Service manual

CP-850FX Service Manual
Europe R&D
65
5.10.1.2 Auxiliary/Drive Winding
After the control circuit starts its operation, the power
supply is gained by rectifying and smoothing the
voltage of the auxiliary winding D.
Fig.3 shows the start-up voltage waveform of the
Vcc Terminal. The auxiliary winding voltage does
not rise up to the set voltage after the control circuit
starts its operation, and the Vcc terminal voltage
starts falling. However, because the operation stop
voltage is set as low as 10.6V(Max), the auxiliary
winding voltage D reaches stabilizing voltage before
falling to the operation
stop voltage, and the control circuit continues its ope ration.
The auxiliary winding voltage, at the normal power
supply operation, is to be set the number of windings
for both the ends voltage of C3 to be higher than the operation stop voltage [Vcc(OFF)
10.6V(MAX)] and lower than the OVP operation voltage [Vcc(OVP) 25.5V(MIN)].
Besides, in an actual power supply circuit, the Vcc terminal voltage might be varied by the value
of secondary output current as shown in Fig.4. This is caused by the small circuit current of
STR-W6700 itself and C3 is charged up to the peak value by the surge voltage generated
instantly after the MOSFET is turned OFF.
In order to prevent this, it is effective to add a
resistor having several to several tens ohms (R7) in
series to a rectifier diode as shown in Fig.5. The
optimum value of the additional resistor should be
determined in accordance with the specs of a
transformer since the Vcc terminal voltage is varied
by the structure difference of transformers.
Furthermore, the variation ratio of the Vcc terminal
voltage becomes worse due to an inaccurate
coupling between primary and secondary windings
of the transformer (the coupling between the
auxiliary winding D and the stabilizing output
winding for the constant voltage control). Thus, for
designing the transformer, the winding position of
the auxiliary winding D needs to be studied carefully.
補助巻線電圧
制御回路動作開始
時間
Vin(AC)→ON
V
(TYP )
(MAX )
起動不良時
図 ３ 起動時 Ｖ 端子電圧波形例
10 .6V
CC
18 .2V
CC
V
Ｉ
D
D2
V
追加
S/GND
R7
が無 い場合
R7 が有 る場合
R7
C3
図 ５ 出力電流 Iou t の影響 を受 け に く い
補助電源回路
図 ４ 出力電流
Iou t-V 端子電圧
ou t
STR-W6700
3
4
CC
CC
CC
Fig.3. Waveform of VCC Terminal Vol. at Start-up
Control Circuit Operation Start
Aux. Winding Vol.
Operation Failure
Time
Fig.4. Output Current IOUT – VCC Terminal Vol.
Without R7
With R7
Fig.5. Auxiliary Power Supply Circuit
not affected by Output Current
I
OUT
Addition
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