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PULSE WIDTH MODULATION
The term PULSE WIDTH MODULATION describes
how much time is devoted to conduction in the posi-
tive 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 22 microseconds
each and allowing only 3 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.
THEORY OF OPERATION
E-9 E-9
PRO-CUT 80
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1
An IGBT group consists of two IGBT modules feeding one transformer primary winding.
MINIMUM OUTPUT
MAXIMUM OUTPUT
22
50
22
3
48
50
sec
sec
sec
sec
sec
sec
sec
sec
FIGURE E.8 – TYPICAL IGBT OUTPUTS