Service manual
CP-850FX Service Manual
Europe R&D
70
5.10.3 FB Terminal (Pin. 6)
The operation of FB terminal is divided into normal (constant voltage control circuit operation)
and stand-by operation control. Refer to item No. 8.6 for the controlling at stand-by operation.
5.10.3.1 Constant Voltage Control Circuit
The STR-W6700 series adopts the current mode controlling circuit for the constant voltage
control, which proves its superiority in a heavy load. The MOSFET drain current peak value ( ≒
ON time) is varied comparing FB terminal voltage and HIC’s internal VOCPM. During the OFF-
time, Quasi-Resonant operation synchronized to the reset signal from a transformer is applied.
While, where no reset signal is supplied from the transformer, the fixed oscillation frequency
(approx. 22kHz) is applied by the HIC’s internal oscillation circuit. The timing chart is shown in
Fig.12, and the internal circuit diagram at the constant voltage control is shown in Fig. 13.
respectively.
V
OCPM
I
DS
V
OCPBD(LIM)
GND
GND
V
FB
D
ROCP
+
FB
－
＋
－
＋
＋
－
＋
－
OCP
VOCPM
VFB
V
CC
OSC
D
S/GND OCP/BD
FB
I
DS
START 18V
BURST 11V
BURST
－
＋
－
＋
SW1
過負荷 通常負荷
極軽負荷
図12 定電圧制御(擬似共振信号は無視) 図13 定電圧制御回路（原理図）
The constant voltage controlling circuit makes the control signal (FB current) flowing from the
secondary side error-amplifier input to No.6 terminal by the photo-coupler. The input FB current
is transformed into Feedback voltage VFB by the HIC internal resistor (SW1 is ON at normal).
While, the reversed voltage waveform (VOCPM) of the drain current waveform is input to the
input terminal of the FB comparator. It is the current mode controlling circuit that controls the
peak value of the drain current by the FB comparator.
The FB current shall be decreased to nil value at the overload in Fig.12. At that time, the drain
current is controlled under the current value regulated by the Overcurrent Protection Circuit. At
the transition period from the normal load to the lowest load in Fig.12, the drain current is
decreased since the FB current increases and VFB rises. Where the VFB exceeds the FB
terminal threshold voltage (VFB(OFF), 1.5V TYP) such as at the lowest load, the thinned-out
oscillating operation starts and the HIC controls the secondary side output voltage so as not to
raise the secondary side output voltage.
Fig.12. Constant Control Voltage
Fig.13. Constant Voltage Control Circuit
(Theory)
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