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power IC, the switching FET is turned off. This time, a current decreasing at a gradient of (Vpfc-Vin (rms))/L flows
through the rectifier diodes. When the current value becomes zero, the FET is turned on again. Current critical operation
is realized by these operations. The PFC circuit controls these operations so that the envelope of the peak current flowing
through the PFC choke becomes a current proportional to the sine half-wave waveform of an AC power voltage when
viewed in the half-wave section of a commercial frequency. As a result, the power factor is improved because the
waveform of an input voltage becomes similar to that of an input current.
Main Power
The insulated main power circuit supplies power to the unit.
Main power output
• OUT_24V
This voltage is output from the copper winding of the transformer (T6500).
Power supply method
Separately-excited composite current resonance insulated converter
Components
• Main power IC (IC6500)
• Transformer (T6500)
• Primary-side switching FET (Q6501, Q6502)
• Resonance capacitor (C6514: PP capacitor)
• Current limiting resistors (R6524, R6525: oxide metal film resistors)
• OUT_24V, Secondary-side smoothing diode (D6511, D6512)
• OUT_24V, smoothing capacitor (C6517, C6519: electrolytic capacitor)
Setting output voltages
• OUT_24V: 24.5 V
Overvoltage protection
This circuit detects an overvoltage by using the zener voltage of the zener diode (D6139) connected to the output voltage
(OUT_24V), and outputs a signal to the POWER ON circuit from the protection circuit (latch circuit) consisting of
Q6105 and Q6106 to stop the main power circuit (setting voltage: approx. 33 V).
Overcurrent protection
This circuit detects an overcurrent flowing through the primary-side resonance capacitor by using the current limiting
resistors, and inputs a signal to the OCP pin of the main power IC to stop switching operation. After that, the power IC
is rebooted twice. If the protection circuit is activated in spite of this reboot, the main power circuit stops in the latch
state. (Setting current: a 10 A current flowing through the secondary-side OUT_24V line)
Description of circuit
The main power circuit employs a separately-excited composite current resonance insulated converter. This power circuit
realizes high efficiency and low noise. When the PFC voltage reaches approx. 320 V with the VCC voltage supplied
from the SUB power circuit, switching operation of the two FETs starts.
The main power circuit configures a half bridge with two switching FETs, a resonance capacitor, and a transformer. The
center-tapped secondary winding of the transformer outputs a voltage of OUT_24V. When OUT_24V is output, this
voltage is monitored by the feedback circuit (IC6501, PH6500) and is fed back to the main power IC. Upon receiving
this voltage, the main power IC keeps the duty cycle constant and oscillates the frequency to continue resonance operation
and stabilize the power voltage.
BVM-E171
7-4
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