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E-11
THEORY OF OPERATION
E-11
POWER WAVE 455/POWER FEED 10
microsecond. The negative portion is the
other IGBT group
1
. The dwell time (off time)
is 48 microseconds (both IGBT groups off).
Since only two microseconds of the 50-
microsecond time period is devoted to con-
ducting, the output power is minimized.
MAXIMUM OUTPUT
By holding the gate signal on for 24
microseconds each, and allowing only two
microseconds 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 that is under the curve indicates that
more power is present.
1
An IGBT group consists of two IGBT
modules feeding one transformer primary
winding.
PULSE WIDTH
MODULATION
The term Pulse Width Modulation is used to
describe how much time is devoted to con-
duction in the positive and negative portions
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 sig-
nal, the IGBT is turned on and off for differ-
ent durations during the cycle. The top
drawing in Figure E.9 shows the minimum
output signal possible over a 50-microsec-
ond time period.
The positive portion of the signal represents
one IGBT group
1
conducting for one
FIGURE E.9 — TYPICAL IGBT OUTPUTS.
(+)
1 µsec
48 µsec
50 µsec
1 µsec
(-)
(+)
(-)
2 µsec
24 µsec
50 µsec
24 µsec
MINIMUM OUTPUT
MAXIMUM OUTPUT
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