13-4
2-2-2 Amp Terminal (pin 6), Oscillator, Regulated
Voltage Control Circuit
The oscillator utilizes charging and discharging characteristics
of C1 and C2 contained in a HIC (abbreviation of Hybrid IC)
and generates a pulse signal to turn on and off the MOS FETs.
The voltage regulation control under the switching power
supply operation is conducted by varying on time period of
the MOS FET with the off period fixed except under light
load condition. The on time period control is carried out by
directly varying output pulse width of the oscillator.
Fig. 13-8 shows operations of the oscillator under HIC single
operation (without voltage regulation control), and C2 is
charged at a specified voltage (about 5V) when the MOS
FET is on due to operation of the oscillator. On the other
hand, charging for C1 starts at almost zero through R2 and the
voltage across C1 increases with a slope determined by
multiplication of C1 and R2.
When the voltage across C1 reaches about 0.75V (TC =
25°C), the oscillator output is inverted and the MOS FET is
turned off.
At the same time C1 is rapidly discharged by a circuit inside
the oscillator and the voltage across C1 becomes almost zero.
With the MOS FET turned off, C2 starts discharging through
R3 and the voltage across C2 drops with a slope determined
by multiplication of C2 and R3.
When the voltage across C2 drops to about 3V, the oscillator
output is inverted again and the MOS FET is turned on. At the
same time, C2 is rapidly charged up to about 5V again.
Operations described in the previous page are repeatedly
conducted and the on/off operations of the MOS FET is also
repeated. In this way, C3 is charged with a voltage shown in
Fig. 13-9 for the off period from the ND coil through RQ10,
RQ09, RQ08 and DQ04. For the next on period, the voltage
across C3 is higher than the IOS terminal threshold voltage of
0.75V, the MOS FET is not turned on, but turned on after the
voltage becomes lower than 0.75V after the period of T,
thereby realizing a delay operation.
CQ09 is a resonant capacitor and CQ12 is a capacitor to delay
turn on time of the MOS FET. The operation described above
is called a partial resonant circuit.
The on time is controlled by flowing a current corresponding
to an output signal of QQ08 error amplifier provided with
secondary output circuit through a photo coupler QQ03 and
by varying the charging current.
The higher the AC voltage of the power supply and the lower
the load current, the higher the flowing current and the
smaller the on time period.
C2 Voltage
across
C1 Voltage
across
To Next Page
Loading...
