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THEORY OF OPERATION
E-12 E-12
POWER WAVE 450
The term PULSE WIDTH MODULATION is
used to describe how much time is devoted to
conduction in the positive and negative por-
tions of the cycle. Changing the pulse width is
known as MODULATION. Pulse Width
Modulation (PWM) is the varying of the pulse
width over the allowed range of a cycle to
affect the output of the machine.
MINIMUM OUTPUT
By controlling the duration of the gate signal,
the FET is turned on and off for different dura-
tions during a cycle. The top drawing above
shows the minimum output signal possible
over a 50-microsecond time period.
The positive portion of the signal represents
one FET group
1
conducting for 1 microsec-
ond. The negative portion is the other FET
group
1
. The dwell time (off time) is 48 micro-
seconds (both FET groups off). Since only 2
microseconds of the 50-microsecond time
period is devoted to conducting, the output
power is minimized.
MAXIMUM OUTPUT
By holding the gate signals on for 20 micro-
seconds each and allowing only 10 microsec-
onds of dwell time (off time) during the 50-
microsecond cycle, the output is maximized.
The darkened area under the top curve can be
compared to the area under the bottom curve.
The more dark area under the curve, the more
power is present.
PULSE WIDTH MODULATION
FIGURE E.11 – TYPICAL FET OUTPUTS
1
A FET group consists of the sets of FET modules grouped onto one switch board.
(+)
(+)
(–)
(–)
1 µsec
1 µsec
48 µsec
50 µsec
MINIMUM OUTPUT
MAXIMUM OUTPUT
20 µsec
10 µsec
20 µsec
50 µsec