low voltage chopper regulator circuit id PWM controlled and synchronized with the H deflection DC/DC
converter output of the DC/DC converter.
The pulse duty is determined by comparing the saw tooth waveform and the feedback voltage through error amp of
IC5A4 with a comparator of IC5A3. This feedback circuit is automatically controlled and generates constant and
stable output voltage.
If the H OSC operation stops, the output voltage will become low. The output voltage of 6V line is limited by low +B
protector.
This circuit generates 6V power supply from the pulse of T551's primary winding and 5V power supply from the 6V
power supply by using 3 terminal regulator of lC864. By using the pulse induced in the secondary windings, 12V,
- 1 OV, -5V and horizontal raster centering power supply can be obtained.
As for 12V power supply, it is obtained as below.
After adding the pulse induced between pin B-0 of T551 and the generated + 6V, it is rectified by D553 and
C 5 6 1 .
Then the rectified voltage of 13V or a little more voltage is obtained. 12V power supply is obtained from this voltage
by using 12V 4 terminal regulator of lC552.
As for -lOV power supply, it is obtained by rectifying the voltage induced between pin a-0 of T551 by D552 and
C559. As for -5V power supply, it is obtained by using zener diode and as for horizontal raster centering power
supply, it is obtained by rectifying the voltage of pin @ and pin @I with their middle point (pin @) connected to the
+B voltage of horizontal deflection.
The voltage of pin @ is rectified by Q513 and C517. The voltage of pin @ is rectified by D512 and C516.
+24V
H OSC -
(Fig 1-4-1) BLOCK-DIAGRAM of the Low Voltage Chopper Regulator
Q552
T551
0551
IC5A3. R5G7
SAW TOOTH
GENERATOR
L551
1C5A5
47
VR551
+6V ADJ
R5F9
R5GO
+24v