Advertisement
1. Power Circuit
1.1 General Description
This power unit uses a switching power supply. The switching frequency is synchronized within the range of
the horizontal frequencies (31kHz to 96kHz).
1.2 EMI Circuit
The EMI circuit comes in a two-stage configuration. The first stage is composed of the choke coil in the
common mode and the capacitor X. The second stage is composed of the four capacitors Y. R101 is a
discharge resistor for the capacitor X. When the power switch is OFF, this resistor is used to discharge the
electric charge accumulated in the capacitor C101.
The EMI circuit is the circuit that is used to prevent the outflow of monitor's switching noise toward the
outside. Its function is effective in minimizing the adverse influence of switching noise to other electronic
devices.
1.3 AC Rectifier and Smoothing Capacitor
The AC input power is rectified by the full bridge type rectifier. This rectifier is composed of the diodes D101
to D104. The AC voltage is converted into a DC voltage by the smoothing capacitor C107.
TH101 is an NTC thermistor intended to reduce the inrush current that is generated when the power switch is
turned ON.
1.4 Degaussing Circuit
The degaussing circuit is composed of the PTC thermistor TH102, the degaussing coil and the relay RL181.
The relay is controlled by the control signal from the MCU.
1.5 Transformer and Energy
1) When a drive pulse is fed from U101 pin 6 (IC KA3842A) to the gate of Q101, a current flows from the
positive side to the negative side of C107 through the primary winding of transformer T101 (Pin 6 and Pin
4) and Q101 D-S. By this operation, energy is stored in T101. When the supply of the driving pulse to the
gate of Q101 is suspended, Q101 is turned OFF and all the diodes connected to the secondary winding is
reversed biased so that the energy stored in T101 is discharged to the secondary side. Q101 is turned ON
and OFF by U101 and this operation is then repeated.
2) The MOS FET Q101 is turned ON and OFF by U101 (KA3842A). KA3842A is a PWM (Pulse Width
Modulation) IC with an operation start voltage of 16V and an operation stop voltage of 10V.
The pin assignment of the KA3842A pulse width modulation IC chip is shown below.
Pin 1: Feedback
Pin 2: Correction
Pin 3: Current sensor
Pin 4: Oscillator
1.6 Overcurrent Protection
If a certain fault occurs and the output power on secondary side becomes too excessive, the peak voltage at
U101 Pin 3 raised. When this voltage reaches 1V, U101 operates so that the ON period of Q101 is shortened.
As a result, the output voltage on secondary side and the voltage in the auxiliary winding (Pin 1 and Pin 2) on
T101's primary side is lowered. When the voltage in the auxiliary winding becomes less than 10V, the
oscillation of the U101 is stopped. However, a charging current flows into C111 through R102, R103, Q102,
and D105. When the voltage at U101 Pin 7 reaches 16V, U101 begins to oscillate again. As a result, U101
repeats the intermittent oscillation (repetition of oscillation and suspension).
Pin 5: Ground (GND)
Pin 6: Pulse output
Pin 7: VCC
Pin 8: VREF (5.1V)
7-3
Advertisement
Chapters
