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PULSE WIDTH MODULATION
The term PULSE WIDTH MODULATION (PWM) is
used to describe how much time is devoted to conduc-
tion in the positive and negative portions of the cycle.
Changing the pulse width is known as MODULATION.
Pulse Width Modulation 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 IGBT
is turned on and off for different durations during a
cycle. The top drawing below shows the minimum out-
put signal possible over a 50-microsecond time period.
The shaded portion of the signal represents one IGBT
group
1
, conducting for 1 microsecond. The negative por-
tion is the other IGBT group. The dwell time (off time) is
48 microseconds (both IGBT groups off). Since only 2
microseconds of the 50-microsecond time period are
devoted to conducting, the output power is minimized.
MAXIMUM OUTPUT
By holding the gate signals on for 24 microseconds each
and allowing only 2 microseconds of dwell or off time
(one microsecond during each half cycle) during the 50
microsecond cycle, the output is maximized. The dark-
ened area under the minimum output curve can be com-
pared to the area under the maximum output curve. The
more darkened area, the more power is present.
E-12 E-12
THEORY OF OPERATION
POWER WAVE 455/R
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FIGURE E.10 – TYPICAL IGBT OUTPUTS
MINIMUM OUTPUT
50
sec
25
sec
MAXIMUM OUTPUT
sec
sec
sec
24
sec
24
sec
sec
1
50
sec
sec
1
1
An IGBT group consists of the sets of IGBT modules grouped onto
one switch board.