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E-8
THEORY OF OPERATION
E-8
PULSE WIDTH MODULATION
The term PULSE WIDTH MODULATION is used to
describe how much time is devoted to conduction 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 signal, the IGBT
is turned on and off for different durations during a
cycle. The top drawing below shows the minimum
output signal possible over a 50-microsecond time
period.
The positive portion of the signal represents one IGBT
group
1
conducting for 1 microsecond. The negative
portion is the other IGBT group
1
. The dwell time (off
time) is 48 microseconds (both IGBT 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 24 microseconds
each and allowing only 2 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 under the curve, the more power
is present.
FIGURE E.7 TYPICAL IGBT OUTPUTS
INVERTEC V250-S
MINIMUM OUTPUT
MAXIMUM OUTPUT
24
50
24
2
48
50
sec
sec
sec
sec
sec
sec
sec
sec
1
An IGBT group consists of two IGBT modules feeding one transformer primary winding.