Service manual

CP-850FX Service Manual
Europe R&D
68
5.10.2.2 Overload Protection
The output characteristics of the secondary side at the time
when the OCP circuit operates, due to the overload of the
secondary side output, is shown in Fig.8. Where the output
voltage falls below the overload mode, the auxiliary winding
voltage of the primary side also falls proportionally, and the
Vcc terminal voltage falls below shutdown voltage to stop the
operation. In that case, as the circuit current is also
decreased simultaneously, the Vcc terminal voltage rise
again by the start-up resistor Rs ‘s charging current, and the
circuit re-operates intermittently at the operation start-up
voltage. However, where the transformer has lots of output
windings and the coupling is not sufficient, and even if the
output voltage is reduced in overload mode, the operation may not be intermittent because the
primary side auxiliary winding voltage does not fall. Although the intermittent operation is not
provided, the operation itself can be protected by the OLP circuit.
In the overload mode (the mode in which the drain current is controlled by OCP operation), the
secondary side output voltage falls. Thus, the error-amplifier and photo-coupler in secondary side
need to be cut off. The STR-W6700 series recognizes the circumstances continuing OCP
operation without FB signal as overload mode, and the SS/OLP terminal voltage starts rising by
ISSOLP(OLP) as shown in Fig.9, and after the SS/OLP terminal voltage continues rising to reach
VSSOLP(OLP) TYP 5V, the oscillation is stopped and turns the latch protection operation.
Power MOSFET
Waveform
Stop
Voltage
Start-up
Voltage
Secondary
Output Voltage
OLP
detection level
V
CC
Secondary
Output Current
Feedback Current
(Feedback Voltage)
SS/OLP
OverLoad
Operation
Stop
Normal
StartUp
SS
V SSOL P( OL P)
OCP
Limit
I SSOL P( OL P)
図 9 過負荷時のﾀｲﾐﾝｸﾞﾁｬｰﾄ
Vou t
出力電圧
出力電流
Iou t
AC
低
AC 高
図 ８ 電源出力過負荷特性
Fig.9. Timing-Chart at Overload
Fig.8. SMPS Output Overload Characteristics
Vout
Iout
AC Low
AC High
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