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4. Power supply
MG5.1E
4.2 Full power factor correction circuit
Personal notes
An explanation of the full Power Factor Correction circuit is now
given. (Figure 4-6) The Power Factor Correction circuit is a
fixed-frequency Pulse-Width modulated boost regulator power
supply. Operating frequency is approximately 60 kilohertz. Due
to the low power consumption of the set during the standby
mode, the Power Factor Correction circuit is turned Off. The
operating voltage, PFC, on Pin 19 of 7102 is turned Off. When
the set is turned On, the operating voltage is switched to Pin 19
of 7102. Once the circuit is in operation, a 5.11 volt internal
reference voltage on Pin 11 is applied to Pin 6 of the IC. Output
of the bridge rectifier is fed to the IC on Pins 4 and 7. Feedback
from the output circuit is fed to Pin 14 where it is multiplied with
the Bridge output. Inputs to the Gilberts Multiplier produce an
error signal that is fed to the current amplifier. The Gilberts
Multiplier multiplies the bridge output voltage with the output
voltage on D438. The 5.11 volt reference is used to provide a
clamping reference for the other inputs. Input current is
sampled by resistors 3130, 3131, 3132, and 3133. This sample
is added to the error signal from the Multiplier going into Pin 8.
The error signal is amplified and then compared to the oscillator
ramp to determine the reset point for the latch. When the
oscillator goes low, the latch is set making output "Q" High. With
the other inputs to the AND gate High, Pin 20 then goes High,
turning 7104 On. By comparing the input and output voltage,
the On time of 7104 is increased when the AC voltage is at its
low point to maintain the 350 volt DC output. When the AC input
voltage is at its peak, 7104 is On for a shorter period.
The IPK circuit connected to Pin 2 is an overcurrent protection
circuit which resets the latch if there is excessive current
through the return resistors. This will reduce the On time of
7104. In a like manner, the OVP, Over Voltage Protection,
circuit will reset the latch removing drive to Pin 20 is the output
voltage exceeds 392 volts.
To check the operation of this circuit when the set is operating
correctly, check the PFCOUT voltage. This voltage should be
approximately 350 volts. If the output voltage is approximately
329 volts, this circuit is not working. Check the operating
voltage on Pin 19 of 7102. Then check the output drive on Pin
20 and at the gate of 7104. There are three grounds on the Hot
side of the supply. Ground HA is connected directly to the
bridge 6000. Ground "B" is the ground for the Power Factor
Correction circuit. If the current sensing resistors 3130 through
3133 should open, this would remove the ground for all the
switching supplies and make the set inoperative. Ground "C" is
the ground for the switching supplies. When ground "B" passes
through the choke 5106, it becomes ground "C".
Startup up voltage for the Full Power supply is taken from the
neutral side of the AC mains.
