5-5. Power Supply Circuit
Power supply block of this circuit board consists of the two switching regulators as shown below.
1. Power factor improvement regulator to conform to the power supply higher harmonics regulation.
2. Main regulator that supplies the necessary voltages to the signal system circuit, deflection system/
high tension system circuits.
1. Power factor improvement block
The power factor improvement block of this power supply block uses the active filter IC module (IC601)
of the current-critical type boost-chopper system in order to conform to the power supply higher harmon-
The power factor improvement block consists of IC601, L602, C608, C617 and the related parts.
IC601 is a module IC that contains the built-in control IC, switching FET, boost diode and input/output
voltage detector inside the module.
Basic operation of the power factor improvement circuit is described below. When the Vcc power is
supplied to this circuit, FET turns on to flow a current in the primary winding of L602 and in FET.
This current increases with the slope of "Vin (rms)/L" where L is inductance of primary winding of L602.
This FET current is monitored by the source-current detecting resistor connected between pin-4 and pin-7
of IC601. When the detection voltage reaches the target value that is set by the multiplier inside the
control IC, the FET is turned off. Then a current that decreases with the slope of "Vd- (rms)/L", flows in
the boost diode. When this current becomes zero, the FET is turned on. The current-critical operation is
realized by the above described circuit operation. (where Vd : voltage across C617. Vin : input power
The complete single cycle of circuit operation as described above is repeated all the time. When observ-
ing the half-cycle period of the commercial power frequency, the FET is turned on and off in the way that
the envelope of the choke current peak value is proportional to the sine waveform of input power voltage.
As the result of the above control, waveform of the input voltage becomes similar to the waveform of
input current. In the other words, the power factor is improved. At the same time, the voltage Vd across
C617 becomes higher than the peak value of the input power voltage, and is maintained to approximately
380 V regardless of the input power voltage.
2. Main regulator
This circuit is the separately excited PWM type fly back converter.
The main power supply circuit consists of the PWM drive IC602, main switching FET Q606, converter
transformer T602, and the rectifier diode and smoothing capacitor that are connected to the secondary
winding of T602.
This circuit operates as follows : The drive pulse that is output from IC602 is buffered by Q617/Q618 and
is sent to Q606 gate that is running in the switching operation. In order to control this switching opera-
tion, the secondary circuit output +135 V is controlled by the constant voltage control with IC603. The
output signal from IC603 passes through the optical isolator PH601 and is returned to IC602 so that ON
DUTY ratio of the drive pulse is changed until the secondary circuit output is controlled to provide the
constant voltage output.
IC602 performs the constant voltage control of the +135 V power line. IC602 receives the control signal
from IC603 through an isolator PH601. This control signal changes the oscillating frequency of IC602 so
that the +135 V power voltage is controlled to a constant voltage.
The secondary winding of T602 generates various output voltages starting from +200V, +15V, _15V,
+6V, _6V and heater voltage.
The output voltages of +15V, _15V, +6V and _6V are regulated to +12V, _12V, +5V and _5V
respectively by the three-terminal regulators.