Daewoo DTF-2950-100D, Service Manual, Page: 77 / 92

CP-850FX Service Manual
Europe R&D 76
At the stand-by mode, as mentioned above, due to the burst-mode of the HIC’s intermittent
operation, the output voltage falls since the HIC stops its operation during the oscillation stop
period. While, during the stand-by operation, the intermittent operation repeating oscillation and
stop through the start-up resistor is provided because the transformer’s auxiliary winding voltage
supplying the power supply to the HIC is extremely decreased. Accordingly, the load except
stand-by load cannot be taken out at the stand-by operation (the period of intermittent operation).
Where the load excluding the stand-by load is imposed on the transition period from the stand-by
to normal operation, the incomplete start-up might be occurred. Thus, for the switching to the
normal mode, it is required to have the sequence (no load at stand-by should be imposed on the
normal mode), and the switching needs to be made after the power supply is completely turned
to the normal mode.
5.10.7 Step-Drive Circuit
The STR-W6700 series reduces noises at Turn-ON by
adopting the step-drive circuit for the MOSFET drive circuit as
shown in Fig. 19. The drive current at Turn-ON is controlled
at low by RG1 first, and it makes the gate voltage increase
gradually, and the gate voltage is increased rapidly through
RG1 + RG2 after 0.8µsec approximately. While, the
MOSFET drive voltage adopts the constant voltage drive
circuit maintained at VDRM=7.6Vtyp, and it is not affected to
VCC. The MOSFET gate electric charge is discharged
rapidly through RG3 when the MOSFET is turned OFF.
That is, in the STR-W6700 series’ drive circuit, the gate
voltage imposed on the MOSFET is shifted with the two steps, which lowers the gate voltage at
Turn ON and controls the surge current flowing at Turn ON, and provides the ideal drive circuit
securing the sufficient gate voltage at normal drive mode.
5.10.8 Maximum ON Time Controlling Function
The MOSFET ON Time is controlled in the transition
mode such as a low input voltage or AC input ON and
OFF. The maximum ON Time is set at about 80% of
the oscillation cycle (= 1/fOSC approx. 45µsec) and
approximately 35µsec (TYP). While, for the design of
power supply, it is also required to monitor the
MOSFET ON time at maximum load and input voltage
minimum.
Fig.19. Step-Drive Circuit
Maximum OnTime
I
DS
V
DS
RG2
Control part
RG1
RG3
D
S/GND
３ ３
DRIV E
REG
１ １
Delay
Fig.20. Maximum ON Time
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