6-4. G Board
Power supply of this monitor consists of the following three switching regulators.
1. The power factor improvement regulator that is used to comply with the power supply high frequency
2. The standby regulator that supplies the power to the control system (such as CPU) and supplies the
heater power to CRT.
3. The main regulator that supplies the powers, which are required by the signal system, by, the process-
ing circuit of deflection and high voltage system, and by the output circuit.
1. The power factor improvement block
The power factor improvement circuit of this monitor uses the current-threshold type boost-chopper
system. Therefore, the output voltage Vpfc is always higher than the peak value of the input power
voltage. Vpfc of this monitor is set for about 370 V.
The power factor improvement circuit consists of IC1, T3, C16, C17 and the related components.
IC1 is a module IC in which the control IC, the switching FET, the boost diode and input/output voltage
detectors are built in.
Basic operation of the power factor improvement block is as follows. When the POWER signal goes
Low and the Vcc power supply is supplied to IC1, the FET is turned on and an electric current starts to
flow in the primary winding of T3 and the FET. This current increases with the slope of Vin (rms)/L
where L is the primary side inductance of T3. This FET current is monitored by the source current
detection resistor that is connected between pin-4 and pin-7 of IC1. When this FET current reaches the
set value that is specified by the multiplier inside the control IC, the FET is turned off. Then an electric
current flow through the boost diode where the current decreases with the slope of -(Vpfc - Vin (rms))/L.
When this current reaches 0, the FET is turned on. The current-threshold operation is realized by the
above described circuit operations.
In other words, the circuit operations that are described as one operation cycle, are performed all the time.
When the circuit operations as described above, are observed only during the half-wave period of com-
mercial power line frequency, the ON/OFF timing of the FET is controlled by the control IC so that the
envelope of the peak values of the choke current is proportional to the half-wave of the sine waveform of
the power line frequency. As the result of this control, waveform of the input voltage and that of the
output voltage become similar so that the power factor is improved. This circuit does not operate during
the standby mode.
2. Standby regulator
The standby regulator consists mainly of IC101, IC102, PH101, T101 and the secondary side rectifier
circuit of T101. IC101 has the built-in switching FET, the PWM controller and protection circuit. The
control terminal of IC101 receives the control signal from IC104 that performs the constant voltage
control over the STBY5V line through the isolator P101. The internal FET is PWM-switched by the
control signal so that the STBY5V in the secondary side of T101 is stabilized. At the same time, the
floating 7.5 V and -7 V are generated as the other supply voltages.
The floating 7.5 V is regulated to 6.3 V by IC102 to be used for the heater power and is supplied to the C
board. The heater power is switched ON/OFF by IC102 that is controlled by the HV CTRL signal of the
E board. -7 V power is stabilized to -5 V by IC103 and is supplied to the comparator IC203.