Lincoln Electric 11141 Service Manual page 46

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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 the
cycle. The top drawing in Figure E.7 shows the mini-
mum output signal possible over a 50-microsecond
time period.
The positive portion of the signal represents one IGBT
group conducting for one microsecond. The negative
1
portion is the other IGBT group1. 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 conducting, the output power
is minimized.
THEORY OF OPERATION
FIGURE E.7 — TYPICAL IGBT OUTPUTS.
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.
An IGBT group consists of two IGBT modules feeding
1
one transformer primary winding.
POWER WAVE ® 355M/405M
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