9
R-1750
R-1751
R-1752
R-1754
Wave-form of main parts are shown in Figure A-2.
During off-time, flyback voltage appears on the primary
winding of the high voltage transformer. The off-time de-
pends on the inductance of the transformer and capaci-
tance of the resonant capacitor C3. Synchronous circuit
generates the timing signal by detecting the flyback volt-
age. The power transistor Q1 is periodically switched on
when the collector voltage returns to zero (0) volt. High
voltage (about 2 KV) appears on secondary winding of the
transformer. The full-wave voltage doubler circuit increase
to about 4 KV and is applied to the magnetron. The effi-
ciency of the inverter unit itself is approx. 93%, and equal to
that of current efficiency. Ferrite core is used for the high
voltage transformer. It is superior to silicon steel core at
high frequency, low magnetic loss. The power level is
determined by the touch control board, and the PWM data
corresponding to the level is set to the IC of the inverter unit
through the control lines. The IC controls on-time of tran-
sistor Q1 by comparing the average value of the primary
current with the magnitude of the power level data. The IC
has the protection circuit which detects over-current of the
main circuit and the high voltage circuit, over-voltage of the
power transistor Q1 and AC power interruption. When
abnormal phenomenon is detected, the IC pauses on the
moment in order to protect the power transistor Q1 from
Figure A-1. Basic Circuit
being destroyed. After approx. 60 millisecond, it starts
generating the driving signal from the minimum pulse width
and expands the length of on-time slowly until the current
of the main circuit reaches the target level.
Figure A-2. Switching Wave-form
Driving signal
Collector
current of Q1
Collector
voltage of Q1
Secondary voltage
of high voltage
transformer
Synchronous
signal
A
A
Choke coil
Bridge diode
High voltage
transformer
Magnetron
R
103
R
102
C11
C12
D11
D12
C3
C2
Power
transistor
Q1
AC 120V / 60 Hz
+ V
IC
Driving
Circuit
Synchronous Circuit
Voltage Detector
Overcurrent and
Overvoltage Detector
Average Current
Synchronous
Signal for AC.
Output Control
Signal
